A. C. Peak— Age of the Rocky Mountains in Colorado. 177 



tbeir deposition. They also show that they were derived from 

 the degradation of Archaean rocks.* West of the Eio Dolores, 

 in 1875 and 1876, I noted Upper Carboniferous rocks resting 

 on the Archaean, and in the lowest bed, which was a coarse con- 

 glomerate. I noted large angular fragments of granitic rock ex- 

 actly like that upon which the bed rested. Ascending, I noticed 

 the beds becoming finer, showing that the shore line had 

 advanced to the eastward. Farther east the red beds (Trias) 

 rest on the Archaean for at least 20 miles (the length of the 

 Unaweep Canon) and probably more, with no older sedimen- 

 taries between them. 



In New Mexico, Dr. Newberry found the Carboniferous rocks 



by when they were formed, f 



It is evident therefore that there was Archaean land above 

 the level of the Carboniferous sea, and that it was subsiding as 

 the age progressed. 



This subsidence continued through Mesozoic time. Along 

 the eastern trout of the Colorado or Front Eange, with one or 

 two exceptions where thev have been removed by erosion, the 

 Bed H.-ds (Triasi rest upon the uranite, (see Reports of 1873.) 

 At the base of the series, Mr. Marvine found conglomerates of 

 which he says, " It is in these lower parts, indeed, that the 

 beds are so" directly made up of the material of older rocks 

 near by that a verv I : •> in some instances 



i md it [ tii, ult to distinguish si - - ot saw - n li m 



the underlying granites. "J In my own district, south of Mr. 

 Marvine's, I found the lower bed of the Trias, a light colored 

 conglomerate, containing pieces of unchanged granite. Above 

 this conglomerate, the" sandstones became fine-grained and 

 uniformh i< 1 m . .. h sin u l,n h i I pro 



greased westward. inSoul P ere rest on 



tl" g i for, oil t east sid Dal i - q ie-tsomt, 



while on the west side the Paleozoic rocks are inter- 

 red as I have already stated. Continuing the section west- 

 ward to the Paleozoic strata be- 

 neath the Trias, bul : the Trias 

 and the und iIm V , n ,l M ut .wltkCVt ohmms 

 superimposed on the "Tanite. Following down the Gunnison 

 «•'- Hud the underlying rocks coming in gradually until the 

 Red Be,K sho« , <ti !j( , on th. -ranif West of the Elk 

 Mountains, between the Gunnison River and the Rio 

 Dolores, tha Trias rests on the granite, gradually thicken- 



