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Geology of Mount Washington, Vancouver Island British Columbia Carson, David John Temple

Abstract

Mount Washington rises abruptly to the west of the Coastal plain of Vancouver Island near Courtenay, British Columbia. It is ( -shaped possessing two cirques which face northeast. The basement rocks of the Mount Washington area consist of several thousand feet of massive basic to intermediate volcanics of the Triassic Vancouver group. A layer composed of gently dipping Upper Cretaceous shales, sandstones, and minor conglomerate and coal overlies the Triassic rocks on the Coastal plain, and outliers of this layer are present on the higher areas west of the plain. Dioritic intrusions cut the Triassic and Upper Cretaceous rocks. The higher portions of Mount Washington are composed mainly of Upper Cretaceous rocks. These, and the Triassic rocks underlying them have been domed by the intrusion of a centrally located quartz diorite stock. Numerous offshoots of this stock are present in the Upper Cretaceous rocks surrounding it. At its west border there are two breccia pipes. Copper-bearing quartz veins are present on the west side of the mountain in the vicinity of the stock. These were formed at high temperatures in a near-surface environment. The distribution of the breccias and sill-like intrusions at Mount Washington suggests that the development of the stock was highly restricted as it moved upward through the Triassic volcanics and that on reaching the Upper Cretaceous sediments it encountered much less resistance so that it spread laterally to form dykes, sills, and laccoliths(?). The present investigation provides, the only detailed geological mapping done in the Mount Washington-Constitution Hill area, and its contributions are as follows: (1) additions to the knowledge of the stratigraphy and structure of the rocks in the area; (2) information on the probable methods of emplacement of the dioritic intrusions, and the relationships among these intrusions; (3) the existence, extent, and nature of the breccias; (4) additions to the knowledge of the character of the mineral deposits, including the occurrence of the mineral wehrlite.

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