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A geological reconnaissance of Bowie Seamout Herzer, Richard Howard 1970

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A GEOLOGICAL RECONNAISSANCE OF BOWIE SEAMOUNT BY RICHARD HOWARD HERZER B.Sc. Queen's U n i v e r s i t y , 1 9 6 6 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE In the Department of ' Geology We ac c e p t t h i s t h e s i s as conforming t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1 9 7 0 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree tha permiss ion fo r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l ga in s h a l l not be a l lowed without my w r i t t e n p e r m i s s i o n . Depa rtment The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8 , Canada Date Frontispiece - Photograph taken on the eastern flank of Bowie Seamount at a depth of 3 » C 0 0 f t . ABSTRACT Bowie Seamount, a submerged v o l c a n o s i t u a t e d o f f the west c o a s t of Canada a t 5 3 ° l 8 ; N, 1 3 5 ° 3 9 ' W, has a r e l i e f o f 1 0 , 0 0 0 f t . and comes to w i t h i n 1 0 0 f t . of the ocean s u r f a c e . I t i s made up of a s e r i e s of i n t e r s e c t i n g . r i d g e s which t o g e t h e r g i v e the mountain an o v e r a l l n o r t h e a s t - southwest e l o n g a t i o n . I t appears t o be a c o m b i n a t i o n c e n t r a l and f i s s u r e type v o l c a n o which has been b u i l t up over a system o f i n t e r s e c t i n g f r a c t u r e s i n the o c e a n i c c r u s t . Two t e r r a c e s form the f l a t t e n e d summit of the v o l c a n o a t approximate depths of 4 5 and. 1 3 0 fathoms. These are thought to be the remains o f p l a t f o r m s produced by combined wave e r o s i o n and s h a l l o w - w a t e r v u l c a n i s m d u r i n g l a t e Q u^ernary time when sea l e v e l was lower than i t i s today. The l a s t phase o f v o l c a n i c a c t i v i t y on the summit o c c u r r e d a f t e r the f o r m a t i o n of the upper t e r r a c e no more than 1 8 , 0 0 0 y e a r s ago. Samples dredged from the upper h a l f of the v o l c a n o i n c l u d e : p i l l o w fragments, fragments o f n o n - p i l l o w e d f l o w s , p i l l o w b r e c c i a s , bombs, t u f f s , a s h , and u n s o r t e d t e p h r a . The r o c k s are m a i n l y a l k a l i o l i v i n e b a s a l t s , accompanied by r a r e a n d e s i t e s which, presumably, were d e r i v e d by d i f f e r e n t i a t i o n of the b a s a l t i c magma. F e l d s p a t h i c and g a b b r o i c i n c l u s i o n s , many of which appear- t o be cumulates, are common i n the b a s a l t ; I c e - r a f t e d r o c k s a re r a r e on the summit o f Bowie Seampunt but a r e common on i t s n e a r e s t neighbour - Hodgkins Seamount. A ferro-manganese d e p o s i t , a p p a r e n t l y over 1 m i l l i o n y e a r s o l d , t h a t e x i s t s on the summit of Hodgkins Seamount, suggests t h a t t h i s peak i s r e l a t i v e l y much o l d e r than the.summit a r e a of Bowie Seamount. P a l a g o n i t e appears t o form as the i n i t i a l phase of w e a t h e r i n g of g l a s s y b a s a l t s i n the a r e a o f study but the p r o d u c t s o f more advanced w e a t h e r i n g a re m o n t r a o r i l l o n i t e and z e o l i t e s . Rock fragments t h a t have been rounded by c h e m i c a l w e a t h e r i n g a re common. TABLE OF CONTENTS I . - I n t r o d u c t i o n 1 I I . F i e l d Work A . 1 9 6 7 c r u i s e k B. 1 9 6 8 c r u i s e 5 I I I . Geomorphology A. Topography 1 3 B. T h e o r e t i c a l Development o f f l a t - t o p p e d seamounts. 1 7 C. Geomorphic Development of Bowie Seamount 2 3 IV. P e t r o l o g y A. Lava f l o w s 3 8 B. I n c l u s i o n s 5 1 C. Tephra 6 6 D. V e s i c l e L i n i n g s and amygdules °? E. P o s t - v o l c a n i c d e p o s i t s 9 ^ F. Chemical w e a t h e r i n g 9 7 V. Summary and C o n c l u s i o n s 1 0 2 V I . R e f e r e n c e s 1 0 7 V I I . Appendices L I S T _ p ^ TABLES Table I - Chemical c o m p o s i t i o n s o f r o c k samples 43 Table I I - Normative c o m p o s i t i o n s of r o c k s t a p l e s 4 4 Table I I I - Modal c o m p o s i t i o n s o f a n a l y s e d r o c k samples 45 LIST OF I L J J J ^ A T I O N o FIGURES F i g u r e 1 - Index map 2 F i g u r e 2 - Sample L o c a t i o n Map 6 F i g u r e 3 - S h i p ' s T r a c k 8 F i g u r e 4 - Topographic d e t a i l o f summit 1 0 F i g u r e 5 - Topographic d e t a i l o f summit ( 1 9 6 9 ) 1 1 F i g u r e 6 - S h i p ' s t r a c k over summit 1 2 F i g u r e 7 - Topography of Bowie Seamount 1 4 F i g u r e 8 - S i l h o u e t t e s o f Bowie Seamount 1 6 F i g u r e 9 - Model f o r f o r m a t i o n o f f l a t - t o p p e d Seamounts I 2 0 F i g u r e 1 0 - Model f o r f o r m a t i o n o f f l a t - t o p p e d Seamounts I I 2 1 F i g u r e 1 1 - Kolocene sea l e v e l changes 2 o F i g u r e 1 2 - Suggested s t r u c t u r e o f Bowie Seamount J>k F i g u r e 1 3 - O r i g i n o f d i f f e r e n t l a v a s from the same magma chamber 65 PLATES. P l a t e 1 - Sonar p r o f i l e s over the summit 1 8 P l a t e 2 - Reworked l a p i l l i d e p o s i t s 26 P l a t e 3 - D e p o s i t of p i l l o w fragments 3 5 P l a t e 4 - D e p o s i t o f s m a l l p i l l o w cru.-;t frag- ..ts 3 6 P l a t e 5 - Types o f f l o w r o c k . 3 9 P l a t e 6 - T e x t u r a l change from t a c h y l y t e t o sideromelane • 4 0 P l a t e 7 - M i c r o s c o p i c t e x t u r e s o f the b a s a l t s 4 7 P l a t e 8 - Photomicrographs o f the a n d e s i t e s 5 0 P l a t e 9 - Pyroxene cumulate 5 3 P l a t e 1 0 - Andesine - o l i v i n e cumulate 5 5 P l a t e 1 1 - . P l a g i o c l a s e - hornblende cumulate 5 7 P l a t e 1 2 - H a r r i s i t i c overgrowth on a n o r t h o s i t e x e n o l i t h 60 P l a t e 1 3 - P l a g i o c l a s e - magnetite x e n o l i t h 6 2 P l a t e 1 4 - Graphic a l t e r a t i o n t e x t u r e of hornblende 6 4 P l a t e 1 5 - Bombs found on the seamount 6 8 P l a t e 1 6 - Bombs found on the seamount 6 9 P l a t e 1 7 - Bombs found on the seamount 7 1 P l a t e 1 8 - Bottom photographs o f t u f f o c c u r r e n c e 7 4 P l a t e 1 9 - Cementation of the t u f f s 7 5 P l a t e 2 0 - Cementation o f the t u f f s 7 6 P l a t e 2 1 • - Cementation o f the t u f f s ^ 7 7 P l a t e 2 2 - Loose l a p i l l i d e p o s i t o f the 4 5 fathom t e r r a c e 7 9 P l a t e 2 3 - R i p p l e marks i n s h e l l m a t e r i a l 8 0 ) P l a t e Zk - Bedded ash 82 P l a t e 2 5 - P i l l o w b r e c c i a 85 P l a t e 2 6 - Bottom photographs i n a r e a o f p i l l o w b r e c c i a °° P l a t e 2 7 - Rubbly s u b s t r a t e on the seamount's f l a n k ^7 P l a t e 2 8 - V e s i c l e l i n i n g s and amygdules 9 0 P l a t e 2 9 - V e s i c l e l i n i n g s and amygdules 9 2 P l a t e 3 0 - V e s i c l e l i n i n g s and amygdules 9 3 9 6 P l a t e 3 2 - P a l a g o n i t e i n hand specimen 9 8 P l a t e 3 3 - A l t e r a t i o n • 1 0 1 P l a t e 3 1 - Ferro-manganese n o d u l e s from Hodgkins Seamount ACKNOWLEDGEMENTS The author wishes to express his thanks to Drs, K.C. McTaggart, R.E. Garrison and J.W. Murray who acted as thesis advisors. The enthusiastic co-operation of the o f f i c e r s and crew of CNAV ENDEAVOUR and of fellow U.B.C. students and technicians i s also appreciated. Thanks must go to Dr. W.H. White and his technical s t a f f at U.B.C. for t h e i r K - Ar age ^ determination, to Dr. B.E.B. Cameron of the Geological Survey of Canada for his work on the microfauna, to Dr. J.G. Souther of the Geological Survey for his help and advice on the cruise and for providing rock analyses, and f i n a l l y to Dr. J.A. Scrimger of D.R.E.P. for his w i l l i n g exchange of data on the seamount. Financial support for t h i s project was kindly provided by the National Research Council of Canada, the International Nickel Company of Canada and the Deans Research Fund of the University of B r i t i s h Columbia.- The author i s indebted to the personnel of Defence Research Establishment P a c i f i c for their willingness and help i n providing the ship. Without the support of these organizations this project could not have been undertaken. 1 INTRODUCTION A. PurjD ojb e_and Scojp e_of the Study The a r e a of study encompasses Bowie Seamount, a submarine v o l c a n o i n the n o r t h e a s t P a c i f i c Ocean 1 0 0 m i l e s west of the Queen C h a r l o t t e I s l a n d s , B r i t i s h Columbia. The purpose of the p r e s e n t i n v e s t i g a t i o n was to determine the form of the seamount, the g e o l o g i c p r o c e s s e s t h a t have c o n t r i b u t e d to i t s development, and the age of i t s f o r m a t i o n . I t s p r o x i m i t y to t h e coast and the nearness of i t s summit to the s u r f a c e ( 1 5 fathoms) make i t w e l l s u i t e d f o r g e o l o g i c i n v e s t i g a t i o n . 3. G e o l o g i c S e t t i n g The seamount, (53°lS»N, 1 3 5 ° 3 9'W) (Scrirnger 1 9 6 9 ) i s s i t u a t e d on the c o n t i n e n t a l r i s e on the s o u t h e r n p a r t o f the P r a t t - W e l k e r c h a i n ( F i g u r e I ) i n the G u l f o f A l a s k a Seamount P r o v i n c e (Menard, 1 9 6 4 ) . The P r a t t - Welker group i s a l i n e o f seamounts and guyots which s t r i k e s s o u t h e a s t from Kodiak Seamount i n the A l e u t i a n Trench towards the Queen C h a r l o t t e I s l a n d s o f f the west c o a s t o f Canada. E a s t o f Bowie Seamount, a l o n g the c o n t i n e n t a l margin, l i e s the Queen C h a r l o t t e F a u l t ( S t . Arnand, 1 9 5 7 and S u t h e r l a n d Brown, 1 9 6 8 ) which may be a major s t r u c t u r a l element of the E a s t P a c i f i c R i s e . The r e l a t i o n s h i p s o f the P r a t t - W e l k e r c h a i n and Bowie Seamount to the E a s t P a c i f i c R i s e , to the c o n t i n e n t a l 150° 145° 140° 135° 130" 125° INSET MAP BASED ON U.S. NAVAL OCEANOGRAPHIC CONTOUR SHEETS: 410 AND 411. |35° |3Q° 125° 3 margin and to the r e c e n t l y d e l i n e a t e d magnetic anomaly bands i n the r e g i o n are s t i l l not c l e a r . P r e v i o u s Work on the'Seamount Only one p r e v i o u s geologic- e x p e d i t i o n has been made to Bowie Seamount, the r e s u l t s of which were p u b l i s h e d i n a b s t r a c t form ( H u r l e y and Nayudu, 1 9 6 1 ) . These a u t h o r s found an e x t e n s i v e t e r r a c e near the summit between 1 2 0 and 1^0 fathoms, above which, s m a l l p i n n a c l e s r i s e an a d d i t i o n a l 1 0 0 fathoms. They r e c o v e r e d p a l a g o n i t i c m a t e r i a l from the t e r r a c e and f r e s h b a s a l t from the peaks and co n c l u d e d t h a t the peaks were formed by p o s t - g l a c i a l v u l c a n i s m . In 1 9 6 2 , Nayudu proposed t h a t the t e r r a c e was not e r o s i o n a l but a p r o d u c t of p r i m a r y v u l c a n i s m . E n g e l and E n g e l ( ( 1 9 6 3 ) i n c l u d e d an a l k a l i b a s a l t from Bowie Seamount i n t h e i r a n a l y s e s o f n o r t h e a s t P a c i f i c b a s a l t s . Other work on the seamount has been l i m i t e d t o i n f r e q u e n t c r o s s i n g s d u r i n g g e n e r a l b a t h y m e t r i c s u r v e y s c a r r i e d out by the U.S. Navy and the U.S. Coast and Geodetic Survey around the time of the Second World War. D. Scope of_ the F i e l d j f t o j r k The f i e l d work was c a r r i e d out over a t o t a l of about two weeks d u r i n g two s h o r t v i s i t s to Bowie Seamount i n the summers of 1 9 6 7 and 1 9 6 8 . The v e s s e l used f o r both c r u i s e s was the Canadian N a v a l A u x i l i a r y V e s s e l ENDEAVOUR, furnish e d , by Defence Research E s t a b l i s h m e n t P a c i f i c i n E s q u i m a l t , B r i t i s h Columbia. J u s t over 600 m i l e s of echo sounding l i n e s were run i n a p a t t e r n over the seamount and bottom samples were o b t a i n e d from seventeen l o c a l i t i e s and underwater photographs from f o u r , I I . FIELD WORK A. 1 9 6 7 C r u i s e The f i r s t phase of the f i e l d work on Bowie Seamount was completed d u r i n g t h r e e days of a two-week c r u i s e i n 1 9 6 7 devoted p r i m a r i l y to a c o n t i n u o u s s e i s m i c p r o f i l i n g survey of the c o n t i n e n t a l s h e l f o f f Canada's west c o a s t . At t h a t t i m e , a p r e l i m i n a r y survey was made by echo sounder f o l l o w e d by dredge and grab sa m p l i n g s t a t i o n s i n the s h a l l o w waters of the summit. Si n c e s u f f i c i e n t l y a c c u r a t e f i x i n g of p o s i t i o n s by LORAN or by c e l e s t i a l n a v i g a t i o n v/as i m p o s s i b l e , n a v i g a t i o n was to have been e f f e c t e d by f r e q u e n t r a d a r f i x e s on an aluminum r e f l e c t o r mounted on a t a l l spar buoy which was moored on the summit of the seamount. U n f o r t u n a t e l y , l o s s o f the r e f l e c t o r i n rough weather s h o r t l y a f t e r the buoy was s e t out, n e c e s s i t a t e d r e s o r t i n g t o n a v i g a t i o n by dead r e c k o n i n g w i t h o c c a s i o n a l LORAN f i x e s and s i g h t i n g s of the c r i p p l e d buoy, 5 The proposed s h i p ' s t r a c k was i n the form of an e l o n g a t e M a l t e s e c r o s s w i t h two superimposed c o n c e n t r i c c i r c l e s . T h i s p a t t e r n was r e t a i n e d d u r i n g the sonar survey u s i n g the s h i p ' s l o g to e s t i m a t e the d i s t a n c e covered and the headings on the Gyro compass to approximate the a c t u a l course. V i s u a l c o n t a c t s w i t h the the buoy were r e c o r d e d . . A f u r t h e r c l o s e p a t t e r n o f f o u r m u t u a l l y a d j a c e n t one-mile square s r u n m one v i c i n i t y of the buoy i n o r d e r to o b t a i n d e t a i l e d t o p o g r a p h i c c o n t r o l i n the a r e a to be sampled. The c o m p i l a t i o n o f the c o n t i n u o u s sounding l i n e s , o b t a i n e d a f t e r c o r r e c t i o n s were made f o r c l o s u r e on the b a s i s of f i v e v i s u a l f i x e s on the buoy, r e v e a l e d t h a t the s h i p had been s e t so f a r t o the s o u t h e a s t by wind and ocean c u r r e n t s d u r i n g the survey t h a t o n l y h a l f of the seamount was a c t u a l l y mapped. S i x dredge h a u l s and seven grab samples were taken on the c e n t r a l peaks and the t e r r a c e s ( F i g u r e 2 ) . 3 • 1 9 6 , 8_Cr u i se D u r i n g the 1 9 6 8 c r u i s e , the e n t i r e seamount was r e s u r v e y e d . More a c c u r a t e and d e t a i l e d t o p o g r a p h i c coverage was o b t a i n e d and sample c o n t r o l was extended out onto the f l a n k s ( F i g u r e 2 ) . A magnetic survey was r u n i n c o n j u n c t i o n w i t h the sounding survey by the  7 Department o f Geophysics at t h i s U n i v e r s i t y ( M i c h k o f s k y , • 1 9 6 9 ) . The magnetic survey r e q u i r e d v e r y c l o s e s p a c i n g of l i n e s and c o n s e q u e n t l y a t o t a l of 4 7 5 m i l e s of s i m u l t a n e o u s magnetic and sounding l i n e s were o b t a i n e d . Employing the same b a s i c n a v i g a t i o n t e c h n i q u e as t h a t of the p r e v i o u s summer, t h a t i s , r a d a r f i x e s on a s i n g l e s t a t i o n a r y t a r g e t , the s h i p ' s t r a c k was planned i n the form of an expanding r e c t a n g l e superimposed on a s e r i e s of s i x t e e n r a d i a t i n g l i n e s , the whole r e s e m b l i n g a r e c t a n g u l a r s p i d e r web ( F i g u r e 3 ) . The r a d a r t a r g e t , anchored on the summit of the seamount, n a t u r a l l y formed the c e n t r e o f the p a t t e r n . S i n c e the s u c c e s s of the e n t i r e v e n t u r e depended on a c c u r a t e and r e l i a b l e n a v i g a t i o n a more e l a b o r a t e n a v i g a t i o n system than t h a t employed i n 1 9 6 7 was used. A r a d a r t r a n s p o n d e r (Appendix A) was mounted on top of a 2 0 f t . l o n g s t e e l spar buoy, 8 f t of which p r o j e c t e d above the water. The u n i t , t r i g g e r e d by the beam from the i n t e r r o g a t i n g r a d a r aboard the s h i p , r e p l i e d a u t o m a t i c a l l y w i t h a s t r o n g s i g n a l which produced a d i s t i n c t i v e double b l i p on the s h i p ' s r a d a r s c r e e n . T h i s n a v i g a t i o n system, which proved t o be h i g h l y r e l i a b l e , f u r n i s h e d a range and b e a r i n g to the buoy a t any time which were a c c u r a t e - t o w i t h i n 0 . 2 m i l e s and 0 . 5 degrees r e s p e c t i v e l y at d i s t a n c e s up t o 11+ m i l e s . r3' I JO- FIGURE 3- . B O W I E S E A M O U N T SHIP'S TRACK CONTOUR INTERVAL - 100 fmt SCALE ' NAUTICAL WILES KILOMETRES .i 0 I . 2 S « 5 • T I5J* I/O'-.. /J&OO' 135'50 I35'*0 I3S'30':. " 9 I n t e r m i t t e n t weak s i g n a l s were p i c k e d up when the s h i p was once l 6 m i l e s from the buoy. D u r i n g the s u r v e y , f i x e s were taken every h a l f m i l e or a t even c l o s e r i n t e r v a l s , the p o s i t i o n was p l o t t e d , and c o r r e c t i o n s were made i n the s h i p ' s h e a d i n g to c o u n t e r a c t the v a r y - i n g s e t . 3y t h i s method, the s h i p was a b l e t o f o l l o w c l o s e l y the planned s h i p ' s t r a c k , w i t h the r e s u l t t h a t the e n t i r e survey a r e a was u n i f o r m l y covered w i t h a c c u r a t e c o n t i n u o u s sounding p r o f i l e s ( F i g u r e 3 ) . A d e t a i l e d s u r v e y of the summit above the 2 0 0 fathom l e v e l was made a t the end o f the 1 9 6 8 c r u i s e ( F i g u r e 4 ) , and i n March o f 1 9 6 9 } the summit was r e s u r v e y e d by Defence Research E s t a b l i s h m e n t P a c i f i c u s i n g CSS PAR1ZEAU ( F i g u r e 5 ) . The maps r e s u l t i n g from these two independent s u r v e y s agree t o w i t h i n 0 . 