1888.] 503 [Crosby. 



of Canada, the Rocky Mountains, and probably the Sierra Nevada, 

 were then in existence ; and the areas of the intervening basins first 

 invaded by the Paleozoic sea must have been in a general way 

 those now covered by the lowest Paleozoic sediments, the Kewee- 

 naw or Grand Canon beds, following the classification of Walcott. 



At the close of the Keweenawan age, as Walcott has shown, and 

 during the deposition of the lower and middle Cambrian strata in 

 New England, New Brunswick and Newfoundland, the Kewee- 

 nawan beds in the Lake Superior and Grand Canon areas were el- 

 evated and extensively eroded, before the deposition upon them of 

 the upper Cambrian or Potsdam strata. The Potsdam was not only 

 spread over the eroded Keweenawan series, but also, as already 

 indicated, over extensive Archaean areas which had not been cov- 

 ered by the Keweenawan sea. The Potsdam thus indicates a more 

 general subsidence of the land than occurred during Keweenawan 

 time ; and the Paleozoic ocean may now be regarded as fairly es- 

 tablished over the Archaean continent. 



The enormous volumes of the Paleozoic sediments in the Alle- 

 ghany region and in Utah and Nevada, although diminishing in 

 thickness in both cases away from the shore line, conclusively prove 

 (1) that both the eastern and western margins of the Paleozoic sea 

 were areas of a progressive subsidence continuing, with unimpor- 

 tant interruptions, through the whole of Paleozoic time ; and (2) 

 that over these marginal areas deposition so nearly kept pace with 

 the subsidence that the comparatively small oscillations of the 

 crust attending the downward movement were sufficient to convert 

 the areas of deposition into areas of erosion, and explain the slight 

 unconformities of the Paleozoic formations already noticed. 



That the profound subsidence permitting the accumulation of 

 from six to eight miles of sediments along the ancient shore lines 

 was not limited to the marginal portions of the Paleozoic sea, is 

 proved by the thinness and the largely calcareous nature of the 

 Paleozoic formations in the central areas of the continent, and also, 

 it is believed, by the great hiatus without unconformity between 

 the Potsdam and Carboniferous. The Rocky Mountains were al- 

 most completely submerged before the close of the Potsdam period. 

 Although it may be true, as Emmons and others have suggested, 

 that the highest points of the range have been dry land continu- 

 ously since Archaean times, the nearly universal extension of the 

 great Carboniferous limestone, save where it has been removed by 



