90 NEW YORK STATE MUSEUM 



according to the comprehensive summary of Suess (v. 3, pt 2, 

 p. 441 ff.) a large block has broken down and remained so free from 

 later folding that the Cambrian is still resting horizontally upon the 

 eroded surface of the Precambrian. This condition is especially 

 well shown in the Gallatin range, Buffalo plateau and the Snowy 

 range (Bear Tooth in the east), the Bridge range and the Great 

 and Small Belt ranges. It is, as Suess expresses it, an entirely 

 foreign block in the Rocky mountains. It was in this region that 

 the late Precambrian deposition of the Belt terrane, reached its 

 greatest thickness, and the rocks though separated from the follow- 

 ing Cambrian by a distinct unconformity are but little disturbed 

 and metamorphosed. The earlier Proterozoan rocks, however, 

 and especially the Archeozoic rocks, are here as everywhere in the 

 world, intensely folded. They are described as vertical by Arnold 

 Hague and his associates (see Hague, 1899, p. 4), and it is stated 

 that " the pronounced lamination or schistosity of the whole body 

 of rocks is quite uniform in its position, the layers standing at high 

 angles or nearly vertical, with a general north-south trend." 



Farther south in the Laramie-Sherman folio (Wyoming, see 

 Darton et al., 19 10), the dominating structure of the Archean 

 according to Blackwelder is the schistosity, and this also shows a 

 general trend east by north, averaging N 70 E; and the boundary 

 lines of the Archean rocks also strike north by east. 



Summing up the somewhat confusing evidence from the Pre- 

 cambrian rocks of the Rocky mountains, we may state as a fairly 

 true conclusion that the original strike of the Precambrian rocks 

 seems to have been in north-south direction, with an original tendency 

 to turn northeast. Upon this original direction has in many 

 localities been imposed the later northwest direction of the Rocky 

 mountain folding, which has in places turned the folding and folia- 

 tion even east- west. 



If we compare this original north-south direction of folding of 

 the Rocky mountain Precambrian with that observed in the north- 

 western part of the Canadian shield, which is also north-south, 

 we can hardly avoid the conclusion that the Rocky mountain Pre- 

 cambrian folding thus found to be directed exactly parallel to the 

 western Laurentian folding, is a western continuation of the same 

 system of Precambrian folding. 



We have seen before that the Colorado plateau acted as a block 

 against which the Rocky mountain folds coming like waves from 

 the north were reflected and bent. The Colorado river has eroded 

 through this plateau and exposed one of the most wonderful sections 