2 n a u t i c a l m i l e s ( S c r i m g e r , 1 9 6 9 ) , a f i g u r e which does not exceed the i n t r i n s i c e r r o r of the r a d a r n a v i g a t i o n system used on the survey. The U.B.C. survey employed a s e r i e s of p a r a l l e l t r a c k s spaced a q u a r t e r of a m i l e a p a r t w i t h n a v i g a t i o n f i x e s t a k e n every q u a r t e r m i l e . To t h i s was added d a t a from the sounding l i n e s which c r o s s e d the summit d u r i n g the l a r g e s c a l e survey of the seamount ( F i g u r e 6 ) . The D.R.E.P. survey used s i m i l a r p a r a l l e l l i n e s spaced one q u a r t e r of a m i l e a p a r t w i t h n a v i g a t i o n a l f i x e s taken a t the b e g i n n i n g and end of eac FIGURE k BOWIE SEAMOUNT TOPOGRAPHIC DETAIL OF SUMMIT AREA ABOVE 200 FATHOMS CONTOUR INTERVAL - 20 FATHOMS CONTOURS CONTROLLED BY SOUNDING LINES < 1/2 MILE APART CONTOURS CONTROLLED BY SOUNDING LINES 1 -3 MILES APART SCALE . NAUTICAL MILES i i i i i r 1 i i i 1/2 0 1/2 I I 1/2 2 KILOMETRES V II M l I I I 13 5° 39 200'W FIGURE 5 - Topogrophic map of the summit of Bowie Seamount ( after Scrimger, 1969) . / 'J LOCATIONS OF PROFILES APPEARING IN PLATE Lines run in 1968 Line run in 1967. (location approximate) F 1 3 l i n e . D r edging a t f o u r s t a t i o n s and underwater photo- graphy at four o t h e r s t a t i o n s were s u c c e s s f u l l y under- taken d u r i n g the 1 9 6 8 c r u i s e but a t t e m p t s at s a m p l i n g i n deep water w i t h grab, snapper, dredge and c o r e r met w i t h l i t t l e s u c c e s s . I I I . GE OMOR PI-IOLO G Y A- Topography , C o m p i l a t i o n of the sounding r e c o r d s r e v e a l e d a c l o s e agreement i n depths at sounding l i n e i n t e r s e c t i o n s , u s u a l l y to w i t h i n 1 0 fathoms. Only a t 7 out of a t o t a l of 6 7 i n t e r s e c t i o n s d i d depths d i s a g r e e by 5 0 fathoms .or more. The o c c a s i o n a l d i s c r e p a n c y of t h i s magnitude i s t o be expected i n vie?; o f the p o t e n t i a l s o u r c e s of e r r o r . The.maximum s l o p e measured on the seamount i s 3 0 ° but most s l o p e s range between 1 0 ° and 2 0 °.• The water depths range from 1 5 to 1,800 fathoms. I t i s e s t i m a t e d t h a t the steep s l o p e s , the g r e a t d e p t h s ; the n a v i g a t i o n e r r o r o f 0 . 2 miles,- and the 3 0 ° spread of the sonar beam c o u l d e a s i l y combine to produce an e r r o r of 1 0 0 fathoms or more. A l l the major d i s c r e p a n c i e s d i d , i n f a c t , occur i n deep water or where steep s l o p e s and' r a p i d l y changing bottom topography p r e v a i l e d . The map ( F i g u r e 7 ) was contoured by hand. Where the d i s c r e p a n c y i n depths at a - p r o f i l e i n t e r s e c t i o n was  1 5 l a r g e enough to produce a s i g n i f i c a n t s h i f t i n the contour l i n e s , the l i n e w i t h the l e a s t r e l i a b l e b a t h y m e t r i e r e c o r d was d i s c a r d e d . D u r i n g p a r t s of the s u r v e y , m a l f u n c t i o n s i n the sonar equipment r e s u l t e d i n d i s c o n t i n u o u s r e c o r d s , hence the l e s s r e l i a b l e bathymetry r e f e r r e d t o above. I f a major d i s c r e p a n c y o c c u r r e d where r e l i a b l e l i n e s c r o s s e d each o t h e r , the c o n f l i c t i n g soundings were d i s c a r d e d and the c o n t o u r s were drawn to s a t i s f y s u r r o u n d i n g p o i n t s which were i n agreement. A l t h o u g h the survey a r e a d i d not extend f a r enough eastward t o i n c l u d e a l l of Bowie Seamount, the r e s u l t i n g map shows t h a t the seamount g e n e r a l l y has the form o f a n o r t h e a s t - s t r i k i n g mountain 1 3 m i l e s wide on tne average and a t l e a s t 3 0 m i l e s l o n g ( F i g u r e 7 ) which r i s e s a b r u p t l y from the g e n t l y s l o p i n g ocean f l o o r ( F i g u r e 8 ) . The r e g i o n a l survey by the U.S. Navy ( F i g u r e 1 , i n s e t ) i n d i c a t e s t h a t the n o r t h e a s t t r e n d o f Bowie Seamount meets the northwest t r e n d o f Hodgkins, Davidson and Denson Seamounts a p p r o x i m a t e l y a t r i g h t a n g l e s on i t s northwest s i d e , and t h a t s e v e r a l o t h e r seamounts on t h i s p a r t of the P r a t t - W e l k e r c h a i n are . s i m i l a r l y e l o n g a t e i n these two d i r e c t i o n s . . The h i g h e s t peak, which stands 1 0 , 0 0 0 f t . above the ocean f l o o r , dominates the southwestern h a l f of the seamount and i s e l o n g a t e d i n a n o r t h e a s t - s o u t h w e s t VERTICAL SCALE 0 500 1000 1500 2000 0 1000 2000 3000 NW HORIZONTAL SCALE NAUTICAL MILES 0 1 2 3 KILOMETRES D I 2 3 4 5 6 7 SE FIGURE 8. S I L H O U E T T E S O F B O W I E S E A M O U N T 1 7 d i r e c t i o n . To the n o r t h e a s t ,t a lower r i d g e c o n t i n u e s a l o n g the same t r e n d but a t a s l i g h t n o r t h e r l y o f f s e t , - and comprises most of the r e m a i n i n g h a l f o f the mountain. Together, these two f e a t u r e s c o n t r o l the o v e r a l l n o r t h e a s t - s o u t h w e s t e l o n g a t i o n of Bowie Seamount. A s i m i l a r r i d g e s t r i k e s eastward from the main mass or summit r i d g e , f o r m i n g a major spur which, l i k e the n o r t h e a s t r i d g e , extends beyond the e a s t e r n l i m i t of the map a r e a . To the south o f t h i s r i d g e , a s m a l l l o b e p r o j e c t s beyond the f o o t of the mountain i n t o the s u r r o u n d i n g p l a i n o f the ocean f l o o r . The e a s t e r n f l a n k of the seamount i s f u r t h e r c o m p l i c a t e d by a s e r i e s of s m a l l peaks and v a l l e y s . The f l a t t e n e d summit i s seen i n F i g u r e s k and 5 to be, i n e f f e c t , a d i s c o n t i n u o u s t e r r a c e at a depth of 1 3 0 fathoms which i s surmounted by a s e r i e s of peaks, most of which are a l i g n e d a l o n g the n o r t h e a s t t r e n d c h a r a c t e r i s t i c o f the seamount i t s e l f . F i g u r e s 4 and 8 , and P l a t e 1 r e v e a l t h a t these summit h i l l s a r e , i n t u r n , cut by a t e r r a c e between 3 5 and 5 5 fathoms. B. T h e o r e t i c a l Development o f F l a t - T o p p e d Seamounts T r a d i t i o n a l l y , f l a t - t o p p e d searnounts have been thought o f e i t h e r as the t r u n c a t e d and s u b s i d e d foundations of a n c i e n t v o l c a n i c i s l a n d s , or as a t o l l s t h a t have PLATE 1 - Sonar p r o f i l e s o f the suraa&t of Bowie Seamount. C l e a r l y v i s i b l e a r e : the lower t e r r a c e between 1 2 0 and I ' - L O fathoms ( p r o f i l e s E-F, G ~ H and J - K ) , the summit h i l l s which r i s e above i t ( p r o f i l e s E - F and J - K ) , the upper t e r r a c e between 3 5 and 5 5 fathoms which forms the f l a t t e n e d t o p s o f the summit h i l l s ( p r o f i l e s 1 A-B, C-D, E-F and J - K ) , and the s m a l l , rugged p i n n a c l e s which p r o j e c t , i n turn", above the l a t t e r t e r r a c e ( p r o f i l e s A-B, C-D and J - K ) . The l o c a t i o n ox the p r o f i l e s i s shown i n F i g u r e {$ ( o v e r l a y ) . 18 1 9 submerged due to a too r a p i d r e l a t i v e r i s e of sea l e v e l . Nayudu ( 1 9 6 2 ) proposed t h a t the l a r g e r seamounts become more or l e s s f l a t - t o p p e d due to t h e growth o f a l a r g e dome of magma w i t h i n the v o l c a n o beneath a p y r o c l a s t i c mantle ( F i g u r e 9 ) . H i s argument i s based on the f o l l o w i n g e v i d e n c e : 1 . Guyats have g e n e r a l l y steep s l o p e s , i n the o r d e r of 1 8 ° to 2 5 ° which would i n d i c a t e t h a t they were composed o f f r a g m e n t a l m a t e r i a l r a t h e r than l a v a flows.. He c i t e s the o c c u r r e n c e of p a l a g o n i t e t u f f s on the t e r r a c e s of Bowie and Cobb Seamounts as s u p p o r t i n g e v i d e n c e . 2 . S e i s m i c r e f r a c t i o n work on B i k i n i A t o l l ( R a i t t , 1 9 5 4 ) has shown the e x i s t e n c e of a h i g h v e l o c i t y , p r o b a b l y b a s a l t i c c o r e , capped by a lower v e l o c i t y , p o s s i b l y p y r o c l a s t i c l a y e r , upon which the r e e f l i m e s t o n e i s b u i l t . Jones ( 1 9 6 6 ) and K j a r t a n s s o n ( 1 9 6 6 ) proposed t h a t f l a t - t o p p e d seamounts are submarine analogues of the P l e i s t o c e n e i n t r a g l a c i a l v o l c a n o e s o f I c e l a n d and B r i t i s h Columbia ( F i g u r e 1 0 ) . These f l a t - t o p p e d v o l c a n o e s are thought to have grown w i t h i n l a k e s melted through an i c e cap by v o l c a n i c heat ( K j a r t a n s s o n , 1 9 4 3 and Mathews, 1 9 4 7 ) . The base Of such a vo l c a n o i s u s u a l l y composed of p i l l o w l a v a s , p i l l o w b r e c c i a s and h y a l o c l a s t i t e s which formed by q u i e t e f f u s i o n o f 20 - ' sea level I. . sea level 2 . Figure 9 - Method of fo rmat ion of f l a t - t o p p e d seamounts suggested by Nayudu (1962) . Stage I : Eruption on sea floor produces a mound of c last ic debris derived from the aqueous brecciation of lava f lows. Stage 2 : Growth of a dome beneath the clastic mound. The clastic mantle continues to grow, fed by lava reaching the surface. Stage 3 : Continued growth as in stage 2 with the development of more complex structure. product of successive eruptive phases during (d)'Terraced' and subsided. (b) Emerged. (a)Submerged. FIGURE 1 0 - Method of formation of f la t - topped seamounts suggested by Jones (1966). 2 2 b a s a l t i c l a v a i n deep water where perhaps the c o n f i n i n g p r e s s u r e was g r e a t enough to prevent p h r e a t i c and v o l c a n i c e x p l o s i o n s (McBirney, 1 9 6 3 ; K j a r t a n s s o n , 1 9 6 6 ; and Jones, 1 9 6 7 in-Moore and F i s k e , 1 9 6 7 ) . The upper r e a c h e s o f the v o l c a n o are dominated, by c l a s t i c v o l c a n i c d e b r i s which commonly o c c u r s i n s t e e p , o u t w a r d - d i p p i n g beds, and above t h i s , the f l a t mountain top i s formed by a s h i e l d of s u b a e r i a l b a s a l t f l o w s . The c l a s t i c m a t e r i a l i s a p p a r e n t l y the product p a r t l y o f p h r e a t i c e x p l o s i o n s , which c h a r a c t e r i s t i c a l l y accompany b a s a l t i c e r u p t i o n s i n s h a l l o w v a t e r , and p a r t l y of b r e c c i a t i o n o f s u b a e r i a l f l o w s where they e n t e r e d the waters o f the l a k e ( F u l l e r , 1 9 3 1 ) • • . . I f the l a v a s h i e l d i s absent, i t i s presumed e i t h e r t h a t i t was removed by p r o l o n g e d wave e r o s i o n , e x p o s i n g the c l a s t i c p l a t f o r m , or t h a t i t never e x i s t e d and t h a t the v o l c a n o succeeded o n l y i n b u i l d i n g a broad, wave-washed p l a t f o r m of t e p h r a . K j a r t a n s s o n ' s ( 1 9 6 6 ) work on S u r t s e y suggests t h a t t h i s young v o l c a n o may be a r e c e n t marine analogue o f the I c e l a n d i c t a b l e - mountains. Moore and F i s k e ( 1 9 6 9 ) observed t h a t the s u b s t r u c t u r e o f K i l a u e a V o l c a n o , H a w a i i , may s i m i l a r l y f i t t h i s model. R a i t t ' s s e i s m i c d a t a on B i k i n i A t o l l a p p a r e n t l y s u p p o r t s t h i s model as w e l l as i t does Nayudu's. 2 3 C. Geomorphic Development of Bowie Seanio_unt_ Because r o c k s dredged from the seamount care almost e x c l u s i v e l y b a s a l t v i t r o p h y r e s w i t h a wide v a r i e t y o f v o l c a n i c t e x t u r e s and s t r u c t u r e s , the seamount i s presumed t o be of v o l c a n i c o r i g i n . In the a u t h o r ' s o p i n i o n , submarine e r o s i o n on a s c a l e l a r g e enough to a f f e c t the g r o s s v o l c a n i c morphology of the seamount has not o c c u r r e d . L i t t l e i s known of the v o l c a n i c s u b s t r u c t u r e of Bowie Seamount but the prominent r i d g e s t h a t make up the v o l c a n o suggest t h a t i t grew from e r u p t i o n s a l o n g a system of f i s s u r e s ( F i g u r e 7, o v e r l a y ) . E x t r u s i o n s a l o n g a main n o r t h e a s t - s t r i k i n g set o f f i s s u r e s p r o b a b l y formed the l o n g summit and n o r t h e a s t r i d g e s . S u b o r d i n a t e f e e d e r s p r o b a b l y u n d e r l i e the spurs on the v o l c a n o ' s s o u t h e a s t e r n f l a n k . A second major f r a c t u r e i s assumed to u n d e r l i e Hodgkins and Davidson Seamounts, which extend northwestwards i n a l i n e from Bowie Seamount ( F i g u r e s 1 and 7 ) . The i n t e r s e c t i o n of the n o r t h e a s t - t r e n d i n g f r a c t u r e beneath Bowie w i t h the n o r t h w e s t - t r e n d i n g f r a c t u r e o f the Hodgkins - Davidson system r a y have a f f o r d e d the magma r e l a t i v e l y easy a c c e s s to the s u r f a c e and t h u s have accounted f o r the huge v o l c a n i c p i l e of the summit r i d g e . Bowie Seamount, t h e r e f o r e , appears 2 4 to bo a c o m b i n a t i o n c e n t r a l and f i s s u r e type v o l c a n o . The network of i n t e r n a l c o n d u i t s i n a vo l c a n o of t h i s s i z e and shape must be f a r from simple and many o f the s m a l l e r peaks on the seamount's f l a n k s may be p a r a s i t e cones. The n a t u r a l 1 0 ° to 2 0 ° s l o p e s of the summit r i d g e are t r u n c a t e d .around 1 4 0 fathoms, from which depth a broad t e r r a c e s l o p e s g e n t l y up to a depth o f 1 2 0 fathoms ( F i g u r e 8 and P l a t e 1 ) . Above t h i s p l a t f o r m r i s e h i l l s 3 0 0 to 7 0 0 f t . h i g h which b a r e l y a l t e r the f l a t - t o p p e d s i l h o u e t t e of the 1 0 , 0 0 0 f t . h i g h v o l c a n o . The fo u r h i g h e s t of these summit h i l l s a r e , i n t u r n , t e r r a c e d between 3 5 and 5 5 fathoms ( F i g u r e 4 and P l a t e 1 ) . S m a l l , rugged peaks p r o j e c t an a d d i t i o n a l 1 0 0 t o 2 0 0 f t . above t h i s upper t e r r a c e l e v e l , the t a l l e s t coming to w i t h i n 1 0 0 f t . , of the ocean s u r f a c e ( S c r i m g e r , 1969)• The s u r f a c e m a t e r i a l on the t e r r a c e s and summit h i l l s i s made up m a i n l y of t e p h r a and b r o k e n f l o w - r o c k . The f r e s h n e s s of the v o l c a n i c e j e c t a and the nearness o f the t e r r a c e s t o sea l e v e l suggest t h a t the t e r r a c e s were produced by combined wave e r o s i o n and s h a l l o w water v u l c a n i s m d u r i n g l a t e Quaternary sea l e v e l l o w s , and t h a t the summit h i l l s mark the c e n t r e s of e r u p t i o n . A d d i t i o n a l evidence s u p p o r t i n g wave e r o s i o n i s seen i n 2 5 the d i s t i n c t l y abraded and winnowed n a t u r e of some of the l a p i l l i d e p o s i t s . Two such d e p o s i t s wore d i s c o v e r e d : one on or j u s t below the 4 5 fathom t e r r a c e (sample 6 7 - 2 , Appendix 3) and the o t h e r j u s t o f f the 1 3 0 fathom t e r r a c e (sample 67 - 7, Appendix B). These are both composed o f w e l l rounded l a p i l l i and c o n t a i n abraded p h e n o c r y s t s and s h e l l fragments; such s h e l l fragments b e i n g very conspicuous i n sample 67 - 2 ( P l a t e 2 ) . A l t h o u g h the s o r t i n g o f the g r a i n s cannot be s a i d t o be good i n e i t h e r d e p o s i t , i t i s much b e t t e r i n sample 67.-- 7 than i n any o t h e r l a p i l l i d e p o s i t found on the seamount. The p r e s e n t depth of the t e r r a c e s s u g g e s t s t h a t they must have been formed at some time i n the past when e i t h e r the seamount s t o o d h i g h e r above the g e o i d or sea l e v e l s t o o d l o w e r . An e s t i m a t e d maximum sea l e v e l l o w e r i n g of 5 4 0 f t . i s g i v e n by Shepard ( 1 9 6 3 ) - A low P l e i s t o c e n e s t a n d , t h e r e f o r e , c o u l d c o n c e i v a b l y have accounted f o r the upper t e r r a c e , whose depth v a r i e s from 2 1 0 t o 3 3 0 f t . but a p p a r e n t l y not f o r the lower t e r r a c e , which has a depth o f 7 2 0 to 8 4 0 f t . I t i s r e a s o n a b l e to suppose, however, t h a t the seamount i t s e l f may have s u b s i d e d s i n c e the f o r m a t i o n o f the lower t e r r a c e . Subsidence of o c e a n i c v o l c a n o e s i s commonplace and guyots are common on the P r a t t - V.'elker c h a i n . PLATE 2 - Reworked L a p i l l i D e p o s i t s (a.) Weakly cemented v o l c a n i c conglomerate- found i n the v i c i n i t y o f the 4 5 fathom t e r r a c e . The r o c k i s composed of w e l l - rounded l a p i l l i , f r o s t e d p h e n o c r y s t s , and abraded s h e l l fragments (white i n the photograph) ( S t a t i o n 67 - 2 ) . (b) Loose d e p o s i t of w e l l - r o u n d e d l a p i l l i •and f r o s t e d p h e n o c r y s t s , found near the edge of the 1 3 0 fathom t e r r a c e . The s o r t i n g of t h i s d e p o s i t i s b e t t e r than t h a t of any o t h e r on the- seamount. The thumb-tack i n the lower r i g h t hand c o r n e r p r o v i d e s the s c a l e ( S t a t i o n 67 - 7) 26 2 7 I g n o r i n g s u b s i d e n c e , one can e s t i m a t e a maximum age f o r the upper t e r r a c e from p u b l i s h e d Quaternary sea l e v e l c u r v e s ( F i g u r e 1 1 ) (Kenney, 1 9 6 4 , Curray, 1 9 6 5> and Hopkins, 1 9 6 7 ) . The upper t e r r a c e i s composed o f u n s t a b l e , l o o s e or weakly cemented m a t e r i a l and i t seems c e r t a i n t h a t the t e r r a c e , which has a depth of 2 1 0 to 3 3 0 f t . , would not s u r v i v e i f sea l e v e l were t o drop f o r l o n g t o the l e v e l o f i t s l a t e W i s c o n s i n 1 low o f 4 0 0 f t . T h i s t e r r a c e then c o u l d not have been i n e x i s t e n c e p r i o r t o the low st a n d o f 1 8 , 0 0 0 to 2 2 , 0 0 0 y e a r s 3 . P . and must, t h e r e f o r e , have formed d u r i n g the Holocene t r a n s g r e s s i o n . A c c o r d i n g to t h e sea l e v e l c u r v e s , Holocene sea l e v e l was at the p r e s e n t upper t e r r a c e l e v e l between 1 3 , 0 0 0 and 1 8 , 0 0 0 y e a r s ago. I f the vo l c a n o has, i n f a c t , s u b s i d e d s i n c e t h a t t i m e , the t e r r a c e must be even younger, The proposed mjoximum age o f the upper t e r r a c e i s , t h e r e f o r e , 1 8 , 0 0 0 y e a r s . No s a t i s f a c t o r y age c o u l d be e s t a b l i s h e d f o r the lower t e r r a c e but t h e r e i s a h i g h p r o b a b i l i t y t h a t i t too was formed d u r i n g the P l e i s t o c e n e . I c e - r a f t e d m a t e r i a l i s v i r t u a l l y absent on both t e r r a c e s , but o c c u r s i n abundance on the n o r t h f l a n k and on the summit of nearby Hodgkins Seamount (Appendix B). The summit o f Bowie Seamount may, t h e r e f o r e , have stood' h i g h enough 2 8 M E T R E S BELOW PRESENT SEA L E V E L d u r i n g low P l e i s t o c e n e s t a n d s to prevent the passage of d e b r i s - l a d e n i c e b e r g s which, d u r i n g such p e r i o d s , must have c a l v e d i n g r e a t numbers from g l a c i e r s r e a c h i n g the sea. I f the l o w e r . t e r r a c e were a p r e - P l e i s t o c e n e f e a t u r e i t would n e v e r t h e l e s s have had ample o p p o r t u n i t y t o accumulate a c o n s i d e r a b l e q u a n t i t y o f i c e - r a f t e d d e b r i s . Furthermore the t e p h r a d e p o s i t s and t u f f s o f b oth t e r r a c e s c o n t a i n o n l y r e c e n t and P l e i s t o c e n e m i c r o - faunas (Cameron, 1 9 6 9 p e r s o n a l communication). P o s t - t e r r a c e v u l c a n i s m might have obscured a p r e - P l e i s t o c e n e fauna, i f such e x i s t e d .on the lower t e r r a c e , and might a l s o have b u r i e d any accumulated g l a c i a l m a t e r i a l . However, i t would a l s o t e n d to obscure the t e r r a c e i t s e l f V u l c a n i s m has, i n f a c t , c o n t i n u e d • u n t i l so r e c e n t l y on the summit t h a t the s u r v i v a l o f a p r e - P l e i s t o c e n e t e r r a c e would seem h i g h l y u n l i k e l y . I n view of the e x c e l l e n t p r e s e r v a t i o n of the 1 3 0 fathom t e r r a c e , i t i s p r o b a b l e t h a t t h i s f e a t u r e was a l s o formed d u r i n g l a t e P l e i s t o c e n e t i m e . Wave e r o s i o n d u r i n g p e r i o d s o f low P l e i s t o c e n e sea l e v e l s was e v i d e n t l y i n s t r u m e n t a l i n the f o r m a t i o n of these t e r r a c e s . However, evidence of p o s t - g l a c i a l v o l c a n i c a c t i v i t y i n d i c a t e s t h a t the summit morphology i s not due t o wave e r o s i o n a l o n e . 