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Show PEE-TEETIAEY VOLCANIC EOCKS. 93 contemporaneous rocks of the Carboniferous in England and Scotland basalt, while others who desire to be non-committal call them traps, which may mean either diabase, basalt, or dolerite, or even augite-andesite. Professor Geike* specially mentions basalt and dolerite as among the inter-bedded and contemporaneous Carboniferous traps of Great Britain, and so eminent a geologist is certainly not liable to confuse his technical terms. Mr. Jukes also mentions the basalts of the South Staffordshire coal-fields (Rowley Rag) as being of Carboniferous age. Still more ancient are certain basalts of the northern peninsula of Michigan, of which the fragments are found abundantly in the drifts of Wisconsin and Illinois. These were all erupted prior to the Potsdam period; and though they are usually called greenstones, many of them are certainly basalt. Sir W. Logan and T. Sterry Hunt mention doleritesf of Archaean age in Canada (Grenville), much of it very fine-grained and sometimes amygdaloidal, and Sir William pronounced it to have been erupted prior to the Silurian, which is seen to overlap the denuded dikes in which it occurs. Prof. J. W. Dawson speaks of basalts J of Triassic age extensively developed along the eastern shore of the Bay of Fundy, especially in the vicinity of Cape Blomidon. The oldest volcanic rocks from the Rocky Mountain Region of which I have any knowledge, are found in rounded pebbles of the Shinarump conglomerate, which lies at the top of the series to which Professor Powell has given that name, and which is supposed to be of Triassic or Permian age. These are fragments of a very fine-grained basalt, quite indistinguishable from the water-worn pebbles of the latest Tertiary basalts. Numerous cases might be cited of the occurrence of augitic rocks with a volcanic texture erupted prior to Tertiary time, and far back, indeed, into the Archaean, though unquestionably the augitic rocks of earlier epochs possess in the great majority of cases the granitic textureâ€"in short, may very properly be called diabase. It is difficult to resist the conclusion resulting from the various accounts of these rocks that their textures depend chiefly upon the conditions of cooling. Where this has been rapid, as, for instance, in cases of contact with dike-walls, the magmas have been * Address British Association, Dundee meeting, 1887. t Geology of Canada, 18(53, pp. 36, 653. t Acadian Geology, pp. 94,98. |
