PRE-TEKTIABY VOLCANIC EOCKS. 93 



contemporaneous rocks of the Carboniferous in England and Scotland 

 basalt, wliile others who desire to be non-committal call them traps, which 

 may mean either diabase, basalt, or dolerite, or even augite-andesite. Pro- 

 fessor 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 cer- 

 tain 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 Will- 

 iam 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. Daw- 

 son speaks of basalts f 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 Pro- 

 fessor Powell has given that name, and which is sixpposed to be of Tri- 

 assic 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 texture — 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, 1S67. 

 t Geology of Canada, 18(53, pp. 36, 653. 

 t Acadian Geology, pp. 94,98. 