3 0 The summit peaks are s t r i k i n g i n t h e i r a l i g n ment p a r a l l e l t o the l o n g a x i s o f the seamount ( F i g u r e 5 ) , an a l i g nment which was p r o b a b l y c o n t r o l l e d by the s t r u c t u r e o f the v o l c a n o . I f these are e r o s i o n a l remnants o f a once g r e a t e r peak t h a t has been deeply d i s s e c t e d , they would have to be made of r e l a t i v e l y more r e s i s t a n t r o c k t o p r e s e r v e t h e i r s t r u c t u r a l a l i g n m e n t , i n which ca s e , they might be a l i n e of v o l c a n i c p l u g s or dykes. Work on p a r t of the c e n t r a l L-shaped system o f peaks ( F i g u r e 2 ) has i n d i c a t e d t h a t the b u l k of the s u r f a c e cover' i s composed of l o o s e or weakly cemented t e p h r a and l e s s e r amounts of f r a g m e n t a l f l o w r o c k . The s h a l l o w e s t p i n n a c l e was found by d r e d g i n g and d i v i n g t o be composed o f s t r o n g l y cemented f e r r u g i n o u s t u f f s . Some peaks, t h e n , are not s i m p l y v o l c a n i c p l u g s . The i n c o h e r e n c e o f the m a t e r i a l making up the c e n t r a l summit peaks p r e c l u d e s the p o s s i b i l i t y of t h e i r s u r v i v a l f o r l o n g as an i s l a n d d u r i n g the low sea l e v e l s t a n d which formed the lower t e r r a c e . R i c h a r d s ( 1 9 6 5 ) r e p o r t e d t h a t a t e p h r a sea c l i f f w i t h an average h e i g h t o f 2 0 0 f t . , formed by the 1 9 5 2 e r u p t i o n o f Barcena on I s l a San B e n e d i c t o , west o f Mexico, was cut back by wave e r o s i o n a t an average r a t e o f 5 . 5 f t . per day. An a s s o c i a t e d aa l a v a f l o w was s i m i l a r l y eroded at an average d a i l y 3 1 r a t e of 0J+ f t . per day.. At such r a t e s , the summit peaks, i f they were i s l a n d s , would have been d e s t r o y e d w i t h i n a year.' I t i s p r o b a b l e , t h e r e f o r e , t h a t the summit h i l l s were formed by v o l c a n i c e r u p t i o n s a f t e r the f o r m a t i o n and submergence o f a summit p l a t e a u , which now e x i s t s i n r e l i c form as the 1 3 0 fathom t e r r a c e . By the same t o k e n , the 4 5 fathom t e r r a c e l e v e l may be a l a t e r p l a n a t i o n s u r f a c e capped by l a t e t u f f cones. The t e p h r a on the summit o f the seamount a r e , f o r the most p a r t , e x t r e m e l y f r e s h (Chapter IV), as would be expected from such r e c e n t v u l c a n i s m . The evidence on the summit i m p l i e s , t h e r e f o r e , t h a t e r u p t i v e a c t i v i t y c o n t i n u e d a f t e r the f o r m a t i o n and submergence of the upper t e r r a c e l e s s than 1 8 , 0 0 0 y e a r s ago. T h i s c o n f i r m s the c o n c l u s i o n reached by H u r l e y and Nayudu ( 1 9 6 1 ) t h a t p o s t - g l a c i a l v u l c a n i s m formed the summit peaks on the 1 3 0 fathom t e r r a c e . T h i s apparent c l o s e a s s o c i a t i o n i n time between the l a t e s t e p i s o d e s o f v u l c a n i s m and the f o r m a t i o n o f the summit t e r r a c e s at sea l e v e l s u g g e s t s t h a t Bowie Seamount i s not s i m p l y the t r u n c a t e d f o u n d a t i o n o f an o l d v o l c a n i c i s l a n d t h a t was f i n a l l y washed away a f t e r a l o n g p e r i o d o f wave e r o s i o n . I n s t e a d , i t appears more l i k e l y t h a t the v o l c a n o reached sea l e v e l d u r i n g the l a s t 3 2 s t a g e s o f i t s a c t i v i t y and s t r u g g l e d f o r a w h i l e to b u i l d i t s e l f up above the sea s u r f a c e . Any v o l c a n i c i s l a n d s which may have thus been formed were p r o b a b l y r e p e a t e d l y p l a n e d o f f by wave a c t i o n ever the course of months or perhaps y e a r s , g i v i n g r i s e , i n the end, to f l a t p l a t f o r m s composed o f p y r o c l a s t i c and e r o s i o n a l d e b r i s , which now remain as t e r r a c e s . I f ephemeral i s l a n d s d i d , i n f a c t , e x i s t the b r e c c i a t i o n o f s u b a e r i a l l a v a f l o w s e n t e r i n g the sea ( F u l l e r , 1 9 3 1 ) say a l s o have c o n t r i b u t e d s u b s t a n t i a l l y t o the growth of the s e p l a t f o r m s by b u i l d i n g out f o r s e t beds of f l o w - f o o t b r e c c i a s . The be h a v i o u r o f Bowie Seamount d u r i n g l a t e P l e i s t o c e n e and Holocene time may, t h e r e f o r e , have been s i m i l a r t o t h a t o f modern submarine v o l c a n o e s such as C a p e l i n h o s ( Z b y s z e w s k i , i 9 6 0 ) and S u r t s e y ( T h o r a r i n s s o n , 196k and 1 9 6 5 ) which have been observed' e r u p t i n g a t sea l e v e l . The p o s t - P l e i s t o c e n e r i s e i n sea. l e v e l , the subsidence of the v o l c a n o and the t a p e r i n g o f f o f v o l c a n i c a c t i v i t y a p p a r e n t l y brought an end to i t s b r i e f s u b a e r i a l h i s t o r y and i t has p r o b a b l y remained i n a c t i v e ever s i n c e . The o r i g i n o f the t e r r a c e s appears to f i t the model suggested by Jones ( 1 9 6 6 ) and K j a r t a n s s o n ( 1 9 6 6 ) r a t h e r than t h a t proposed by Nayudu ( 1 9 6 2 ) . The t e r r a c e s are known to be p r e s e n t l y covered l a r g e l y by c l a s t i c d e b r i s and the summit peaks are a p p a r e n t l y composed of t u f f s 3 3 but whether a l a v a s h i e l d ever e x i s t e d on the summit o f the seamount i s not known. P i l l o w l a v a s and p i l l o w b r e c c i a s are a p p a r e n t l y more common on the s l o p e s of the seamount than on the summit (Chapter I V ) . A m o d i f i e d v e r s i o n o f Jones's diagram d e p i c t i n g the suggested p r e s e n t s t r u c t u r e of the summit o f Bowie .Seamount appears i n F i g u r e 1 2 . L a t e f l a n k e r u p t i o n s appear to have o c c u r r e d , p o s s i b l y a t the time o f the f i n a l summit e r u p t i o n s , f o r d e p o s i t s of broken p i l l o w s of v e r y f r e s h , b l a c k , g l a s s y b a s a l t , v i r t u a l l y f r e e of o r g a n i c growth or a l t e r a t i o n , are found 8 0 0 fathoms down the west f l a n k near a s m a l l cone on the same l i n e a r t r e n d as the summit peaks ( s t a t i o n 6 8 - 7 ) ( P l a t e s 3 and 4 , and F i g u r e 2 ) . b o u l d e r o f v e s i c u l a r b a s a l t , f i s h e d from the upper t e r r a c e at s t a t i o n 6 8 - 9 was dated by the whole-rock p o t a s s i u m - argon method and found t o be 7 5 > 0 0 0 - 1 0 0 , 0 0 0 y e a r s o l d . The use of the potassium ~ argon method f o r d a t i n g young, g l a s s y , submarine l a v a s has come under f i r e r e c e n t l y . P o t a s s i u m makes up l o s s than Z% of most b a s a l t s and K ^ , the r a d i o a c t i v e i s o t o p e , c o n s t i t u t e s about 1 / 8 4 0 0 o f the t o t a l p o t a s s i u m . The h a l f l i f e o f 1'}''® i s 1 , 3 1 0 m i l l i o n y e a r s and t h e r e f o r e the amount of r a d i o g e n i c argon i n a b a s a l t l e s s than NW SE FIGURE 1 2 - Suggested structure of Bowie Seamount according to the Jones hypothesis. The diagram represents a hypothetical northwest - southeast cross - section through the main body of the seamount. \ \ \ Tuff, ash, and possible f low-foot breccia. Pillow lava, pillow breccia and hyaloclastites. PLATE 3 - P i l l o w fragments on the west f l a n k of Bowie Seamount at S t a t i o n 6 8 - ?. The diameter of the compass i s 3 i n c h e s and the l e n g t h o f the vane i s 1 0 i n c h e s . 35 PLATE 4 - Loose d e p o s i t o f s m a l l p i l l o w c r u s t fragments and l a r g e r p i l l o w fragments on the west f l a n k ( S t a t i o n 6 8 - 7 ) . In the lower photograph, the t h i n cover of f i n e v o l c a n i c and p e l a g i c sediment has been d i s t u r b e d by the camera frame and the compass. The diameter of the compass i s 3 i n c h e s and the l e n g t h of the vane i s 1 0 i n c h e s . 36 1 m i l l i o n y e a r s o l d i s e x c e e d i n g l y s m a l l . Furthermore v o l c a n i c g l a s s w i l l l o s e argon w i t h the passage of t i m e , r e s u l t i n g i n a younger than a c t u a l date ( H i l l s and Baadsgaard, 1 9 6 7 ) . F l u i d i n c l u s i o n s , common i n p h e n o c r y s t s , may c o n t a i n argon, which w i l l g i v e an anomalously o l d age (as much as + 8 8 0 11.Y.) (Funkhausor et a l , 1 9 6 6 ) . G l a s s produced by r a p i d quenching of submarine l a v a s may c o n t a i n t r a p p e d argon - 1+0 which d i d not have a chance to escape by v e s i c u l a t i o n , o r , i f the h y d r o s t a t i c p r e s s u r e i s g r e a t enough, argon w i l l be s i m i l a r l y r e t a i n e d due t o l a c k o f v e s i c u l a t i o n , both s i t u a t i o n s y i e l d i n g o l d e r than a c t u a l ages (Dalrymple and Moore, 1 9 6 8 ) . To a v o i d as much e r r o r as p o s s i b l e , a v e s i c u l a r r o c k , f r e e of c h i l l e d s u r f a c e phenomena and f l u i d i n c l u s i o n s , which was found i n s h a l l o w water ( 6 0 fathoms) was s e l e c t e d f o r the age dating.. S i n c e Bowie Seamount was a p p a r e n t l y s t i l l a c t i v e towards the end o f the P l e i s t o c e n e i t i s o f g r e a t i n t e r e s t t o know when i t f i r s t appeared on the sea f l o o r . On the b a s i s o f magnetic d a t a , M i c h k o f s k y ( 1 9 6 9 ) b e l i e v e s t h a t the main body o f the seamount was formed d u r i n g a p e r i o d o f geomagnetic r e v e r s a l , p u t t i n g i t s age back at l e a s t 0 . 6 9 m i l l i o n y e a r s (Cox, 1 9 6 9 ) . I t s age beyond t h i s can o n l y be guessed a t . 3 8 IV. PETROLOGY• A. Lava Flows 1. Types and d i s t r i b u t i o n o f flov; r o c k • Rocks which appear t o have o r i g i n a t e d by q u i e t f l o w were r e c o v e r e d from n i n e s t a t i o n s s c a t t e r e d over the seamount (Appendix B ). On the b a s i s o f t h e i r p h y s i c a l appearance, these r o c k s f a l l i n t o t h r e e c a t e g o r i e s : p i l l o w fragments, t o p s o f n o n - p i l l o w e d l a v a f l o w s , and h i g h l y j o i n t e d n o n - v e s i c u l a r r o c k . Most o f these r o c k s o c c u r r e d as s u r f a c e r u b b l e i n the areas sampled. In o n l y one case ( s t a t i o n 67 - 4) were samples t a k e n from what appeared t o be a coherent l a v a f l o w , ( i ) P i l l o w Fragments About h a l f o f the samples o f flo w r o c k a r c fragments o f p i l l o w s . Most of these have the c h a r a c t e r - i s t i c p y r a m i d a l shape ( P l a t e 5a) and a quenched o u t e r s u r f a c e marked by a t r a n s i t i o n from a m i c r o l i t c - r i c h , t a c h y l y t e v i t r o p h y r e i n the i n t e r i o r t o a c l e a r sidero;- melane v i t r o p h y r e at the s u r f a c e ( P l a t e 6 ) . P a l a g o n i t e r i m s are uncommon, but may be l o c a l l y w e l l developed ( P l a t e 3 2 ) . Sideromelane i s thought to form at the s u r f a c e of a subaqueous f l o w where b a s a l t i c l a v a i s c h i l l e d so q u i c k l y t h a t i r o n i s r e t a i n e d i n s o l u t i o n , i m p a r t i n g an amber c o l o u r t o the t r a n s l u c e n t g l a s s , whereas t a c h y l y t e forms PLATE 5 - Types of f l o w - r o c k found on Bowie Seamount (a) P i l l o w fragments. The l o c a t i o n s of the samples a r e , from l e f t t o r i g h t : S t a t i o n 6 8 - 7 , S t a t i o n 6 8 - 1 , and S t a t i o n 6 7 - 2 . I n t a c t p i l l o w s were found a t S t a t i o n 6 8 - 1 . (b) Fragment from the s u r f a c e of a f l a t , n o n - p i l l o w e d l a v a f l o w . ( S t a t i o n 6 8 - 1 ) (c) B l o c k l a v a or t a l u s m a t e r i a l eroded from an o u t c r o p p i n g dyke or d i s s e c t e d f l o w . ( S t a t i o n 6 7 - 7 )  PLATE 6 - The t e x t u r a l change from t a c h y l y t e t o s i d e r o m e l a n c i n the c h i l l e d s u r f a c e of a v e s i c u l a r b a s a l t i c l a v a . The s u r f a c e o f the r o c k i s out of view, j u s t beyond the top of the photograph. ( P l a n e l i g h t ) ( T h i n s e c t i o n 68 - 7 - B). (x27)  j u s t below the s u r f a c e where the l a v a c o o l s l e s s r a p i d l y , a l l o w i n g the p r e c i p i t a t i o n of f i n e m agnetite which r e n d e r s the g l a s s b l a c k and opaque (Peacock, 1 9 2 8 ) I t i s d i f f i c u l t t o t e l l whether these p i l l o w fragments r e p r e s e n t the p r o d u c t s o f p i l l o w s which have broken up i m m e d i a t e l y a f t e r f o r m a t i o n , or of p i l l o w s w hich have d i s i n t e g r a t e d d u r i n g p o s t - v o l c a n i c e r o s i o n . P i l l o w fragments formed 1 0 0 % o f the l o o s e cover a t S t a t i o n 6 8 - 7 on the s t e e p l y s l o p i n g west f l a n k of the seamount ( P l a t e s 3 and 4 ) and were a l s o found i n abundance a t S t a t i o n 6 8 - 1 on the east f l a n k ( P l a t e s 2 5 and 2 6 ) . ( i i ) Tops o f n o n - p i l l o w e d f l o w s Fragments o f l a v a f l o w s w i t h o u t p i l l o w s t r u c t u r e were r e t r i e v e d at s e v e r a l l o c a t i o n s (Appendix B). The t o p s o f thes e f l o w s e x h i b i t the t a c h y l y t e - t o - sideromelane c h i l l t e x t u r e d e s c r i b e d above and may have a t h i n s k i n of p a l a g o n i t e . Bread c r u s t f r a c t u r i n g i s common on the upper s u r f a c e ( P l a t e 5 b ) . ( i i i ) A n g u l a r , n o n - v e s i c u l a r r o c k Whether t h i s type of r o c k r e p r e s e n t s a f l o w o f " B l o c k Lava", as d e s c r i b e d by Washington ( 1 9 2 6 ) and F i n c h ( 1 9 3 3 ) , or t a l u s m a t e r i a l from an exposed dyke o r p l u g cannot be a s c e r t a i n e d . The o n l y two specimens are 4 2 h i g h l y but i r r e g u l a r l y j o i n t e d , a n g u l a r , n o n - v e s i c u l a r b l o c k s ( P l a t e 5 c ) , the o n l y massive v o l c a n i c r o c k o b t a i n e d from the seamount. 2 • £^ t r o g r a p h y o f _ t h c_ _La v as Ba_salt_ The r o c k s of Bowie Seamount are mai n l y b a s a l t i c , a l t h o u g h a n d e s i t i c v a r i e t i e s do occur r a r e l y . The b a s a l t s are of the a l k a l i o l i v i n e type (Yoder and T i l l e y , 1 9 6 2 ) ( T a b l e s 1 and 2 ) and are a p p a r e n t l y s i m i l a r i n c o m p o s i t i o n t o o t h e r o c e a n i c b a s a l t s from s i m i l a r environments ( c f . E n g e l and E n g e l , 1 9 6 3 , 1 9 6 4 , 1 9 6 6 and En g e l et a l . , 1 9 6 5 ) - T h e i r o c c u r r e n c e on a h i g h v o l c a n i c s t r u c t u r e i s c o m p a t i b l e w i t h the h y p o t h e s i s o f Enge l and Engel ( 1 9 6 4 ) , which l i m i t s such r o c k s t o the l a r g e r o c e a n i c v o l c a n o e s . T h e i r r e s u l t s , and the r e s u l t s o f p r e v i o u s workers, suggest t h a t a l k a l i o l i v i n e b a s a l t s r e p r e s e n t l a t e phases of v u l c a n i s m which crown v o l c a n o e s formed p r i m a r i l y of t h o l c i i t i c b a s a l t . The b a s a l t s l i s t e d i n the t a b l e s were dredged from the summit r i d g e of Bowie Seamount i n depths s h a l l o w e r than 5 0 0 fathoms. I t i s p o s s i b l e t h a t the r o c k s on the lower r e a c h e s of the seamount are t h o l c i i t i c . The a l k a l i o l i v i n e b a s a l t , which i s the most wide- s p r e a d r o c k t y p e , i s a "dark g r e y , v e s i c u l a r , p o r p h y r i t i c Oyide T i 0 2 , i 2 o 3 r ?7 - 7 . 53.5 1.07 19.8 1.7 67.j:„.6 -„12. 45.4 2.60 18.5 1.0 oo ~,..4-„. 48 ;0 2.43 18.5 1.5 6 3 „ 9 45.2 3-10 16.4 . 0 . 6 PV 50 45-33 3-53 15.50. 1 .81 FeO 6.7 9.7 7.5 10 . 3 10.64 MnO 0.25 0.19 0 .15 0.18 0.23 NgO 1. 5.9 5.0 6.9 6.77 CaO 3.6 8.5 8.5 8.1 8.50 ^ a 2 0 5.1 -7 O 3 . 0 4.0 3-8 4.30 ICO 3.1 1 . 9 1 .6 2.4 2.36 H 20 1 . 1 0 . 3 0 . 6 0.6 0.21 0.43 0 .70 0.07 0.71 0.73 c o 2 <0.1 <0.1 <0.1 < 0 . 1 TABL'3 I - Chemical c o m p o s i t i o n s of r o c k samples from Bowie Soamount The f i r s t two p a r t s of a sample number i n d i c a t e the s t a t i o n at which the sample was found ( F i g u r e 2 ) . Sample .67 - 7 i s an a n d e s i t o . The o t h e r s a r c a l k a l i o l i v i n e b a s a l t s . 67 - 6 - 12 i s a x e n o c r y s t - r i c h b a s a l t , from which the x e n o c r y s t s were removed p r i o r to a n a l y s i s . ^ PV 50 i s a b a s a l t from Bowie Seamount, a n a l y z e d by Engol.and Engel (19b3). The o t h e r a n a l y s e s wore made by the L a b o r a t o r i e s o f the G e o l o g i c a l Survey o f Canada, Ottawa. • l i n e r a l i 7 . _ - . .7. 67 " ' 6 - 12 .68 . - 8 9.9.. ' Jr'V 50 A l b i t o 4 4 . 0 1 1 8 . 86 2 5 . J-5 1 5 . . 2 0 13- . 6 2 A n o r t h i t o 1 3 . 62 2 8 . 0 8 2 8 . 36 2 1 . 1 3 16 . 