60 SCIENCE PROGRESS. 



North America and in other areas on that continent ; but 

 they have been, for the most part, so modified by dynamic 

 and other metamorphosing agents, that superficially they 

 differ widely as a whole from the fresh Tertiary lavas so 

 magnificently displayed to the west of the Rocky Mountains. 

 Their significance has thus been overlooked both by 

 Wernerians, such as Sterry Hunt, and by the holders of 

 certain extreme " metamorphic " theories, such as Dana 

 and Logan, the one school regarding the schistose and 

 foliated crystalline rocks as something sui generis, the other 

 considering them as necessarily altered sediments. To this 

 it must be added that petrology in America has drawn its 

 inspiration largely from Germany, and not a few of the 

 younger workers have been trained partly in the laboratories 

 of Leipzig, Heidelberg, etc. 



Nevertheless a number of American geologists, Selwyn, 

 Wadsworth, R. D. Irving, G. H. Williams, Van Hise and 

 others, have clearly recognised these ancient volcanic rocks 

 as lavas and tuffs of various petrographical types, whose 

 differences from corresponding products of Tertiary and 

 Recent age result merely from the vicissitudes through 

 which they have passed in their long life-history ; and the 

 description of the abundant material from this point of view 

 has already been begun. The ancient volcanic rocks in 

 America about which we have at present most information 

 are situated in the country about Lake Superior and in what 

 we may call, in a broad sense, the Appalachian region, 

 occupying an enormous extent in the east of the United 

 States and Canada. 



In the Lake Superior region it has been recognised, 

 chiefly owing to the work of Wadsworth, Pumpelly, Irving, 

 Williams, and Van Hise, that volcanic rocks of various 

 types play an important part in the constitution of some 

 of the extensive pre-Cambrian formations there developed. 

 Some of these rocks have undergone more or less profound 

 alterations, and in the detailed study of them much still 

 remains to be done. 



As regards the eastern region, Diller gave in 1881 a 

 somewhat detailed account of the petrographical characters 