1 2 O r t h o c l a s e 1 8 . 90- 1 1 . 12 9 . 4 5 lb. . 4 6 13 . 9 0 N e p n o l i n e 7 . 3 8 5 - 11 9 • 37 12 . 2 1 D i o p s i d c o o. 26 1 2 . 12 12 . 3 0 17 . 5 3 Hyper sthc-no 1 1 . 15 - O l i v i n e 2 . 87 - 1 7 . 59 1 2 . 7 0 10 . 1 6 15 • 41 M a g n e t i t e 2 . 55 1 . 62 0 9 0 . 9 3 2 • 5 5 I l m e n i t o 2 . 13 5 . 02 2 . 8 9 b . 0 4 6 . 6 9 A p a t i t e 1 . 0 1 1 . r  :~> oo I . 60 1 . 6 8 Corundum 3 . 16 - TABLE 2 - Normative c o m p o s i t i o n s o f r o c k samples from Bowie Seamount The f i r s t two p a r t s of a sample number i n d i c a t e the s t a t i o n at which the sample was found ( F i g u r e 2 ) . Sample 67 - 7 i s an a n d e s i t e . The o t h e r s are a l k a l i o l i v i n e b a s a l t s . 67 - 6 - 12 i s a x e n o c r y s t - r i c h b a s a l t from which the x e n o c r y s t s w.*ro removed -prior to a n a l y s i s . PV 50 i s a b a s a l t from Bowie Seamount a n a l y z e d bv Englc and Gngel ( 1 9 6 3 ) . Component 67. - 6 - 1 2 68.. r _ l . ~ . 3. 63....- 9. py_. P l a g i o c l a s e 19 20 0 10 1 0 . , 0 Pyroxene 1 J? T X J , 0 O l i v i n e 1 2 1 4 4. ,1 Opaque 1 < 1 < 1 ^> 2 . .1 Groundless ( g l a s s ) 70 47 4 2 52 5 2 . .6 V e s i c l e s _ 2 8 48 2 0 . . 0 TABLE 3 - Modal c o m p o s i t i o n s o f r o c k samples from Bowie Seamount PV 50 i s reproduced from p u b l i s h e d r e s u l t s of En g e l and En g e l (1963). The o t h e r s are v i s u a l e s t i m a t e s made by the a u t h o r . -p- 46 r o c k w i t h c o n s p i c u o u s p h e n o c r y s t s of o l i v i n e and p l a g i o c l a s e . B l a c k c l i n o p y r o x e n e p h e n o c r y s t s are a l s o p r e s e n t but a r c not con s p i c u o u s because of t h e i r dark c o l o u r . The p h e n o c r y s t s are g e n e r a l l y no l a r g e r than 5mm. i n diameter. In t h i n s e c t i o n , these b a s a l t s e x h i b i t some m i n e r a l o g i c a l v a r i a t i o n s and a gr e a t range o f t e x t u r a l t y p e s , p r o b a b l y r e f l e c t i n g many d i f f e r e n t environments. A l l have a m a t r i x of e i t h e r pure g l a s s or m i c r o l i t e s w i t h i n t e r s t i t i a l g l a s s . The m i n e r a l s a re o l i v i n e , l a b r a d o r i t e (An 62 - 7 0 ) , c l i n o p y r o x e n e , and magnet i t e . These may occur i n d i v i d u a l l y as p h e n o c r y s t s , t o g e t h e r as g l o m e r o p h e n o c r y s t s , or swarming i n the m a t r i x as m i c r o l i t e s and c r y s t a l l i t e s . ' In g e n e r a l , the m i c r o - p h e n o c r y s t s were i n apparent e q u i l i b r i u m w i t h the magma and are w e l l formed and unzoncd but the l a r g e r p h e n o c r y s t s ( v i s i b l e i n hand specimen) show s t a g e s i n t h e i r h i s t o r y when t h e r e was a growth h i a t u s or p a r t i a l r e s o r p t i o n by the magma ( P l a t e 7 ) . A unique a l k a l i o l i v i n e b a s a l t o f a p p a r e n t l y r e s t r i c t e d a r e a l e x t e n t dominates the e a s t e r n h a l f o f the survey a r e a on the summit. T h i s r o c k i s d i s t i n g u i s h e d by an abundance of l a r g e x e n o c r y s t s o f p l a g i o c l a s e (An 54 - 6 4 ) and hornblende, 1 to 2 cm. a c r o s s , and PLATE 7 - Sonic m i c r o s c o p i c t e x t u r e s of the a l k a l i o l i v i n e b a s a l t s . (a) Zoned, a l t e r e d p h e n o c r y s t s and c u h e d r a l , u n a l t e r e d m i c r o p h e n o c r y s t s i n a m a t r i x o f i s o t r o p i c g l a s s . (Crossed n i c o l s ) . ( T h i n s e c t i o n 6? - 5 - C). ( x 2 ? ) (b) X e n o c r y s t of b a s a l t i c hornblende w i t h a r e a c t i o n r i m of magnetite and ort h o p y r o x e n e . ( P l a n e l i g h t ) . ( T h i n s e c t i o n 67 - k). ( x 2 7 ) (c) Pounded x e n o c r y s t of p l a g i o c l a s e , rimmed by a f r e s h o u t e r zone o f more c a l c i c c o m p o s i t i o n . ( C r o s s e d n i c o l s ) . ( T h i n s e c t i o n 67 - k)• ( x 2 7 )  48 c o a r s e - g r a i n e d , w h o l l y or p a r t l y c r y s t a l l i n e i n c l u s i o n s , 2 t o 5 cm. i n diameter. P h e n o c r y s t s o f o l i v i n e , c l i n o p y r o x e n e , and p l a g i o c l a s e (An 62 - 66 ) , under ^>mm. i n d i a m e t e r , are more common i n the r o c k but are not consp i c u o u s . The host r o c k , i n which the x e n o c r y s t s and i n c l u s i o n s are found, i s an a l k a l i o l i v i n e b a s a l t s i m i l a r t o the o t h e r s on the seamount (Table 1, 67 - 6 - 12). I n i t , the g r e a t hornblende x e n o c r y s t s , many o f which are fragmented, have r e a c t e d s t r o n g l y , p r o d u c i n g t h i c k r i m s o f magnetite and or t h o p y r o x e n e , and the l a r g o p l a g i o c l a s e x e n o c r y s t s bear evidence o f s i m i l a r ' i n s t a b i l i t y i n the magma ( P l a t e 7b and c ) . The i n c l u s i o n s , most of which are f e l d s p a t h i c or g a b b r o i c w i t h a l a r g e amount of hornblende, were s i m i l a r l y i n d i s e q u i l i b r i u m w i t h the magma a t the time o f e r u p t i o n . The o c c u r r e n c e of abundant i n c l u s i o n s i n an a l k a l i o l i v i n e b a s a l t i s normal, but the gr e a t predominance of f e l d s p a t h i c and g a b b r o i c t y p e s over u l t r a m a f i c t y p e s i s unu s u a l . These i n c l u s i o n s w i l l be d i s c u s s e d i n a l a t e r s e c t i o n . Andes_ite_s A n d e s i t i c r o c k s were found at o n l y two l o c a t i o n s on Bowie Seamount: S t a t i o n s 6 7 - 7 and 68 - 5. Sample 67 - 7 i s d i s t i n g u i s h e d i n hand specimen by 49 i t s t o t a l l a c k of v e s i c l e s and h i g h degree of j o i n t i n g . On a f r e s h , broken s u r f a c e , f i n e n e e d l e s of p l a g i o c l a s e are c o n s p i c u o u s i n a grey groundmass. In t h i n s e c t i o n , i t j.s seen t o be made up dom i n a n t l y of more or l e s s f l o v / - o r i e n t e d p l a g i o c l a s e l a t h s (An 45 - 5 0 ) w i t h r e l a t i v e l y few c r y s t a l s o f o l i v i n e , pyroxene, m a g n e t i t e , and a p a t i t e ( P l a t e 8 a ) . None o f the m i n e r a l s shows any i n d i c a t i o n of i n s t a b i l i t y i n the magma. The ground- mass i s g l a s s y but c o n t a i n s such an abundance o f m i c r o l i t e s t h a t i t i s almost masked. The c o m p o s i t i o n of t h i s r o c k i s seen i n T a b l e s 1 and 2 to l i e between mugearite ( c f . Muir and T i l l e y , 196.1) and t r a c h y t e . The normative p l a g i o c l a s e was c a l c u l a t e d t o be An 2 2 . 6 . The r o c k has a p p a r e n t l y been c h i l l e d as q u i c k l y as any o f the s u r f a c e l a v a s f o r a zone o f si d e r o m e l a n c e x i s t s i n the ground mass a l o n g one margin o f a b l o c k w h i l e t a c h y l y t e forms most o f the i n t e r i o r . I t i s p o s s i b l e , t h e r e f o r e , t h a t t h i s r o c k may have o r i g i n a t e d i n a b l o c k l a v a f l o w (Washington, 1 9 2 6 and F i n c h , 1 9 3 3 ) . I t s h i g h l y d i f f e r e n t i a t e d c o m p o s i t i o n would c e r t a i n l y f a v o u r the f o r m a t i o n o f such a h i g h l y v i s c o u s f l o w (Macdonald, 1 9 6 7 ) . Without f u r t h e r e v i d e n c e , however, the p o s s i b i l i t y s t i l l remains t h a t these a n g u l a r r o c k samples may have formed p a r t o f a t a l u s near an eroded dyke. PLATE 8 - Photomicrographs of the a n c l o s i t e s (a) N o n - v e s i c u l a r a n d e s i t i c r o c k from S t a t i o n 6 7 - 7 . P h e n o c r y s t s o f p l a g i o c l a s e , m a g n e t i t e and c l i n o p y r o x e n e are v i s i b l e i n the m a t r i x of p l a g i o c l a s e m i c r o l i t c s and g l a s s . ( P l a n e l i g h t ) ( T h i n s e c t i o n 67 - 7 - A ) ( x 2 7 ) (b) Hornblende - b e a r i n g a n d e s i t e from S t a t i o n 6 8 - 5 - S m a l l diamond-shaped c r y s t a l s of o l i v i n e and a d a r k c r y s t a l of hornblende are surrounded by f l o w - o r i e n t e d p l a g i o c l a s e and i n t e r s t i t i a l g l a s s . The l i g h t areas are v e s i c l e s ( Plane l i g h t ) ( T h i n s e c t i o n 68 - 5 ) ( x 2 7 ) 5o 51 The s i n g l e dredge h a u l on the n o r t h e a s t r i d g e t u r n e d up the o n l y o t h e r a n d e s i t i c r o c k found on the seamount. I t i s uniraposing i n hand specimen, b e i n g a p a l e grey, a p h a n i t i c , f i n e l y v e s i c u l a r r o c k . Under, the microscope ( P l a t e 8 b ) , I t i s seen t o be almost w h o l l y composed of f l o w - o r i e n t e d p l a g i o c l a s e l a t h s (An 51 - 47) w i t h i n t e r s t i t i a l g l a s s and r a r e , e u h e d r a l or s k e l e t a l c r y s t a l s of o l i v i n e and pyroxene. Brown, b a s a l t i c h o r n b l e n d e i s p r e s e n t as s c a t t e r e d e u h e d r a l p h e n o c r y s t s , some o f w h i c l i are surrounded by a v e r y t h i n r e a c t i o n r i m of magnetite and orthopyroxene. The r e a c t i o n p r o d u c t s , a p p a r e n t l y formed by the d e h y d r a t i o n of hornblende, may have formed as a r e s u l t o f a drop i n water p r e s s u r e when the magma was e x t r u d e d . A n d e s i t e s o c c u r i n s m a l l p r o p o r t i o n s on many l a r g e o c e a n i c v o l c a n o e s and are thought t o be h i g h l y d i f f e r e n t i a t e d d e r i v a t i v e s of the a l k a l i o l i v i n e b a s a l t which makes up such v o l c a n o e s . B • I n c l u s i o n s The i n c l u s i o n s are c r y s t a l l i n e , s h a p e l e s s masses 2 to 5 cm. a c r o s s . Some of those examined p e t r o g r a p h - i c a l l y have t e x t u r e s t y p i c a l of cumulates (Wager et a l . I 9 6 0 ) , igneous r o c k s which a p p a r e n t l y have formed by the a c c u m u l a t i o n o f c r y s t a l s on the f l o o r of a magma chamber. 52 Others are o f l e s s d i s t i n c t i v e t e x t u r e and c o u l d be e i t h e r x e n o l i t h s of w a l l - r o c k or i n d i s t i n c t cumulates. Only one u l t r a m a f i c i n c l u s i o n (a p y r o x e n i t e ) was found. The r e s t a r c f e l d s p a t h i c or g a b b r o i c and many are r i c h i n hornblende. The climopyroxene i n these i n c l u s i o n s i s p a l e green i n t h i n s e c t i o n , u n l i k e the b u f f - c o l o u r e d c l i n o p y r o x e n c s common t o t h e l a v a s , ( i ) Pyroxene cumulate T h i s i n c l u s i o n ( P l a t e 9 ) i s an aggregate of zoned, c l i n o p y r o x e n c s and some o l i v i n e w i t h i n t e r s t i t i a l p o i k i l i t i c p l a g i o c l a s e and m i c r o p o r p h y r i t i c s i d c r o m e l a n e . The m a f i c m i n e r a l s which, f o r the most p a r t , a r c e u h e d r a l , are randomly o r i e n t e d and l o o s e l y packed i n i n t e r s t i t i a l v e s i c u l a r g l a s s . There i s no s i g n of f r a c t u r i n g or r e s o r p t i o n on the exposed g r a i n b o u n d a r i e s to' i n d i c a t e the r e m o v a l o f p r e - e x i s t i n g m i n e r a l s from these v o i d s . A p p a r e n t l y t h e n , t h i s u l t r a m a f i c i n c l u s i o n was always an aggregate o f e u h e d r a l c r y s t a l s w i t h i n t e r s t i t i a l magma and i s not the remnant o f a c o m p l e t e l y c r y s t a l l i n e i g n eous r o c k . I t i s probable t h a t i t formed as a cumulate on the f l o o r o f a magma chamber or perhaps as a nodule o f aggregated c r y s t a l s , suspended f r e e l y i n the magma. The c l i n o p y r o x e n e and o l i v i n e , b e i n g the most e u h e d r a l phases, were presumably the f i r s t t o PLATE 9 - Pyroxene cumulate ( T h i n s e c t i o n 6 ? - 3 - C) (a) P a r t i a l l y i n t e r g r o w n cumulus of c l i n o p y r o x e n e c r y s t a l s . The s p e c k l e d m a t e r i a l i s m i c r o l i t e - r i c h , v e s i c u l a r , i n t e r s t i t i a l g l a s s . The l i g h t a r e a s are m a i n l y h o l e s and c r a c k s i n the s l i d e but some are p l a g i o c l a s e and o l i v i n e c r y s t a l s . ( P l a n e l i g h t ) ( X 2 ) (b) Cumulus pyroxenes and a few i n t e r s t i t i a l p l a g i o c l a s e c r y s t a l s . I s o t r o p i c g l a s s forms the b l a c k background. (Crossed n i c o l s ) ( x 2 7 ) 53 54 c r y s t a l l i z e and g r a v i t a t e t o the r e g i o n o f a c c u m u l a t i o n . Both a p p a r e n t l y accumulated as c r y s t a l s at the same time s i n c e n e i t h e r e x e r t s a dominant c r y s t a l l o g r a p h i c i n f l u e n c e on the o t h e r . Growth o f the pyroxenes and the few o l i v i n e s c o n t i n u e d a f t e r d e p o s i t i o n , r e s u l t i n g i n the p a r t i a l c o a l e s c e n c e of a d j a c e n t c r y s t a l s . A l t h o u g h some e a r l y p l a g i o c l a s e appears t o have s e t t l e d out w i t h the f e r r o m a g n e s i a n cumulus m i n e r a l s , the g r e a t e r p a r t o f i t i s p o i k i l i t i c and i n t e r s t i t i a l , e n c l o s i n g or p a r t l y e n c l o s i n g pyroxene euhedra. C r y s t a l l i z a t i o n had not yet gone t o c o m p l e t i o n when the cumulate was removed . from i t s s i t e o f f o r m a t i o n , i n c o r p o r a t e d i n t o the moving magma, and e j e c t e d onto the s u r f a c e o f the vol c a n o as a cored bomb (Brady and Webb, 1 9 4 3 ) , t h i n l y mantled and permeated by quenched siderornelane ( i i ) Andesine - o l i v i n e cumulate The m i n e r a l s making up t h i s i n c l u s i o n a r c andesine (An 3 0 - 3 4 ) , f c r r o - h o r t o n o l i t e (Fa 8 0 ) , a p a t i t e , and opaque i r o n o x i d e ( P l a t e 1 0 ) . The c o m p o s i t i o n o f the o l i v i n e was determined by X-ray d i f f r a c t i o n u s i n g the method of Yoder and Sahama ( 1 9 5 7 ) . Such an assemblage o f s o d i c p l a g i o c l a s e and i r o n - r i c h o l i v i n e p r o b a b l y r e p r e s e n t s a d i f f e r e n t i a t e o f a dry magma somewhere i n the system o f magma r e s e r v o i r s below the s u r f a c e . The PLATE 10 - Andosino - O l i v i n e cumulate. ( T h i n s e c t i o n 67 - 6 - E) (a) O l i v i n e Fa-80 (upper r i g h t ) , a p a t i t e ( c e n t r e ) , and opaque o x i d e (lower l e f t ) , s urrounded by p l a g i o c l a s e f e l d s p a r A n - 3 0 - 3 4 . M e l t i n g of the p l a g i o c l a s e around the o l i v i n e and o x i d e g r a i n s i s r e a d i l y apparent. (P l a n e l i g h t ) ( x 2 7 ) (b) I n t e r l o c k i n g g r a i n s o f t h e p l a g i o c l a . s e cumulus. (Crossed n i c o l s ) ( x 2 7 ) (c) O l i v i n e ( c e n t r e ) on the margin o f the i n c l u s i o n , e x h i b i t i n g a s t r o n g r e a c t i o n w i t h the magma (lower l e f t ) . The l i g h t e r c o l o u r e d g l a s s , produced by m e l t i n g of the m i n e r a l s w i t h i n the i n c l u s i o n , i s v i s i b l e i n the space between the o l i v i n e and the l a r g e p l a g i o c l a s e (upper r i g h t ) ( P l a n e l i g h t ) ( x 2 7 )  c r y s t a l s are l a r g e and w e l l formed but have i n t e r - l o c k i n g g r a i n b o u n d a r i e s ( P l a t e 10b), a t e x t u r e which the a u t h o r b e l i e v e s t o be the r e s u l t o f c o n t i n u e d growth and c o a l e s c e n c e of aggregated p h e n o c r y s t s which had s e t t l e d out o f the magma onto a s u r f a c e of a c c u m u l a t i o n . A change i n the environment, e.g. a p r e s s u r e decrease d u r i n g movement of the o v e r l y i n g magma., caused i n c i p i e n t m e l t i n g of the m i n e r a l s i n the cumulate, e s p e c i a l l y o f the s o d i c p l a g i o c l a s e around o l i v i n e and i r o n o x i d e g r a i n s , which do not appear themselves t o have been so s u s c e p t i b l e to r e m e l t i n g ( P l a t e 10a). P i e c e s of the cumulate, t r a n s p o r t e d t o the s u r f a c e as i n c l u s i o n s d u r i n g an e r u p t i o n , r e a c t e d w i t h t h e magma at t h e i r margins ( P l a t e 10c) but r e t a i n e d w i t h i n them the i n t e r - s t i t i a l l i q u i d formed by the m e l t i n g of the andesine and o l i v i n e . Quenching of both the l a v a and the i n t e r s t i t i a l l i q u i d on r e a c h i n g the s u r f a c e , produced two d i s t i n c t l y d i f f e r e n t g l a s s e s ; a dark s i d e r o m e l a n e , which surrounds the i n c l u s i o n and a much l i g h t e r g l a s s w i t h i n the i n c l u s i o n ( P l a t e 1 0 c ). ( i i i ) P l a g i o c l a s e - hornblende cumulate- In t h i s cumulate, randomly o r i e n t e d c a l c i c p l a g i o c l a s e g r a i n s are e n c l o s e d i n l a r g e ( 1 - 2 cm), p o i k i l i t i c b a s a l t i c hornblende c r y s t a l s ( P l a t e 11a). Very s m a l l amounts o f o l i v i n e , a p a t i t e and s k e l e t a l , P l a g i o c l a s e - hornblende cumulate ( T h i n S e c t i o n 67 - 6 - G) (a) P l a g i o c l a s e c r y s t a l s o f cumulus surrounded by p o i k i l i t i c hornblende ( P l a n e l i g h t ) (x2) (b) and (c) M i c r o f r a c t u r c d and rounded s k e l e t a l p l a g i o c l a s e shOYri_ng d i s p l a c e m e n t of a d j a c e n t segments of the same c r y s t a l . P o i k i l i t i c hornblende surrounds the f e l d s p a r . ( C r o ssed n i c o l s ) ( (b) x27, (c) x 80)  58 m a g n e t i t e - r i c h c l i n o p y r o x e n e are a l s o i n c l u d e d by the p o i k i l i t i c h ornblende. The p l a g i o c l a s e , i s rounded and o c c u r s both as s m a l l d i s c r e t e g r a i n s and a s . l a r g e s k e l e t a l c r y s t a l s . The l a t t e r show evidence of h a v i n g once been fragmented and then h e a l e d . In p l a c e s , t h e r e i s a s l i g h t d i f f e r e n c e i n o r i e n t a t i o n o f a d j a c e n t u n i t s o f a s i n g l e p l a g i o c l a s e c r y s t a l i n a p o i k i l i t i c h ornblende ( P l a t e l i b and c ) . M i s a l i g n m e n t of t w i n l a m e l l a e and a c o r r e s p o n d i n g d i s p a r i t y i n e x t i n c t i o n p o s i t i o n between d i f f e r e n t a r e a s w i t h i n a v i s i b l y c o n t i n u o u s p o r t i o n o f a p l a g i o c l a s e g r a i n i s a l s o apparen These phenomena can b e s t be e x p l a i n e d by m i c r o f r a c t u r i n g and h e a l i n g w i t h i n the c r y s t a l . Each p l a g i o c l a s e u n i t i n the p o i k i l i t i c hornblende i s i n d i v i d u a l l y zoned, even i f s e v e r a l a d j a c e n t u n i t s appear to.belong to the same l a r g e c r y s t a l . The hornblende, on the o t h e r hand, shows no m i c r o f r a c t u r i n g or z o n i n g . The p l a g i o c l a s e i s t h e r e f o r e o l d e r than the hornblende and appears to be an a c c u m u l a t i o n o f s k e l e t a l c r y s t a l s which, at one p o i n t , were broken and then rounded by r e s o r p t i o n i n an u n f a v o u r a b l e magmatic environment. The open spaces and m i c r o f r a c t u r e s i n the f e l d s p a r cumulus were l a t e r f i l l e d by r e l a t i v e l y few, l a r g e , p o i k i l i t i c c r y s t a l s o f b a s a l t i c hornblende. The r e s u l t i s s i m i l a r to the heteradcumulate o f Wager et a l . ( I 9 6 0 ) . Growth o f such l a r g e , u n i f o r m 59 c r y s t a l s would n e c e s s a r i l y i n v o l v e the f r e e d i f f u s i o n o f the component i o n s of hornblende from the magma to r e l a t i v e l y r a r e n u c l e i i i n the i n t e r c u m u l u s l i q u i d . The apparent age d i f f e r e n c e between the p l a g i o c l a s e and the hornblende p r e c l u d e s the p o s s i b i l i t y t h a t t h i s t e x t u r e r e p r e s e n t s a e u t e c t i c c r y s t a l l i z a t i o n of the two phases. The p o s s i b i l i t y remains t h a t hornblende has p a r t l y r e p l a c e d the p l a g i o c l a s e . ( i v ) A n o r t h o s i t e x e n o l i t h w i t h h a r r i s i t i c overgrowth The core of t h i s type of i n c l u s i o n c o n s i s t s o f c o a r s e l y c r y s t a l l i n e p l a g i o c l a s e (An 60 - 6 5 ) , some opaque i r o n o x i d e and a r e l a t i v e l y s m a l l amount of i n t e r s t i t i a l g l a s s . A change i n the magmatic environment a t some time caused c o n s i d e r a b l e r e s o r p t i o n around the exposed margins of the i n c l u s i o n and, to a s m a l l e r degree, a l o n g g r a i n b o u n d a r i e s w i t h i n i t ( P l a t e 12). Such a change c o u l d be most s i m p l y e x p l a i n e d by a s i m p l e p r e s s u r e drop or temperature i n c r e a s e i n the system. A f t e r b e i n g p a r t l y r e s o r b e d and rounded, the p l a g i o c l a s e was f i n a l l y i s o l a t e d by a f r e s h o u t e r zone of more c a l c i c c o m p o s i t i o n . C r y s t a l growth d u r i n g t h i s second stage-was not r e s t r i c t e d to p l a g i o c l a s e a l o n e . S k e l e t a l c r y s t a l s of o l i v i n e and o f c l i n o p y r o x e n e , shot through w i t h m a g n e t i t e , a p p a r e n t l y n u c l e a t e d on the s u r f a c e of the i n c l u s i o n and grew away from the s u r f a c e . A djacent s k e l e t a l c r y s t a l s are PLATE 12 - H a r r i s i t i c overgrowths on a n o r t h o s i t e x e n o l i t h . The p l a g i o c l a s e o f the i n c l u s i o n o c c u p i e s the l e f t c o r n e r of the photo. The m i n e r a l s f o r m i n g the h a r r i s i t e are c l i n o p y r o x e n e , which appears grey w i t h a rough s u r f a c e , and o l i v i n e , which appears w h i t e i n the photograph. L a t e r - f r o m e d p o i k i l i t i c p l a g i o c l a s e i s v i s i b l e among the s k e l e t a l o l i v i n e and c l i n o p y r o x e n e c r y s t a l s , and as a l i g h t e r r i m on the p l a g i o c l a s e o f the i n c l u s i o n . The b l a c k a r e a s w i t h i n the c r y s t a l s are composed o f g l a s s and magnetite c r y s t a l s . The b l a c k background i s i s o t r o p i c v e s i c u l a r g l a s s . ( C r o ssed n i c o l s ) ( T h i n s e c t i o n 67 - 1 - C) ( x 2 7 ) 60 61 t h o r o u g h l y intermoshecl and i n p l a c e s are r o o t e d t o the p l a g i o c l a s e i n c l u s i o n by p o i k i l i t i c growth o f the o u t e r zone of p l a g i o c l a s e . S m a l l , p r o b a b l y l a t e p l a g i o c l a s e c r y s t a l s a l s o e x h i b i t a p o i k i l i t i c t e x t u r e around p a r t s o f the s k e l e t a l f e r romagnesians. T h i s i n t r i c a t e second stage t e x t u r e appears to be a v a r i a t i o n of the h a r r i s t i c or c r e s c u m u l a t e t e x t u r e of Wager et a l . (I960) and Wager (1963) (v) P l a g i o c l a s e - magnetite x e n o l i t h T h i s i n c l u s i o n ( P l a t e 1 3 ) c o n s i s t s of i n t e r g r o w n p l a g i o c l a s e (An 7 0 ) , magnetite and a p a t i t e c r y s t a l s w i t h a good d e a l o f i n t e r s t i t i a l s i d e r o m e l a n e . The p l a g i o c l a s e i s h i g h l y f r a c t u r e d and sideromelane f i l l s many o f the c r a c k s . R e s o r p t i o n o f the p l a g i o c l a s e has t aken p l a c e a l o n g exposed g r a i n b o u n d a r i e s , p o s s i b l y d u r i n g t r a n s p o r t i n the magma p r i o r t o e r u p t i o n . The sideromelane i n and around the i n c l u s i o n i n d i c a t e s t h a t r a p i d quenching took p l a c e a f t e r i t s e j e c t i o n . L o c a l m e l t i n g o f the magnetite may account f o r a d a r k e r s t a i n i n t he sideromelane a d j a c e n t t o i t . ( v i ) Hornblende gabbrc x e n o l i t h s C o a r s e l y c r y s t a l l i n e , g a b b r o i c x e n o l i t h s composed o f p l a g i o c l a s e (An 66 - 7 6 ) , b a s a l t i c hornblende, c l i n o p y r o x e n e , magnetite and minor a p a t i t e , a r c . common PLATE 13 - P l a g i o c l a s e - magnetite x e n o l i t h ( T h i n s e c t i o n 67 - 6 - H) (a) P l a g i o c l a s e ( w h i t e ) s u r r o u n d i n g a pocket of magnetite ( b l a c k ) , a p a t i t e (white g r a i n s w i t h h i g h r e l i e f and "ground g l a s s " s u r f a c e ) , and i n t e r s t i t i a l s ideromelane (d a r k g r e y ) . The p l a g i o c l a s e i s s t r o n g l y r e s o r b e d on i t s margins and s ideromelane i s v i s i b l e i n f r a c t u r e s w i t h i n i t (upper l e f t ) . ( P l a n e l i g h t ) . ( x 2 7 ) (b) A d i f f e r e n t p a r t of the same s e c t i o n viewed through c r o s s e d n i c o l s . P l a g i o c l a s e appears grey and w h i t e . M a g n e t i t e and g l a s s appear b l a c k . The r e s o r b e d margins o f the p l a g i o c l a s e appear as d e n d r i t i c p a t t e r n s . (x27) 62 6 3 i n the i n c l u s i o n - b e a r i n g b a s a l t s . The p l a g i o c l a s e has a p p a r e n t l y been r e s o r b e d by the magma a l o n g exposed g r a i n b o u n d a r i e s and the r e a c t i o n margin i s mantled by a t h i n growth o f f r e s h p l a g i o c l a s e ( P l a t e 1 4 ) . The hornblende, however, appears to have undergone profound changes where i t was exposed to the magma. Those g r a i n s which l i e on t h e borde r of the i n c l u s i o n are permeated w i t h g r a p h i c b l e b s of magnetite and appear t o pass at t h e i r o u t e r margin i n t o g r a p h i c (or s k e l e t a l ) c l i n o p y r o x e n e , which r e t a i n s the o r i e n t a t i o n and magnetite t r a i n s of the hornblende. A c i c u l a r , r a d i a t i n g c r y s t a l s o f orthopyroxene occur w i t h t h i s apparent a l t e r a t i o n i n a few p l a c e s . In many g r a i n s , morj c o n v e n t i o n a l r e a c t i o n r i m s o f f i n e , u n o r i e n t e d orthopyroxene and magnetite a r c p r e s e n t i n s t e a d of t h i s u n usual g r a p h i c r e a c t i o n t e x t u r e but the one type seems t o grade i n t o the o t h e r . The g r a p h i c a l t e r a t i o n zones of the hornblende arc- g e n e r a l l y f u r t h e r c o m p l i c a t e d w i t h what appears to be a h a r r i s i t i c growth of o l i v i n e and a u g i t e , s i m i l a r t o t h a t d e s c r i b e d p r e v i o u s l y , which p r o b a b l y formed i n d e p e n d e n t l y on any o f the exposed c r y s t a l s u r f a c e s o f the i n c l u s i o n . O r i g i n and s i g n i f i c a n c e o f the i n c l u s i o n s The g r e a t v a r i e t y o f b a s a l t s and i n c l u s i o n s s u g g e s t s t h a t a complex system o f magma r e s e r v o i r s e x i s t e d PLATE Ik - Photomicrograph of the g r a p h i c hornblende a l t e r a t i o n w i t h i n t e r m e s h e d h a r r i s i t i c o l i v i n e . In the photographs, hornblende appears brown, pyroxene appears grey w i t h a rough s u r f a c e t e x t u r e , o l i v i n e appears y e l l o w , p l a g i o c l a s e appears grey w i t h a smooth s u r f a c e t e x t u r e , and magnetite appears as s m a l l , b l a c k , g r a p h i c g r a i n s i n the o t h e r m i n e r a l s . The e x t e n s i v e d a r k m a t r i x i s i s o t r o p i c g l a s s . The shape and o r i e n t a t i o n o f the magnetite i s e v i d e n t l y c o n t r o l l e d by c r y s t a l l o g r a p h i c d i r e c t i o n s w i t h i n the hornblende. The pyroxene, which i s s i m i l a r l y o r i e n t e d , i s a l s o thought t o have been d e r i v e d from the hornblende, and s m a l l amounts of i t cavi-be seen as grey areas, w i t h i n the l a t t e r m i n e r a l . The s k e l e t a l o l i v i n e , has a p p a r e n t l y grown cat r i g h t a n g l e s t o the p l a g i o c l a . s e o f the i n c l u s i o n . ( C r ossed n i c o l s ) ( T h i n s e c t i o n 67 - 6 - F) ( M a g n i f i c a t i o n o f upper photo x.?.7; lower photo x 8 0 ) 64 64 beneath Bowie Seamount b o t h i n the c r u s t , and i n the mantle, where the magma must o r i g i n a l l y have been d e r i v e d . Macdonald ( 1 9 4 9 ) i l l u s t r a t e d how l a v a s of v a r i o u s c o m p o s i t i o n s may o r i g i n a t e a t d i f f e r e n t l e v e l s w i t h i n a s i n g l e magma chamber ( F i g u r e .13) and many o f the r o c k s of the seamount are p r o b a b l y r e l a t e d i n t h i s way. S e v e r a l extremes o f d i f f e r e n t i a t i o n however are r e p r e s e n t e d by t h e i n c l u s i o n s . X e n o c r y s t s of hornblende, x e n o l i t h s of hornblende gabbro, and cumulates c o n t a i n i n g p o i k i l i t i c hornblende a t t e s t t o r e l a t i v e l y h i g h water p r e s s u r e s t h a t must have p r e v a i l e d i n some p a r t s o f the magma chamber system. On the o t h e r hand, the u l t r a m a f i c p y r o x e n i t e cumulate and the andesine - o l i v i n e cumulate- r e s p e c t i v e l y r e p r e s e n t e a r l y and l a t e stage d i f f e r e n t i a t i o n o f dry b a s a l t i c magma. I t i s t h e r e f o r e p r o b a b l e t h a t s e v e r a l magma chambers e x i s t e d a t a h i g h l e v e l i n the c r u s t and t h a t d i f f e r e n t i a t i o n proceeded more or l e s s i n d e p e n d e n t l y i n each. From the v a r i e t y o f cumulate t y p e s , i t i s apparent t h a t the magma had s u f f i c i e n t time to d i f f e r e n t i a t e to a h i g h degree, and p o s s i b l y t o b u i l d up a c o n s i d e r a b l e t h i c k n e s s o f accumulated c r y s t a l s on the magma chamber f l o o r s . D u r i n g e r u p t i o n t h e r e i s a decrease i n c o n f i n i n g p r e s s u r e i n the magma, accompanied by the f o r m a t i o n and 65 FIGURE 13 - Diagram illustrating a possible manner of eruption of lavas of differing composition from the same mqgmo 'Ctasanber.- A, andesite; B , basal t ; C, olivine basalt ; D, p i c r i te -basa l t ; E, trachyte. (After Macdonald, 1949.) 66 e x p a n s i o n o f v e s i c l e s . V e s i c u l a t i o n a l s o t a k e s p l a c e i n the i n t e r s t i t i a l l i q u i d w i t h i n the l o o s e l y bound c r y s t a l mush, and, i f the p r o c e s s were v i o l e n t enough, i t c o u l d have broken up the cumulate beds and t r a n s p o r t e d fragments of them upwards i n t o the e r u p t i n g magma, as i n c l u s i o n s . Corroded w a l l - r o c k s , perhaps c o m p r i s i n g o l d cumulate beds or c o n v e n t i o n a l l y c r y s t a l l i z e d r o c k r e i n t r u d c d by a new column o f magma, c o u l d be broken up and i n c o r p o r a t e d i n t o the magma i n the same way. The drop i n p r e s s u r e i n the w a l l - r o c k might a l s o be compounded by a V c n t u r i - e f f o c t c r e a t e d by r a p i d upv/ard movement o f the magma. The c o r r o d e d anorthosi'te and hornblende gabbro i n c l u s i o n s , which are h o s t s to the unu s u a l h a r r i s i t i c t e x t u r e , might be the remains o f f r a c t u r e d , magma-permeated w a l l - r o c k s or c h a m b e r - f l o o r r o c k s which, f o r a tim e , may have su p p o r t e d h a r r i s i t i c c r y s t a l growths b e f o r e e r u p t i o n o f the magma and t h e i r consequent f r a g m e n t a t i o n and d i s p e r s a l C. Tephra P y r o c l a s t i c m a t e r i a l i s more e a s i l y r e c o v e r e d by dredge and grab sampler than i s f l o w - r o c k and t h i s i s r e f l e c t e d i n i t s o c c u r r e n c e i n a l l but two sample h a u l s from the seamount (Appendix B). The author e s t i m a t e s t h a t the r a t i o o f t e p h r a t o f l o w - r o c k on the seamount i s p r o b a b l y a c t u a l l y around 1:1, 67 ( i ) V o l c a n i c bombs A c o l l e c t i o n o f v o l c a n i c bombs was o b t a i n e d a l o n g a f i v e m i l e s t r i p of the summit r i d g e at s i t e s 6 7 - 1 , 6 7 ~ 6 , 6 7 - 5 and 6 8 - 1 ( F i g u r e 2 ) . - . Among the more f a m i l i a r t y p e s a re the f u s i f o r m bomb ( P l a t e 1 5 a ) , which i s f u l l y formed b e f o r e l a n d i n g , and the cow-dung bomb ( P l a t e 15b) which i s s t i l l p a r t l y l i q u i d a t the end of i t s f l i g h t and f l a t t e n s on impact. A few s m a l l cow-dung bombs appear t o have been f l a t t e n e d and t h e n f o l d e d b e f o r e f i n a l l y s o l i d i f y i n g c o m p l e t e l y ( P l a t o 1 5 c ) . The appearance o f the g r e a t m a j o r i t y of bombs, r e p r e s e n t e d by t h e s u i t e from s t a t i o n 67 - 5 , l i e s somewhere between the two t y p e s . The bombs o f t h i s s u i t e range from s m a l l f l a t t e n e d forms r e s e m b l i n g cow-dung bombs to l a r g e , g l o b o s e , h i g h l y deformed i n d i v i d u a l s w i t h complex i n v o l u t e s u r f a c e s but no d e f i n a b l e shape ( P l a t e 1 6 ) . A l t h o u g h the d e f o r m a t i o n o f those bombs v/as a p p a r e n t l y due to impact w i t h a r e s i s t a n t s u r f a c e , t h e r e are no i m p r i n t s or a d h e r i n g r o c k fragments to c o n f i r m t h a t the impact was w i t h a s o l i d v o l c a n i c s u b s t r a t e , n e i t h e r i s i t c l e a r which s i d e o f the bomb r e c e i v e d the impact. T h e i r mode o f o r i g i n remains u n c e r t a i n . A l l the bombs have a quenched s k i n o f sideromelane which may show some bread c r u s t f r a c t u r i n g . N o r m a l l y , PLATE 15 - V a r i e t i e s of bombs found on Bowie Seamount (a) F u s i f o r m bomb ( S t a t i o n 68 - 1) (b) Cow-dung bombs. The upper bomb was found at S t a t i o n 6 7 - 1 , the lower l e f t one at S t a t i o n 6 7 - 6 , and the lower r i g h t hand one at S t a t i o n 67 - 5- (c) F o l d e d cow-dung bomb ( S t a t i o n 67 - 6 ) .  PLATE 16 - V a r i e t i e s of bombs found on Bowie Seamount ( c o n t i n u e d ) (a) and (b) G r a d a t i o n from l a r g e bombs w i t h deformed s u r f a c e s t o the s m a l l , f l a t , cow-dung v a r i e t y . A l l the specimens are from S t a t i o n 67 - 5 .  70 v e s i c l e s are deformed near the s u r f a c e , becoming l e s s so towards the mid d l e . The v e s i c l e s o f the i n t e r i o r are commonly s l i g h t l y l a r g e r than those .of the c h i l l e d margin, and, i n extreme ca s e s , the i n t e r i o r i s v i r t u a l l y h o l l o w ( P l a t e 1 7 a ) . Those bombs which a r c not f l a t t e n e d , are c o n c e n t r i c a l l y and r a d i a l l y f r a c t u r e d ( P l a t e 1 7 b ) , and i n d i v i d u a l s have been found t h a t have c o m p l e t e l y or p a r t l y l o s t t h e i r quenched s k i n due t o s p a l l i n g a l o n g these f r a c t u r e s . The c o m p o s i t i o n s of t h e bombs vary from p l a c e t o p l a c e and are s i m i l a r t o t h a t o f the r o c k s i n the immediate v i c i n i t y . Cored bombs c o n t a i n i n g i n c l u s i o n s are r a r e . The presence of bombs on the summit i m p l i e s t h a t l a v a f o u n t a i n i n g and e x p l o s i v e e r u p t i o n s took p l a c e t h e r e d u r i n g the l a t e s t a g e s of a c t i v i t y on the summit. In view o f the summit's p r e s e n t nearness to sea l e v e l and i t s p r o b a b l e emergence i n the p a s t , many, i f not a l l , o f the bombs o f the summit would have been e j e c t e d from e i t h e r above or j u s t below sea l e v e l . Many, t h e r e f o r e , p r o b a b l y t r a v e l l e d t h r ough the a i r a l o n g a t l e a s t p a r t . of t h e i r t r a j e c t o r i e s , e v e n t u a l l y f a l l i n g i n t o the sea or onto the s u r f a c e of young v o l c a n i c i s l a n d s which may have e x i s t e d t h e r e at times d u r i n g the p a s t . PLATE 17 - V a r i e t i e s of 'bombs found on Bowie Seamount ( c o n t i n u e d ) (a) Hollow cow-dung bomb ( S t a t i o n 6 7 - 6 ) (b) C o n c e n t r i c and r a d i a l f r a c t u r e s i n a f u s i f o r m bomb. The quenched sideromelane s k i n appears l i g h t e r than the dark co r e . The specimen i s ' from S t a t i o n 6 8 - 1 . 7 1 72 S i n c e these i s l a n d s a r c presumed to have boon eroded away, the present d i s t r i b u t i o n o f the bombs on the seamount may be due not o n l y to primary v o l c a n i c d i s p e r s a l but a l s o to secondary d i s p e r s a l by y/avc e r o s i o n . I t i s the a u t h o r ' s o p i n i o n , however, that few o f the s u b a c r i a l l y formed bombs c o u l d have s u r v i v e d i n t a c t the wave a c t i o n which d e s t r o y e d the ephemeral i s l a n d s on which they l a y . The m a j o r i t y o f the bombs now found on the t e r r a c e s p r o b a b l y c o n s t i t u t e the remainder which e i t h e r l a n d e d i n the sea or o r i g i n a t e d e n t i r e l y under water. T h e i r p r e s e n t l o c a t i o n , i f t h i s i s the c a s . , s h o u l d be the r e s u l t o f p r i m a r y v o l c a n i c d i s p e r s a l . The geographic r e s t r i c t i o n o f l a r g e numbers of o b v i o u s l y r e l a t e d bombs, such as the s u i t e from s t a t i o n 6 7 - 5 ( P l a t e 1 6 ) , can be e x p l a i n e d i n t h i s way. A p p a r e n t l y t h e n , bombs, which ar c t r a d i t i o n a l l y thought o f as b e i n g s u b a c r i a l l y formed s t r u c t u r e s , may be capable o f f o r m i n g i n s h a l l o w water. Perhaps the unusual s u i t e o f bombs from s t a t i o n 6 7 ~ 5 ( P l a t e 1 6 ) i s t y p i c a l o n l y o f s h a l l o w water e r u p t i o n s where e j e c t a may t r a v e l through both a i r and water b e f o r e f i n a l l y coming t o r e s t on the s u r f a c e o f the volca.no. ( i i ) T u f f s and t h e i r u n c o n s o l i d a t e d c o u n t e r p a r t s L a p i l l i and ash, and weakly cemented t u f f s 7 3 d e r i v e d from them, were found i n a l l but one l o c a l i t y on the summit and a t t h r o e o f the f i v e s t a t i o n s on the f l a n k s (Appendix B). Some of the t u f f a c e o u s m a t e r i a l may be h y a l o c l a s t i c r a t h e r than p y r o c l a s t i c , t h a t i s , formed by s h a t t e r i n g g l a s s d u r i n g the aqueous c h i l l i n g o f l a v a ( R i t t m a n , 1962), r a t h e r t h a n by e x p l o s i v e v u l c a n i s m . However, i t was d i f f i c u l t i n some cases t o d i s t i n g u i s h between the two o f them and the term " p y r o c l a s t i c " , as used i n t h i s paper, may t h e r e f o r e cover both t r u e p y r o c l a s t i c and h y a l o c l a s t i c p r o d u c t s . L a p i l l i and L a p i l l i t u f f s Most o f the t e p h r a form m a s s i v e l y bedded, u n s o r t c d a c c u m u l a t i o n s o f l a p i l l i , some o f which may be cemented. The p a r t i c l e s are g e n e r a l l y composed of v e s i c u l a r h y a l o c r y s t a i l i n c s i d e r o m e l a n e , though t a c h y l y t e and h o l o c r y s t a l l i n o fragments may be p r e s e n t i n s m a l l amounts. Cemented d e p o s i t s , i . e . t u f f s , which a re i n the p r o c e s s of f o r m a t i o n on the s u r f a c e of the soanount were found o n l y on the summit d u r i n g the pr e s e n t survey ( P I - t o 1 3 ) , and t h e r e , t h e i r o c c u r r e n c e i s s p o r a d i c . The cementing agents are manganese o x i d e s ( P l a t e 1 9 ) , i r o n o x i d e s ( P l a t o 2 0 ) , or carbonate of o r g a n i c o r i g i n ( P l a t e 2 1a). The carbonate cement i s r e s t r i c t e d t o the s h a l l o w summit peaks, which are w e l l w i t h i n the p h o t i c zone and are t h i c l l y e n c r u s t e d w i t h s e s s i l e marine l i f e , PLATE 18 - Bottom photographs of a cemented t u f f d e o o s i t on the 130 fathom t e r r a c e ( S t a t i o n 63"- 2) (a) The cemented s u r f a c e c r u s t of the t u f f , undercut at i t s margin, st a n d s 6 i n c h e s above the d e p o s i t o f l o o s e t e p h r a i n the channel t o the r i g h t . Sponges grow i n abundance on the s o l i d s u b s t r a t e , but not on the l o o s e m a t e r i a l i n the c h a n n e l . The d iameter of the compass i s 3 i n c h e s and. the l e n g t h o f the vane i s 10 i n c h e s . (b) L i g h t cover o f sediment, d i s t u r b e d by the compass as i t t r a v e l s over the bottom. The s u r f a c e t e x t u r e o f the t u f f i s r e v e a l e d q u i t e c l e a r l y i n t h i s photograph. The diameter o- the,compass i s 3 i n c h e s and the l e n g t h of the vane i s 10 i n c h e s .  The n a t u r e o f cementation o f the t u f f s (a) Hand, specimen showing b l a c k c o l o u r i m p a r t e d by cementing ferro-manganese o x i d e s (Sample from S t a t i o n 67 - 5 ) . (b) T h i n s e c t i o n showing the b l a c k fer.ro- manganese cement among the l a p i l l i of v e s i c u l a r , microioor'ohyrytic s i d e r o m e l a n e . (Plane l i g h t ) ( T h i n " s e c t i o n 67 - 5 - A) ( x 2 )  c PLATE 20 - The n a t u r e o f the cementation of the t u f f s ( c o n t i n u e d ) (a) Hand specimen showing r e d c o l o u r a t i o n i m p a r t e d "by f e r r u g i n o u s cement. A growth of c o r a l i n e a l g a e c o v e r s the o t h e r s i d e of the r o c k and appears as a w h i t e r i m i n t h i s photograph. (Sample from. S t a t i o n 67 - 3 ) . ~ (b) T h i n s e c t i o n showing l a p i l l i , i n t e r s t i t i a l f e r r u g i n o u s cement ( b l a c k ) and growth o f c o r a l i n e a l g a e ( b l a c k s t r i p a t top of s l i d e . ( P l a n e l i g h t ) ( T h i n s e c t i o n 67 - 3 - B) ( x 2 ) 7 6 PLATE 21 - The n a t u r e of cementation o f the t u f f s ( c o n t i n u e d ) (a) B i n d i n g m a t r i x of o r g a n i c carbonate and s i l i c a c o m p r i s i n g f o r a m i n i f e r t e s t s and sponge s p i c u l e s . (Crossed n i c o l s ) ( T h i n s e c t i o n 67 - 1 - A). (x27) (b) U n i f o r m l y cemented l a p i l l i t u f f ( S t a t i o n 6 7 - 3 ) (c) A s t r o n g l y i n d u r a t e d s u r f a c e c r u s t c a p p i n g an o t h e r w i s e weakly cemented l a p i l l i t u f f ( S t a t i o n 6 7 - 2 )  76 n o t a b l y the r e d c o r a l i n e a l g a LithotJmmnjL on, b a r n a c l e s , and v a r i o u s s i l i c e o u s sponges. The f e r r u g i n o u s and manganiferous cements do not appear t o have any such depth l i m i t a t i o n . Some samples appear to be u n i f o r m l y cemented throughout ( P l a t e 21b) but o t h e r s a r ^ s t r o n g l y cemented o n l y i n the top few c e n t i m e t r e s of the d e p o s i t ( P l a t o 21c) and s i m p l y form a c r u s t o v e r l y i n g u n c o n s o l i d a t e d m a t e r i a l ( P l a t e 1 8 a ) . Loose d e p o s i t s o f l a p i l l i and s h e l l fragments ( P l a t o 22) are v e r y widespread on and j u s t below the upper t e r r a c e . The o r i g i n o f the s h e l l d e b r i s i s u n c e r t a i n , but much o f i t i s s i m i l a r to the fauna p r e s e n t l y l i v i n g on b o u l d e r s found on the t e r r a c e . Much o f t h e s h e l l m a t e r i a l may t h e r e f o r e have been .recently d e r i v e d from b o u l d e r s and r o c k y o u t c r o p s i n the v i c i n i t y of the upper t e r r a c e and i t s c e n t r a l p i n n a c l e s . C u r r e n t s , have, i n p l a c e s , reworked the l o o s e d e p o s i t enough t o produce r i p p l e marks i n the l i g h t s h e l l m a t e r i a l ( P l a t o 2 3 ) . Sediments, both l o o s e and cemented, of w e l l rounded l a p i l l i are a s s o c i a t e d w i t h b oth the 45 fathom and 130 fathom t e r r a c e s ( P l a t e 2 ) . These are thought t o r e p r e s e n t f o s s i l wave-washed d e p o s i t s , (Chapter I I I - B ) . Thin-bedded T u f f Thin-bedded t u f f s composed of v o l c a n i c ash a r c not PLATE 22 - The l o o s e l a p i l l i d e p o s i t o f the 1+5 fathom tcrre.ce w i t h i t s c o n spicuous s h e l l d e b r i s . A thumb-tack a t the bottom o f the lower photograph p r o v i d e s the s c a l e . (Camera S t a t i o n 6 8 - 3 and sample S t a t i o n 6 ? - 12) 7 9 PLATE 2 3 - R i p p l e marks v i s i b l e i n the l i g h t s h e l l m a t e r i a l of the d e p o s i t s on the 45 fathom t e r r a c e . The compass i s 3 i n c h e s i n diameter and the vane i s 10 i n c h e s l o n g ( S t a t i o n 6 8 - 3 ) .  8 1 as common as l i p i l l i t u f f s . The o n l y d e p o s i t found i s on the 130 fathom t e r r a c e i n the a r e a covered by dredge s i t e 6 7 - 6 and touched by 6 7 - 2 and 6 7 - 4 . T h i s t u f f ( P l a t e 2 4 ) i s g e n e r a l l y dark brown but, i n p l a c e s , may be r e d d i s h brown o r ochre c o l o u r e d , depending on the r e l a t i v e amount of f e r r u g i n o u s cement. The samples are f r i a b l e and s l a b b y and none exceeds 4 cm. i n t h i c k n e s s . I t c h a r a c t e r i s t i c a l l y p o s s e s s e s graded bedding w i t h p a r t i c l e s i z e s r a n g i n g m a i n l y from coarse to medium sand but l a p i l l i up t o 1 cm. i n diameter are f r e q u e n t l y embedded i n the base of the u n i t . Whether t h i s g r a d i n g c o n t i n u e s downward below the l a y e r r e p r e s e n t e d by the sample s l a b s i s not known. The v i s i b l y graded p o r t i o n i s u s u a l l y capped by a t h i n n e r l a m i n a t e d u n i t o f medium t o f i n e sand. In t h i n s e c t i o n , the r o c k i s seen t o be a v i t r i c - c r y s t a l t u f f ( P e t t i j o h n , 1957), w h o l l y composed of v i t r o p h y r i c sideromelane shards and c r y s t a l fragments. The g l a s s o f the g r a i n s i s e x c l u s i v e l y sideromelane w i t h a narrow range o f r e f r a c t i v e i n d i c e s . The r e f r a c t i v e i n d e x was determined u s i n g immersion o i l s and the Bccko l i n e method, th e i n d e x o f each o i l b e i n g checked i m m e d i a t e l y p r i o r to use w i t h an Abbe r e f r a c t o m o t e r . The shards were c l e a n e d u l t r a s o n i c a l l y i n d i s t i l l e d water, a s i m p l e t i m e - s a v i n g t e c h n i q u e which, f o r u n a l t e r e d s h a r d s , PLATE 2 4 Thin-bedded v i t r i c - c r y s t a l t u f f (bedded ash) ( S t a t i o n 6 7 -6) (a) S e c t i o n through the t u f f s h o v i n g graded bedding (b) Photonv c r o g r a n h o f the sane t u f f . The b l a c k m a t e r i a l between the shards i s the f e r r u g i n o u s cedent. (Plane l i g h t ) ( T h i n s e c t i o n 67 - 6 - D) (xoO) 82 83 was found to work as w e l l as the c h e m i c a l method o f K i t t r i c k and Hope ( 1 9 6 3 ) . Based on o b s e r v a t i o n o f 2 2 5 g r a i n s , the g l a s s has an i n d e x between 1 . 5 7 and 1 . 5 9 ? w i t h a s t r o n g mode between I . 5 8 O and I . 5 8 6 . The spread, o f t h i s mode i s w i t h i n the l i m i t s one would expect f o r a s i n g l e e.sh f a l l ( W i l c o x , 1 9 6 5 ) . T h i s d e p o s i t must, t h e r e f o r e , have o r i g i n a t e d d u r i n g a s i n g l e v o l c a n i c event. The sedimentary s t r u c t u r e s and p e t r o g r a p h i c p r o p e r t i e s of t h i s p a r t i c u l a r t u f f f u r n i s h some i n s i g h t i n t o i t s o r i g i n . The sequence o f graded and l a m i n a t e d u n i t s resemble the A and 3 i n t e r v a l s o f the t u r b i c l i t c s d e s c r i b e d by Bouma (I962), but t h i s docs not n e c e s s a r i l y mean t h a t the t u f f was d e p o s i t e d by t u r b i d i t y f l o w . The f i n e r v o l c a n i c e j e c t a from a s i n g l e e x p l o s i v e event d u r i n g a p e r i o d o f e x p l o s i v e e r u p t i o n may r a i n v e r t i c a l l y from the e r u p t i v e ash c l o u d , o r may be c a r r i e d l a t e r a l l y i n the base surge (Moore, 1 9 6 7 ) down the f l a n k s o f the cmo. In the case of a submarine e x p l o s i o n which breaches the s u r f a c e , a base surge w i l l spread out a c r o s s t h e s u r f a c e of the water. I t i s easy to imagine an ash d e p o s i t w i t h graded bedding o r i g i n a t i n g from any of these p r o c e s s e s . S t r a i g h t f o r w a r d v e r t i c a l d i f f e r e n t i a l s e t t l i n g of the ash column through a i r or water w i l l produce a graded sequence, as w i l l the s i m i l a r f o u n d e r i n g of a base surge on the sea s u r f a c e when i t s energy i s spent. A submarine base surge, s p r e a d i n g down t h e f l a n k s o f the a c t i v e cone s h o u l d behave much l i k e a t u r b i d i t y c u r r e n t and would g i v e r i s e t o a t u f f w i t h t u r b i d i t e f e a t u r e s . Such a g l a s s y t u f f might a l s o be a h y a l o c l a s t i t c d e r i v e d from the s h a t t e r i n g s u r f a c e s k i n o f a submarine f l o w of p i l l o w l a v a . P i l l o w b r e c c i a A g r a i n - s u p p o r t e d v o l c a n i c b r e c c i a , composed of l a r g e , p y r a m i d a l p i l l o w fragments and s m a l l , a n g u l a r p i l l o w - c r u s t fragments, s e t in. an u n s t r a t i f i e d m a t r i x of v i t r i c t u f f ( P l a t e 2 5 ) , was dredged from the e a s t e r n f l a n k o f the summit r i d g e ( S t a t i o n 68 - 1, Appendix B ) . The b l a c k b a s a l t fragments c o n t r a s t s h a r p l y w i t h the g r e y - green and creamy y e l l o w v a r i e g a t e d m a t r i x . The o u t e r s u r f a c e of t h e r o c k i s covered w i t h a t h i n l a y e r (0 - 2mm.) of b l a c k and brown ferro-manganese. Recovered w i t h i t were v a r i o u s independent p i l l o w fragments and a more or l e s s i n t a c t p i l l o w . Bottom photographs i n the a r e a ( P l a t e 26) r e v e a l e d a r u b b l y s u b s t r a t e w i t h v i s i b l e p i l l o w fragments and what may be i n t a c t p i l l o w s . The m a t r i x of the b r e c c i a was examined p c t r o g r a p h i c a l l y and by means of X-ray d i f f r a c t i o n . Samples were prepar e d f o r X-ray d i f f r a c t i o n u s i n g the c h e m i c a l c l e a n i n g procedure of K i t t r i c k and Hope (1963). P i l l o w b r e c c i a ( S t a t i o n 6 8 - 1 ) (a) P i l l o w fragment e x t r a c t e d from the p i l l o w b r e c c i a . The l i g h t e r c o l o u r e d m a t e r i a l on the upper s u r f a c e i s p a l a g o n i t e . A c o l o u r photograph o f t h e upper s u r f a c e o f t h i s specimen appears i n P l a t e 3 2 . (b) S e c t i o n t h rough the p i l l o w b r e c c i a showing p i l l o w c r u s t fragments i n a t u f fac eou s m a t r i x . 85 PLATE 26 - Bottom photographs i n the a r e a o f o u t c r o p of the p i l l o w b r e c c i a ( S t a t i o n 68 - 1 ) . The diameter o f the compass i s 3 i n c h e s and the l e n g t h o f the vane i s 10 i n c h e s . (a) A b a s a l t p i l l o w ( ? ) . A wedge shaped p i l l o w , fragment i s a l s o v i s i b l e t o the l e f t of the compass. (b) U n i d e n t i f i e d b o u l d e r s i n the neighbourhood of the p i l l o w b r e c c i a occurence.  PLATE 27 - Type of r u b b l y s u b s r a t e which appears t o be most common on the f l a n k s of the summit r i d g e of Bowie Seamount. These photographs were taken a t s t a t i o n 6 8 - 1 . The compass i s 3 i n c h e s i n ' diameter and the vane i s 10 i n c h e s l o n g . 87 o Q O O The grey-green p a r t of the b r e c c i a m a t r i x was found to be composed o f unwcathercd, green g l a s s shards w i t h very few m i c r o l i t c s , sponge s p i c u l e s , and a few r a d i o l a r i a n t e s t s . The r e f r a c t i v e i n d i c e s o f the shards were found to l i e between 1 .585 and 1 .589? a narrow range, which would i n d i c a t e t h a t the shards were d e r i v e d d u r i n g a s i n g l e p e r i o d of v o l c a n i c e r u p t i o n s . The creamy y e l l o w p o r t i o n o f the m a t r i x i s seen, under the microscope, t o c o n s i s t of w h i t e , f i n e - g r a i n e d m a t e r i a l , p a l a g o n i t e , and a s m a l l e r p r o p o r t i o n of sponge s p i c u l e s and rhombic z e o l i t e c r y s t a l s . X-ray d i f f r a c t o g r a m s r e v e a l e d t h a t t h i s p o r t i o n of the m a t r i x i s l a r g e l y made up of X-amorphous m a t e r i a l ( i . e . amorphous to X - r a y s ) , w i t h l e s s e r amounts o f q u a r t z , p l a g i o c l a s e and m o n t m o r i l l o n i t e , and t r a c e s o f c h l o r i t e , i l l i t e mica and amphibole. G l a s s , pala'gonite and the o p a l i n e s i l i c a of the sponge s p i c u l e s p r o b a b l y make up the X-amorphous component. The z e o l i t e s and the m o n t m o r i l l o n i t e are most l i k e l y the a l t e r a t i o n p r o d u c t s o f the p a l a g o n i t e ( B o n a t t i and Nayudu, 1965)« The p l a g i o c l a s e o c c u r s as m i c r o l i t c s i n the g l a s s shards. The q u a r t z may be d c t r i t a l or i t may perhaps have c r y s t a l l i z e d from g e l s and s o l u t i o n s w i t h i n the m a t r i x R o c r y s t a l l i z a t i o n o f sponge s p i c u l e s might a l s o c o n t r i b u t e 89 some c r y s t a l l i n e q u a r t z t o the system. Whether the t u f f a c c o u o ' m e t r i x of the p i l l o w b r e c c i a i s a s y n g e n e t i c h y a l o c l a s t i t e , d e r i v e d at the time o f e r u p t i o n from the d e c r e p i t a t i n g s k i n s o f the p i l l o w s and p i l l o w fragments, w i t h which i t i s now a s s o c i a t e d , or an e x p l o s i o n t u f f d e r i v e d i n d e p e n d e n t l y , was not a s c e r t a i n e d . However, the s i m i l a r i t y , i n t h i n s e c t i o n , o f the p r o p e r t i e s of the p i l l o w c r u s t fragments and the s h a r d s o f the m a t r i x would suggest t h a t t h e m a t r i x i s a c t u a l l y a s y n g e n e t i c h y a l o c l a s t i t e ( c f . R i t t m a n , 1962, C a r l i s l e , 1963, Honnorez, 1963, Solomon, 1969). D. V e s i c l e L i n i n g s and Amygdulcs 1. C h e m i c a l l y p r e c i p i t a t e d l i n i n g s V e s i c l e l i n i n g s and amygdules are g e n e r a l l y r e s t r i c t e d t o m i c r o v c s i c l e s i n the sideromelane o f l a p i l l i and c h i l l e d f l o w or bomb s u r f a c e s which can be seen i n t h i n - s e c t i o n . Only i n r e l a t i v e l y o l d r o c k s , such as "the deeply weathered b a s a l t on Hodgkins Seamount, are the l a r g o i n t e r i o r v e s i c l e s l i n e d or f i l l e d . Many o f the amygdules and l i n i n g s are composed of rod-brown, i s o t r o p i c , i r o n o x i d e s ( P l a t e 2 8 a ) , b l a c k , opaque manganese o x i d e s , or s p a r r y and f i b r o u s c a l c i t e ( P l a t e 28b). Most commonly, however, they c o n s i s t c i t h e r of y e l l o w t o p a l e g r e e n i s h , a n i s o t r o p i c , low b i r c f r i n g e n t , PLATE 28 - Photomicrographs o f v e s i c l e l i n i n g s and amygdules. (a) C o n c e n t r i c a l l y banded growth o f i r o n o x i d e b e g i n n i n g to f i l l a v e s i c l e ( P l a n e l i g h t ) ( T h i n s e c t i o n 68 - 5 ) ( x 2 0 0 ) (b) Amygdules o f s p a r r y c a l c i t e . The round, dark grey a r e a s are empty v e s i c l e s and the b l a c k background i s i s o t r o p i c g l a s s . ( P a r t i a l l y c r o s s e d n i c o l s ) ( T h i n s e c t i o n 67 - .3 - A). ( x 8 0 ) 90 91 f i b r o u s m i n e r a l s which are too f i n e - g r a i n e d t o i d e n t i f y ( P l a t e 2 9 ) , or of i s o t r o p i c , g o lden y e l l o w m a t e r i a l ( P l a t e 30a) w i t h a l l the p r o p e r t i e s o f p a l a g o n i t e , which i s sometimes v i s i b l e i n the g l a s s of the v e s i c l e w a l l . Those substances were i n c l u d e d by Peacock (1928) under the term " p a l a g o n i t e " . The former, which may a c t u a l l y be a f i n e f i b r o u s z e o l i t e growth, he c a l l e d ; ; f i b r o - p a l a g o n i t e " and the l a t t e r " g o l - p a l a g o n i t o " . P a l a g o n i t e i s thought to be a form of h y d r a t e d b a s a l t i c g l a s s formed i n a subaquoou environment but i t s mode of o r i g i n i s i n d i s p u t e . I t i s thought to form e i t h e r by the r e a c t i o n o f hot l a v a w i t h water (Peacock and F u l l e r , 1 9 2 8 ) , or by the g r a d u a l h y d r a t i o n and a l t e r a t i o n of sideromelane (Moore, 1 9 6 6 ) . Found i n these ways, p a l a g o n i t e i s most commonly seen i n among the u n d i s t u r b e d m i c r o l i t c s and v e s i c l e s of the g l a s s i t r e p l a c e s ( P l a t e 5 3 a ) . S e v e r a l of the v a r i a t i o n s i n the c o m p o s i t i o n o f the amygdules and v e s i c l e l i n i n g s p a r a l l e l g e o g r a p h i c a l l y the v a r i a t i o n s i n the t u f f cements d e s c r i b e d above. C a l c i t e amygdules are found o n l y i n the r o c k s ca p p i n g the s h a l l o w summit peaks, and l i n i n g s o f manganese o x i d e s occur o n l y where such m a t e r i a l i s v i s i b l e i n q u a n t i t y on the s u r f a c e (e.g. s t a t i o n 6 8 - 3 , Appendix E). The d i s t r i b u t i o n o f the o t h e r c a v i t y - f i l l i n g s u b s tances i s a p p a r e n t l y random. PLATE 2 9 - Photomicrographs o f v e s i c l e l i n i n g s and amygdules "(continued) (a) V e s i c l e l i n i n g s o f f i n e , f i b r o u s , low- b i r e f r i n g e n t m i n e r a l s ( f i b r o - p a l a g o n i t e ) ( P l a n e l i g h t ) ( T h i n s e c t i o n 67 - 5 - A ) ( x 8 0 ) (b) The same, through c r o s s e d n i c o l s . 92 PLATE 30 - Photomicrographs of v e s i c l a l i n i n g s and amygdules ( c o n t i n u e d ) (a) G e l - p a l a g o n i t o l i n i n g and f i l l i n g v e s i c l e s . The same m a t e r i a l l i n e s the s u r f a c e o f the r o c k fragment i t s e l f and has p a r t l y r e p l a c e d the sideromelane w i t h i n the r o c k . A b r e c c i a m a t r i x of g l a s s shards forms the upper r i g h t h a l f o f the p i c t u r e ( P l a n e l i - . h t ) (Thin s e c t i o n 60 - 1 - C) (x27) (b) S e g r e g a t i o n v e s i c l e s i n the b a s a l t o f Rod.gid.ns Seamount. The secondary l i n i n g s are o f b l a c k t a c h y l y t e , drawn from the m a t r i x of the s u r r o u n d i n g r o c k . A c h i n c o a t i n g of f j b c o u s ' ' n e r a l s l i n e s t i e c a v i t i e s r e m a i n i n g i n the t a c h y l y t . (Plane l i g h t ' ( T h i n s e c t i o n 6 0 - 8 -• A) (x27)  94 2. S e g r e g a t i o n v e s i c l e s Some o f the b a s a l t s on the .summit of Hodgkins Seamount c o n t a i n s e g r e g a t i o n v e s i c l e s ( P l a t e 30b) such as those d e s c r i b e d by Smith ( 1 9 6 7 ) . These a r c secondary l i n i n g s of g l a s s , which' a r c thought to be due t o an i n c r e a s e i n c o n f i n i n g p r e s s u r e on the l a v a a f t e r e x t r u s i o n . The s u b s i d i a r y s u r f a c e s of the s e g r e g a t i o n v e s i c l e s do, however, s u p p o r t . c h e m i c a l l y p r e c i p i t a t e d l i n i n g s o f manganese o x i d e s and f i b r o u s m i c r o c r y s t a l l i n e z e o l i t e s . E• P o s t - v o l c a n i c D e p o s i t s S u r f i c i a l m a t e r i a l o f n o n - v o l c a n i c o r i g i n i s r a r e or o n l y l o c a l l y abundant i n the a r e a . I c e - r a f t e d r o c k s are r a r e on Bowie Seamount and have so f a r been found i n abundance o n l y among the r e l a t i v e l y o l d , s p h e r o i d a l l y weathered b a s a l t s o f the n o r t h e a s t r i d g e . The summit t e r r a c e s appear to be almost d e v o i d o f t h i s f o r e i g n m a t e r i a l , f o r o n l y one i c e - r a f t e d pebble was i d e n t i f i e d d e s p i t e the e x t e n s i v e sampling i n t h a t a r e a . In c o n t r a s t , i c e - r a f t e d m a t e r i a l on the summit of Hodgkins Seamount seems t o be ve r y abundant. G l a c i a l d e b r i s c o n s t i t u t e d f u l l y 50% o f the s i n g l e dredge h a u l taken from t h e r e and ranged from s a n d - s i z e d e t r i t u s t o b o u l d e r s 30 cm. a c r o s s . Copious q u a n t i t i e s o f f e r r o - manganese accompanied t h e g l a c i a l d e b r i s i n the dredge. 95 The ferro-manganose i s found b o t h as c o n c r e t i o n s , p a r t l y - b l a n k e t i n g a l t e r e d b a s a l t b o u l d e r s ( P l a t e Jia) and as s l a b s broken from a r u b b l y s u b s t r a t e ( P l a t e 3 1 b ) . I t t h e r e f o r e p r o b a b l y forms a d i s c o n t i n u o u s pavement i n the a r e a o f d r e d g i n g . I t a p p a r e n t l y reached i t s p r e s e n t maximum t h i c k n e s s of 5cm. b e f o r e the a r r i v a l of the. i c e - r a f t e d m a t e r i a l , f o r the g l a c i a l b o u l d e r s bear l i t t l e more than a brown s t a i n o r , at most, a t h i n veneer a f r a c t i o n of a m i l l i m e t r e t h i c k . I t would appear from t h i s t h a t the ferro-manganesc i s s t i l l a c c u m u l a t i n g but a t a very slow r a t e . Barnes and Dymond (1967) r e c o r d growth r a t e s o f 0 . 5 - 40 mm./10^ y r . f o r o c e a n i c n o d u l e s . The r e s u l t s of o t h e r a u t h o r s , who f a v o u r slow a c c r e t i o n , a re c o m p a t i b l e w i t h t h e s e f i g u r e , and the r a t e of growth of the fcrro-raanganese c r u s t on Hodgkins Seamount a l s o appears to f a l l w i t h i n these l i m i t s . I f the f o r m a t i o n r a t e has been c o n s t a n t throughout i t s growth h i s t o r y , the a l t e r e d r o c k s which support a 5 cm. t h i c k n e s s may be w e l l over 1 m i l l i o n y e a r s o l d . A p p a r e n t l y , then, t h i s p a r t o f the summit of Hodgkins Seamount i s r e l a t i v e l y much o l d e r than t h e summit r e g i o n of Bowie Seamount, which was a c t i v e at l e a s t u n t i l the end of the P l e i s t o c e n e . Ferro-manganese nodules i r o n h odgkins Seamount ( S t a t i o n 68 - 8) (a) A l a r g e ferro-manganese nodule v i t h a n u c l e u s of weathered b a s a l t . (b) The under s i d e of a fragment o f f e r r o - manganese pavewent s h o v i n g a weathered rounded b a s a l t cobble p i c k e d up from the s u b s t r a t e . 96 97 F• Chcmic a l Weathering The r o c k s and t e p h r a found on Bov.de Seamount are v e r y f r e s h and g e n e r a l l y show, at most, r u s t y i r o n s t a i n s but deeper w e a t h e r i n g has a l s o been found. P a l a g o n i t c , f o r i n s t a n c e , may le p r e s e n t as a t h i n s k i n where c h i l l e d margins o f sideromelane e x i s t . P a l a g o n i t c may occur as the i n i t i a l h y d r a t e d phase i n the g r a d u a l c h e m i c a l w e a t h e r i n g o f b a s a l t i c g l a s s (Moore, 1966) but i t i s a l s o thought to be a s y n g e n e t i c h y d r a t i o n and a l t e r a t i o n p r o d u c t , formed when the sideromelane i s s t i l l v e r y hot (Peacock and F u l l e r , 1 9 2 8 ) . In the a r e a of s t u d y , the o c c u r r e n c e of p a l a g o n i t c appears t o be r e s t r i c t e d t o those l o c a l i t i e s where the r o c k s a r c o b v i o u s l y r e l a t i v e l y o l d . For example, p a l a g o n i t c i s the dominant c o n s t i t u e n t o f the t u f f s on Hodgkins Seamount ( P l a t e 3 2 a ) , which, a c c o r d i n g t o suggested r a t e s of a c c u m u l a t i o n of ferro-manganese, may be over 1 m i l l i o n y e a r s o l d . On the o t h e r hand, i n the r e g i o n of the summit t e r r a c e s and peaks of Bowie Seamount, where the r o c k s are known t o be very young, some younger than 1 8 , 0 0 0 y e a r s , p a l a g o n i t i z a t i o n was u s u a l l y absent or o n l y i n c i p i e n t . T h i s p a t t e r n a l s o appears elsewhere on Bowie Seamount. For i n s t a n c e , the p i l l o w s and p i l l o w b r e c c i a s o f the c a s t PLATE 32 - P a l a g o n i t e i n the b a s a l t i c r o c k s (a) P a l a g o n i t e t u f f from-Hodgkins Seamount. G r a i n s o f p a l a g o n i t e and a fragment of weathered b a s a l t are c l e a r l y v i s i b l e i n a cementing m a t r i x o f ferro-manganese. ( S t a t i o n 68 - 8) (b) The p a l a g o n i t i z e d upper s u r f a c e of a p i l l o w fragment from S t a t i o n 68 -- 1. 9 8 99 f l a n k of the summit r i d g o ( S t a t i o n 68 - 1) a r c thought t o bo r e l a t i v e l y o l d , f o r they a r c coated w i t h ferro-mangancse and they support the manganese-coated remains o f l o n g dead s i l i c e o u s sponges, an a p p a r e n t l y e x t e r m i n a t e d fauna which a l s o appears on the o l d r o c k s of Hodgkins Seamount and the n o r t h e a s t r i d g e . Furthermore, i n c i p i e n t w e a t h e r i n g , marked by the p r e c i p i t a t i o n of f e r r i c o x i d e s , has l e f t o n l y a core of f r e s h b a s a l t at the c e n t r e o f each p i l l o w fragment. C o n v e r s e l y , a s i m i l a r d e p o s i t on the west f l a n k o f the summit r i d g e ( S t a t i o n 68 - 7) i s thought to be r e l a t i v e l y young, f o r i t i s f r e e o f fcrro-mangancso and v i r t u a l l y f r e e o f any o r g a n i c growth or s i g n s of w e a t h e r i n g . Here a g a i n , p a l a g o n i t e i s a p p a r e n t l y p r e s e n t o n l y on the o l d e r r o c k s , f o r i t i s p r e s e n t i n the sideromelane of the p i l l o w s k i n s and h y a l o c l a s t i c m a t r i x o f the supposedly o l d e r r o c k s a t S t a t i o n 6 8 - 1 ( P l a t e s 3 0 a , 32b and 3 3 a ) , but absent a l t o g e t h e r i n the younger r o c k s o f S t a t i o n 6 8 - 7 ( P l a t e 6 ) . I t i s the a u t h o r ' s o p i n i o n , t h e r e f o r e , t h a t the p a l a g o n i t e , i n these r o c k s i s a product of the w e a t h e r i n g of c h e m i c a l l y u n s t a b l e s i d e r o m e l a n e . Evidence f o r w e a t h e r i n g o t h e r than p a l a g o n i t e and i r o n s t a i n i n g i s r a r e r . The t u f f a c c o u s m a t r i x o f the p i l l o w b r e c c i a d e s c r i b e d above c o n t a i n s z e o l i t e s , m o n t m o r i l l o n i t e , c h l o r i t e and i l l i t e , w hich are most 100 l i k e l y the p r o d u c t s of w e a t h e r i n g o f the g l a s s shards. Weathered a n d e s i t c on the n o r t h e a s t r i d g e was found by X-ray d i f f r a c t i o n to c o n t a i n a s i m i l a r m i x t u r e o f q u a r t z and c l a y m i n e r a l s . Many of these samples have weathered deeply and as a r e s u l t have become q u i t e rounded ( P l a t e 33b) Most of the r o c k s found so far on Eowic Seamount are f r e s h or o n l y s l i g h t l y weathered, w i t h the e x c e p t i o n o f the weathered r o c k s of the n o r t h e a s t r i d g e . The samples r e c o v e r e d from i t s n e i ghbour, Hodgkins Seamount, a r c r e l a t i v e l y deeply a l t e r e d . I n view of t h e i r apparent age, t h i s a l t e r a t i o n i s p r o b a b l y due to w e a t h e r i n g . The fragments of l a v a , some o f which have weathered i n t o rounded shapes, are a r i c h t an c o l o u r , c hanging g r a d u a l l y t o l i g h t grey towards th e c e n t r e . T h i s c o l o u r change i s not a f u n c t i o n o f c r y s t a l l i n i t y , but of degree of a l t e r a t i o n . The r o c k s examined a r c h o l o c r y s t a l l i n o and the tan zone, g e n e r a l l y about 5cm. t h i c k , i s seen i n t h i n - s e c t i o n to be s i m p l y more t h o r o u g h l y weathered than the l i g h t grey a r e a . An X-ray d i f f r a c t o g r a m of the t a n m a t e r i a l r e v e a l e d , b e s i d e s p r i m a r y p l a g i o c l a s e : s e r p e n t i n e , magnesian c a l c i t e , h e m a t i t e , m o n t m o r i l l o n i t o , and minor q u a r t z , c a l c i t e , a p a t i t e and i l l i t e . The t u f f s o f Hodgkins Seamount a r c composed of y e l l o w p a l a g o n i t c , rhombic z e o l i t e c r y s t a l s and a l t e r e d l a p i l l i ( s i m i l a r t o the a l t e r a t i o n above) cemented w i t h v a r y i n g PLA ">] 33 - Two t ypos of a l t e r a t i o n observed i n the a r e a o f study. (a) P a l a g o n i t i z a t i o n . The sideromelane m a t r i x o f the r o c k has been a l t e r e d t o g o l d and r e d n a l a g o n i t e near the margin . Note t h a t the p a l a g o n i t e does not a f f e c t the m i n e r a l g r a i n s . (Plane l i g h t ) ( T h i n s e c t i o n 68 - 1 - C) (e;27) (b) Weathering. The l i g h t margin of the r o c k has been l a r g e l y a l t e r e d t o c l a y . The t h i n o u t e r s k i n , \ r h i c h f l a k e s o f f r e a d i l y , i s composed e n t i r e l y of c l a y . Such a l t e r a t i o n e v e n t u a l l y cause,:, the r o c k fragment to become rounded as i t i s seen here. ( S t a t i o n 63 - 5).  102 amounts of h y d r a t c d manganese o x i d e s ( P l a t e 3 2 a ) . Other than the p a l a g o n i t e and z e o l i t e s , m o n t m o r i l l o n i t e i s t h e o n l y major w e a t h e r i n g product p r e s e n t , though c h l o r i t e and i l l i b e occur i n tre.ee amounts ( c . f . p i l l o w b r e c c i a m a t r i x from S t a t i o n 6 8 - 1 above). V. SUMMARY AND CONCLUSIONS 1. Bowio Seamount i s a co m b i n a t i o n c e n t r a l and f i s s u r e type o f v o l c a n o , which has formed over an a r e a o f complex f r a c t u r i n g o f t h e c r u s t . The assumed n o r t h e a s t f r a c t u r e set has a p p a r e n t l y been the dominant feeder of the system and e x t r u s i o n s a l o n g at l e a s t 30 m i l e s o f i t s l e n g t h a r c r e s p o n s i b l e f o r the p r e s e n t e l o n g a t e shape o f the mountain The main c e n t r e of e r u p t i o n o c c u p i e s the southwestern h a l f o f the r i d g e where i t c r o s s e s the northwest t r e n d of Hodgkins and Davidson Seamounts and the m a j o r i t y o f the o t h e r seamounts on the P r a t t - Wclkcr c h a i n . The i n t e r s e c t i o n of the two s t r u c t u r e s may have p r o v i d e d the magma w i t h a zone of r e l a t i v e l y easy a c c e s s t o the s u r f a c e r e s u l t i n g i n the c r e a t i o n of a l a r g e r v o l c a n i c p i l e over t h a t area. However, e r u p t i o n s were not r e s t r i c t e d t o the summit and n o r t h e a s t r i d g e s a l o n e . Other l o b e s o f the mountain were p r o b a b l y formed by a c t i v i t y a l o n g s u b o r d i n a t 10.3 f i s s u r e s i n the c r u s t , and p a r a s i t i c f l a n k e r u p t i o n s must a l s o have been common. 2. Two t e r r a c e s e x i s t near the sua: i t of the seamount, one at a depth of 35 t o 55 fathoms, and the o t h e r a t a depth of 120 t o 140 fathoms. They are a p p a r e n t l y P l e i s t o c e n e f e a t u r e s , formed by contemporaneous wave e r o s i o n and s h a l l o w water v u l c a n i s m d u r i n g low sea l e v e l s t ands. The upper t e r r a c e was a p p a r e n t l y formed l e s s than 1 8 , 0 0 0 y e a r s ago and the lower t e r r a c e at an e a r l i e r P l e i s t o c e n e i n t e r v a l . The f i n a l phase o f v o l c a n i e r u p t i o n s took p l a c e a f t e r the f o r m a t i o n and submergence of the l o w e r t e r r a c e which was p r o b a b l y o r i g i n a l l y an u n i n t e r r u p t e d p l a t e a u c a p p i n g the seamount a t sea l e v e l . The peaks which r i s e from the 130 fathom t e r r a c e are thought to be the p r o d u c t s of t h i s a c t i v i t y . These l a t e v o l c a n i c cones were a p p a r e n t l y a l s o t r u n c a t e d when they li - b u i l t up to the new sea l e v e l of 1 3 , 0 0 0 to 1 8 , 0 0 0 y e a r s a but were l a t e r surmounted by f u r t h e r , s m a l l t u f f cones a f t e r they had, i n t u r n , submerged. There i s no evidence to i n d i c a t e f u r t h e r a c t i v i t y a f t e r t h i s e p i - - ^ e . A b o u l d e r of f r e s h b a s a l t from the sum-.it f I - ™ - ! - * a p o t a s s i u m - argon age of 7 5 , 0 0 0 - 1 0 0 , 0 0 0 y e a r s B . P . The main b u l k o f Bowie Seamount, a c c o r d i n g t o m i c h k o f s k y ( 1 9 6 9 ) , s h o u l d be o l d e r than 0 . 6 9 m i l l i o n y e a r s , h a v i n g 104 formed p r i m a r i l y d u r i n g a p e r i o d o f magnetic r e v e r s a l . 3. The v o l c a n i c p i l e i s a p p a r e n t l y composed o f e q u a l amounts o f f l o w r o c k and t c p h r a , which form n a t u r a l s l o p e s of 10° to 20°. The l a v a f l o w s formed as both p i l l o w f l o w s and as normal massive f l o w s , p i l l o w f l o w s b e i n g the most common. Both t y p e s have i n t e n s e l y c h i l l e d upper s u r f a c e s where they came i n t o c o n t a c t w i t h sea water. Many, i f not most, o f the f l o w s a r c broken up i n t o l o o s e r u b b l e , some f o r m i n g b r e c c i a s of p i l l o w fragments i n a m a t r i x of h y a l o c l a s t i c v i t r i c t u f f . U n s o r t c d d e p o s i t s o f l a p i l l i and l a r g e r l a v a b l o c k s a r c the dominant c l a s t i c d e p o s i t s on the s u r f a c e . T u f f , a s h and v o l c a n i c bombs, common i n the r e g i o n of the summit, p r o b a b l y o r i g i n a t e d d u r i n g s h a l l o w water and subaorio.l e r u p t i o n s when the volc a n o was a c t i v e a t sea l e v e l . 4. The f e e d e r s under the seamount open below t o what must be a complex system of magma chambers and c o n d u i t s i n the mantle and c r u s t . The v a r i o u s r o c k t y p e s , which range from a l k a l i o l i v i n e b a s a l t s to a n d o s i t e s , n a y owe t h e i r o r i g i n s to t h e t a p p i n g o f d i f f e r e n t i a t i n g magma a t d i f f e r e n t l e v e l s i n these r e s e r v o i r s . U l t r a m a f i c and f e l d s p a t h i c cumulate i n c l u s i o n s suggest t h a t the magma remained q u i e s c e n t i n r e s e r v o i r s w i t h i n the c r u s t l o n g 105 enough f o r e x t e n s i v e d i f f e r e n t i a t i o n t o take p l a c e . Corroded f c l d s p a t h i c and g a b b r o i c x e n o l i t h s , which may have been d e r i v e d from w a l l - r o c k s and f l o o r - r o c k s , bear f r e s h h a r r i s i t i c c r y s t a l growths. T h i s s u g gests t h a t the w a l l s and f l o o r s of a magma chamber or c o n d u i t w i t h i n the c r u s t were, perhaps, b e i n g s l o w l y a t t a c k e d by the magma v/ h i l e , at the same t i m e , s e r v i n g as s u r f a c e s o f n u c l e a t i o n f o r f r e s h m i n e r a l phases. 5. The p r o c e s s o f c h e m i c a l w e a t h e r i n g i n the a r o a o f study i s not v e r y c l e a r , but g l a s s i s a p p a r e n t l y the most u n s t a b l e phase and i s the f i r s t m a t e r i a l t o be a f f e c t e d . P a l a g o n i t e was g e n e r a l l y absent on the c h i l l e d s i d e r o m e l a n e s u r f a c e s o f f r e s h e x t r u s i v e m a t e r i a l and more common i n a r e a s of o l d e r r o c k s . I t i s , t h e r e f o r e , thought to have formed as the i n i t i a l phase o f w e a t h e r i n g of the s i d e r o m e l a n e . In those a r e a s where r o c k fragments have weathered deeply enough, rounded forms are common. In a l l c a s e s , the main m i n e r a l s produced by w e a t h e r i n g appear to be z e o l i t e s and m o n t m o r i l l o n i t e . 6. L i t h i f i c a t i o n o f t e p h r a d e p o s i t s t o form t u f f s has a p p a r e n t l y taken p l a c e on the s u r f a c e . H y d r a t c d i r o n and manganese o x i d e s commonly form the cements and i n s h a l l o w water, where b i o c h e m i c a l a c t i v i t y i s h i g h , c a l c i t c i s a l s o 106 a cementing agent. The c o n t e n t s o f amygdules and microamygdulcs i n the r o c k s may be s i m i l a r t o t h e cementing compounds i n the v i c i n i t y , though most a r c composed o f " g c l - p a l a g o n i t e " and " f i b r o - p a l a g o n i t o " . Ferro-manganese forms a d i s c o n t i n u o u s pavement of 5 cm. t h i c k c o n c r e t i o n a r y s l a b s i n an a r e a o f r e l a t i v e l y o l d , weathered r o c k s on n e i g h b o u r i n g Hodgkins Seamount. The r a t e of growth o f these c o n c r e t i o n s appears t o be slow and of the same o r d e r of magnitude as t h a t r e c o r d e d f o r s i m i l a r open ocean o c c u r r e n c e s by o t h e r a u t h o r s . The r o c k s on t h e summit o f Hodgkins Soamount, which a r c host to the ferro-manganese, may, t h e r e f o r e , be w e l l over one m i l l i o n y e a r s o l d . 7. I c e - r a f t i n g i s an i m p o r t a n t agent o f s e d i m e n t a t i o n i n the a r e a , e s p e c i a l l y on the summit o f Hodgkins Seamount. The summit of Bowie Seamount i s almost f r e e o f g l a c i a l d e b r i s and i t i s thought t h a t t h i s i s due t o i t s s h a l l o w n e s s o r exposure d u r i n g s t a g e s of the P l e i s t o c e n e epoch, and t o b u r i a l of g l a c i a l m a t e r i a l by more r e c e n t v o l c a n i c d e p o s i t s . 107 VI. REFERENCES Barnes, S.S., and Dymond, J.R., 1967, Rates o f a c c u m u l a t i o n o f ferro-mangancso n o d u l e s ; Nature, V. 213, pp. 1218 - 1219. B o n a t t i , E., and Nayudu, Y.R. 1965, The o r i g i n o f manganese nodules on the ocean f l o o r ; Am. J . S c i . , V. 263, pp. 17 - 39. 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K j a r t a n s s o n , 6., 1966, S t a p a k c n n i n g i n og S u r t s e y , a comparison of t a b l e mountains i n I c e l a n d and the v o l c a n i c i s l a n d o f S u r t s e y o f f the s o u t h coast of I c e l a n d , N a t t u r u f r a c d i n g u r i n n , V. 36, PP. 1 - 34. — ^ 1967, V o l c a n i c forms at the sea bottom, i n I c e l a n d and Mid-Ocean Ridges - " R i t " 38, 209 pp., S o c i e t a s S c i c n t i a r u m I s l a n d i c a , R e y k j a v i k , I c e l a n d . Macdonald, G.A., 1949, Hawaiian P e t r o g r a p h i c p r o v i n c e , B u l l . Geol. Soc. Am., V. 60, pp. 1541 - 1596. „ . ~—- • — , i 9 6 0 , D i s s i m i l a r i t y of c o n t i n e n t a l and o c e a n i c r o c k t y p e s ; J . P e t r o l . , V. 1, pp. 172 - 177. 9 1967, E x t r u s i v e b a s a l t i c r o c k s i n B a s a l t s ; Hess and P o l d c r v a a r t Ed., I n t c r s c i e n c e P u b l i s h e r s (John W i l e y & Sons), New York-London- Sydney, pp. 1 - 61. 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J . , 1957, Sedimentary Rocks; Harper & Row, New York. R a i t t , R.W., 1954, B i k i n i and nearby a t o l l s ; U.S. Geol. Surv. P r o f . Paper 200-K, pp. 507 - 527. R i c h a r d s , A.F., 1965, Geology of the I s l a s R e v i l l a g i g e d o , 3 . E f f e c t s of e r o s i o n on I s l a San B e n c d i c t o 1952 - 61 f o l l o w i n g the b i r t h o f V o l c a n Barcena; B u l l . V o l e , V. 28, pp. 381 - 403. R i t t m a n , A., 1962, Volcanoes and t h e i r a c t i v i t y ; John W i l e y & Sons, Nov/ York-London. I l l S t . Amand, P. , 19.57, G e o l o g i c a l and G e o p h y s i c a l s y n t h e s i s of the t e c t o n i c s of p o r t i o n s of B r i t i s h Columbia, the Yukon T e r r i t o r y and A l a s k a ; B u l l . G e o l. Soc. Am., V. 6 8 , pp. 1343 - 1370. Scrimgor, J.A., 1969, Bowie Seamount - p r e l i m i n a r y s u r v e y f o r i n s t r u m e n t package placement; Defence Research E s t a b l i s h m e n t P a c i f i c T e c h n i c a l Memorandum 69 - 7. Shcpard, F.P. , 1 9 6 3 , Submarine Geology, Harper & Row, Mew York-Evanston-London. Smith, R.E., 1967, S e g r e g a t i o n v e s i c l e s i n b a s a l t i c l a v a ; Am. J . S c i . , V. 2 6 5 , pp. 696 - 713. Solomon, M., 1 9 6 9 , The n a t u r e and p o s s i b l e o r i g i n of the p i l l o w l a v a s and h y a l o c l a s t i t e b r e c c i a s of K i n g I s l a n d , A u s t r a l i a ; Quart. J . Geol. Soc. London, V. 124, pp. 1.53 - 169- S u t h e r l a n d Brown, A., 1 9 6 8 , Geology o f the Queen C h a r l o t t e I s l a n d s , B r i t i s h Columbia: B r i t i s h Columbia). Department of Mines and P e t r o l e u m Resources B u l l . , V. 54 . T h o r a r i n s s o n , S., 1964, The submarine e r u p t i o n o f f the Vostmann I s l a n d s 1 9 6 3 - 6 4 , B u l l . V o l c a n o l . , V. 2 7 , pp. 435 - ^i45. . , 1965, S i t t a f h v e r j u urn S u r t s e y - j a r g o s i d , some f a c t s about the S u r t s e y e r u p t i o n . M a t t u r u f r a c d i n g u r i n n , V. 3 5 , pp. 153 - 181. Wager, L.R., Brown, G.M., and Wadsworth, W.J. I960, Types of Igneous cumulates; J . P e t r o l . , V. 1, up 73 - 8 5 . - - , 1 9 6 8 , Rhythmic and c r y p t i c l a y e r i n g i n m a f i c and u l t r a m a f i c p l u t o n s , i n B a s a l t s ; Hess and P o l d e r v a a r t Ed., I n t r s c i c n c o P u b l i s h e r s (John W i l e y & Sons), New York-London-Sydney, pp. 573 - 622. Washington, H.S., 1926, S a n t o r i n i e r u p t i o n o f 1925: B u l l . G eol. Soc. Am., V. 3 7 , pp. 349 - 3 8 4 . 112 W i l c o x , R.E., 1965? V o l c a n i c ash chrono l o g y i n The Quaternary o f the U n i t e d .States; P r i n c e t o n U n i v e r s i t y P r e s s , P r i n c e t o n , Nov; J e r s e y , pp. 807 -- 816. Yodcr, U.S. J r . , and Sahama, T.G., 1957, O l i v i n e X-ray d e t e r m i n a t i v e c u r v e ; .4m. M i n e r a l o g i s t , V. 4 2 , PP. 475 - 491 . Yoder, H.S., and T i l l e y , C.E., 1962, O r i g i n of b a s a l t magmas: an e x p e r i m e n t a l study of n a t u r a l and s y n t h e t i c r o c k systems; J . P e t r o l . , V. 3 , pp. 342 - 532. Z b y s z e w s k i , G . , I960, L ' c r u p t i o n du v o l c a n do C a p e l i n h o s , I l l e de F a i a l , A z ores, B u l l . V o l c a n o l . , V. 2 3 , pp. 77 - 1 0 0 . Equi-omont usod d u r i n g s h i p w o r k EDO Western Sonar - Model l 8 p w i t h 12 KHz Transducer A l d c n P r e c i s i o n Graphic Recorder - Model 419 G i f f t Depth Recorder - T r a n s c i e v c r A l p i n e G e o p h y s i c a l X-band r a d a r t r a n s p o n d e r - model 427-D Decca Radar - Model 828 V a r i a n A s s o c i a t e s Marine P r o t o n P r e c e s s i o n Magnetometer - model V-4937 E.G. & G. I n t e r n a t i o n a l Camera - model 200, and l i g h t s ource - model 210 Ocean Research Equipment P i n g c r - model 250B P o t t c r s s o n grab sampler Tv/elvo i n c h p i p e dredge D i c t z - Lafond snapper APPENDIX B Sample D e s c r i p t i o n s ^ from Each S t a t i o n S t a t i o n 6 7 - 1 L a p i l l i T u f f - brown; u n s o r t e d , subangular t o rounded l a p i l l i and ash; woakly cemented (cement o f i r o n o x i d e s and o r g a n i c c a l c i u m c a r b o n a t e ) ; mantled by the r o d c o r a l l i n e a l g a L i t h o t h a m n i o n and s i l i c e o u s sponges. Cow-dung Bombs - f l a t , 2 - 3 cm. t h i c k ; bread c r u s t f r a c t u r e on upper s u r f a c e ; i n t e r i o r more or l e s s h o l l o w . I n c l u s i o n - c o a r s e , c r y s t a l l i n e p l a g i o c l a s e , o l i v i n e and mag n e t i t e . S t a t i o n 6 7 - 2 V o l c a n i c conglomerate - y e l l o w i s h brown; p o o r l y s o r t e d , w e l l rounded l a p i l l i , f r o s t e d c r y s t a l g r a i n s , abraded s h e l l fragments; s t r o n g l y cemented s u r f a c e c r u s t ( r e d f e r r u g i n o u s and b l a c k manganiferous cements), weakly cemented below c r u s t ; i c e - r a f t e d cobble cemented i n s u r f a c e c r u s t . P i l l o w fragments - r o u g h l y p y r a m i d a l ; f r e s h v e s i c u l a r b a s a l t ; quenched s u r f a c e o f sid e r o m e l a n e w i t h no s i g n o f p a l a g o n i t e . V i t r i c - c r y s t a l t u f f - dark brown t o r e d d i s h brown and y e l l o w ; sand s i z e g r a i n s ; t h i n - b e d d e d , w i t h graded bed topped by t h i n l a m i n a t e d bed, ( c . f . 67 - 6). U n c o n s o l i d a t e d m a t e r i a l - heterogeneous m i x t u r e o f a n g u l a r , u n s o r t e d c i n d e r s , l a p i l l i ash and some s h e l l m a t e r i a l ( c . f . 67 - 10). I n d e t e r m i n a t e - v a r i o u s a n g u l a r and c l i n k e r y b a s a l t p e b b l e s . S t a t i o n 67 - _^ L a p i l l i t u f f - r o d ; u n s o r t e d , a n g u l a r t o rounded l a p i l l i and ash; s t r o n g l y cemented (cement i s f e r r u g i n o u s , hence the r o d c o l o u r ) ; e n c r u s t e d w i t h r e d c o r a l l i n e a l g a e . L a p i l l i t u f f - dark brown; u n s o r t e d , a n g u l a r t o subrounded l a p i l l i and ash; f r i a b l e , weakly cemented (cement o f o r g a n i c c a r b o n a t e ) . I n c l u s i o n - coarse aggregate o f pyroxene c r y s t a l s . I n d e t e r m i n a t e - v a r i o u s b a s a l t i c p e b b l e s 1 - 4 cm. a c r o s s . S t a t i o n 6 7 - 4 B a s a l t - f l o w t o p ; fragments p r o b a b l y s p a l l e d from the s u r f a c e of a l a v a f l o w ; d a r k grey, g l a s s y b a s a l t o f the same c o m p o s i t i o n as the r o c k s of 67 - 6; quenched s u r f a c e of s ideromelane w i t h t h i n p a l a g o n i t c s k i n ; s l i g h t l y weathered a l o n g f r a c t u r e s . V i t r i c - c r y s t a l t u f f - ochre y e l l o w ; sand s i z e grains.; f i n e l y l a m i n a t e d ; ( c . f . 67 - 6). S t a t i o n 67 - 5 L a p i l l i t u f f - b l a c k ; u n s o r t e d l a p i l l i and ash; o c c u r s as a s l a b b y c r u s t , cemented w i t h manganese o x i d e s . Bombs - range from f l a t t e n e d cow-dung v a r i e t y t o h i g h l y deformed, globose forms with, i n v o l u t e and ropy s u r f a c e s ; quenched s k i n of si d e r o m e l a n e . S t a t i o n 6 7 - 6 V i t r i c - c r y s t a l t u f f - dark brown t o r e d d i s h brown and y e l l o w : sand s i z e g r a i n s ; t h i n - b e d d e d , w i t h graded bed topped by f i n e h o r i z o n t a l l a m i n a t i o n s ; f e r r u g i n o u s cement i m p a r t s the v a r i e t y of c o l o u r s . L a p i l l i t u f f - r e d ; t h i n c r u s t o n l y ; u n s o r t e d fragments; f e r r u g i n o u s cement. P i l l o w fragments - p y r a m i d a l shape; b r e a d - c r u s t y , c h i l l e d , upper s u r f a c e of sideromelane w i t h o u t p a l a g o n i t c ; p l a g i o c l a s e and hornblende - r i c h v e s i c u l a r b a s a l t . Cow-dung bombs - f r e s h and black, w i t h bread c r u s t upper s u r f a c e ; a l s o weathered y e l l o w v a r i e t i e s w i t h h o l l o w c e n t r e s . I n c l u s i o n s - common; fSLdspathic and g a b b r o i c ; c o a r s e l y c r y s t a l l i n e . U n c o n s o l i d a t e d m a t e r i a l - heterogeneous, u n s o r t e d m i x t u r e o f ash, l a p i l l i and c i n d e r s of undetermined a f f i n i t i e s . I n d e t e r m i n a t e - u n c o n s o l i d a t e d m a t e r i a l above; n o n d e s c r i p t b o u l d e r s and c o b b l e s of c o m p o s i t i o n s i m i l a r t o the p i l l o w fragment s. S t a t i o n 6 7 - 7 U n c o n s o l i d a t e d m a t e r i a l - w e l l rounded, s o r t e d l a p i l l i w i t h f r o s t e d c r y s t a l g r a i n s ( c . f . 67 - 2 ) . A n d e s i t i c r o c k - a n g u l a r , b l o c k y , i r r e g u l a r l y j o i n t e d b o u l d e r s of n o n - v e s i c u l a r r o c k s l i g h t l y weathered a l o n g f r a c t u r e s . S t a t i o n 6 7 - 8 - u n s u c c e s s f u l ; hard bottom. S t a t i o n 67 - 9 U n c o n s o l i d a t e d m a t e r i a l - s m a l l amount of c r y s t a l and v i t r i c sand; i n s u f f i c i e n t sample f o r use. S t a t i o n 67 - 10 U n c o n s o l i d a t e d m a t e r i a l - u n s o r t e d , subangular t o rounded, f r e s h , b l a c k l a p i l l i and ash; some fre.gmented but unworn s h e l l m a t e r i a l . B a s a l t - b o u l d e r ; f r e s h , dark grey, v e s i c u l a r b a s a l t w i t h prominent o l i v i n e and e.ugite p h e n o c r y s t s . S t a t i o n 67 - 11 U n c o n s o l i d a t e d m a t e r i a l - b r o w n - s t a i n e d c i n d e r s , l a p i l l i , and ash; i n s u f f i c i e n t sample f o r use. S t a t i o n 67 - 12 U n c o n s o l i d a t c d m a t e r i a l - p o o r l y s o r t e d , subrounded t o rounded, brown l a p i l l i and ash w i t h abundant y e l l o w - s t a i n e d s h e l l m a t e r i a l ; s t a i n i n g o f l a p i l l i and s h e l l m a t e r i a l i s duo t o i r o n o x i d e s . S t a t i o n 67 - 13 - u n s u c c e s s f u l S t a t i o n 67 - Ik U n c o n s o l i d a t e d m a t e r i a l - p o o r l y s o r t e d , subrounded t o rounded, fresh, b l a c k l a p i l l i and ash w i t h abundant s h e l l m a t e r i a l . B a s a l t - b o u l d e r ; fresh,, d a r k gre y , v e s i c u l a r , o l i v i n e b a s a l t , e n c r u s t e d w i t h c a l c a r e o u s organisms. S t a t i o n 67 - 15 U n c o n s o l i d a t e d m a t e r i a l - b r o w n - s t a i n e d c i n d e r s l a p i l l i . and ash; q u a n t i t y too s m a l l t o work w i t h . S t a t i o n 6 8 - 1 Bombs - F u s i f o r m bombs, some undamaged, o t h e r s w i t h the o u t e r s u r f a c e s p a l l e d o f f ; a l s o bombs s i m i l a r to 67 - 5; t h i n p a l a g o n i t e s k i n sometimes p r e s e n t on the s ideromelane quenched s u r f a c e . P i l l o w fragments - p y r a m i d a l fragments w i t h r a d i i of 5 to 30 cm.; a l s o c r u d e l y p i l l o w e d forms; r o c k type i s a dark grey, f i n e l y v e s i c u l a r , o l i v i n e b a s a l t ; s ideromelane o u t e r s u r f a c e w i t h w e l l developed p a l a g o n i t c s k i n ; r o c k s g e n e r a l l y weathered along f r a c t u r e s . • P i l l o w b r e c c i a - p i l l o w fragments, as above, and s m a l l e r p e b b l e - s i z e p i l l o w c r u s t fragments, set i n a m a t r i x of v i t r i c t u f f . Flow top - f l a t , f l o w s u r f a c e , a p p a r e n t l y from non- p i l l o w e d f l o w ; bread c r u s t s u r f a c e o f sideromelane w i t h p a l a g o n i t c s k i n ; r o c k i s weathered as above. S t a t i o n 68 - k U n c o n s o l i d a t e d m a t e r i a l - ponded sediment; 50 - 50 m i x t u r e o f s m a l l l a p i l l i and G l o b i g e r i n a ooze; sample too s a m l l t o use. S t a t i o n 6 8 - 5 A n d o s i t e - weathered c o b b l e s of f i n e l y v e s i c u l a r , l i g h t grey a n d e s i t e , each enveloped i n a sheath of c l a y ; many fragments are rounded due t o deep w e a t h e r i n g . I c e - r a f t e d d e t r i t u s - smooth, round c o b b l e s of g r a n o d i o r i t e , S t a t i o n 6 8 - 6 - u n s u c c e s s f u l : few c h i p s of weathered r o c k and a s m a l l amount of c a l c a r e o u s c l a y ; i n s u f f i c i e n t m a t e r i a l f o r use. S t a t i o n 6 8 - 7 P i l l o w fragments - l a r g e fragments o f p y r a m i d a l shape, a l s o p e b b l e - s i z e fragments of p i l l o w c r u s t s ; f r e s h , d a r k g r e y , h i g h l y v e s i c u l a r b a s a l t w i t h sideromelane c h i l l e d s u r f a c e ; p a l a g o n i t e i s absent. S t a t i o n 6 8 - 8 (Summit o f Hodgkins Seamount) P i l l o w fragments - p a l e grey t o t a n ; d e e p l y weathered, v e s i c u l a r b a s a l t . P a l a g o n i t o t u f f - y e l l o w ; s a n d - s i z e p a l a g o n i t e g r a i n s and s few weathered l a p i l l i cemented w i t h v a r y i n g amounts of ferro-mangancso; rhombic z e o l i t e c r y s t a l s are common. C o n c r e t i o n a r y fcrro-manganose - as nodules w i t h weathered b c i s a l t b o u l d e r n u c l e i ! , and as s l a b s ; composed of l a m i n a t e d and b o t r y o i d a l , dark brown ferro-manganese o x i d e s and h y d r a t e d o x i d e s ; maximum t h i c k n e s s 5 cm. I c e - r a f t e d d e t r i t u s - s i z e range from s a n d - s i z e m a t e r i a l to b o u l d e r s 30 cm. i n dia m e t e r ; wide range of c o m p o s i t i o n s : graded l i t h i c and a r k o s i c a r e n i t e s , p h y l l i t e , . a m p h i b o l i t i c g r e e n s t o n e , t r e m o l i t e - garnet s c h i s t , t u f f , p o r p h y r i t i c d a c i t e , g r a n i t e , d i o r i t e and m a g n e t i t e - r i c h gabbro. None of these r o c k s has a d i s t i n c t i v e enough l i t h o l o g y t o t r a c e t o i t s o r i g i n . I n d e t e r m i n a t e - b a s a l t p e b b l e s o f i r r e g u l a r shape; d e e p l y weathered t o a t a n c o l o u r ; many have weathered i n t o rounded shapes'. S t a t i o n 6 8 - 9 B a s a l t - b o u l d e r ; f r e s h , dark gr e y , v e s i c u l a r , o l i v i n e b a s a l t .

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