A. C. Peak— Age of the Rocky Mountains in Colorado. 175 



present not only in the interior but also west of the mountains, 

 and that where the Paleozoic rocks alone show, the overlying 

 rocks have been removed bv erosion. 



2d. The want of conformability between the Paleozoic Rocks and 

 t!,>: Mesozoic Rocks. I have already noted the conformability of 

 the Paleozoic and Mesozoic in the Park Range, and on the 

 north and west sides of the Sawatch Eange. Prof. Stevenson 

 (pp. 386, 497, 499, 500) notes unconformability between Meso- 

 zo'ic and Paleozoic along the Eastern Eange, (Front or Colorado 

 Range,) on the interior axes of elevation, and in the southern 

 p. it nt his aiea. m the San Juan Mountains, and near Tierra 

 Arnarilla, New Mexico. Speaking of the Front Range, he 

 says: "Along the Eastern Range no Carboniferous rocks were 

 exposed at any locality visited by me, but the}- have been seen 

 . elsewhere by others. They must' be quite unconformable to the 

 Trias, as it overlaps them very greatly." Along the Front 

 Range the Carboniferous shows only where the overlying 

 Triassic has been eroded away, and whatever unconformabihty 

 there is, is an unconformabilitv of subsidence and not of up- 

 heaval. The difference of dip between the Paleozoic strata 

 and overlying Mesozoic is caused bv an abrupt fold and the 

 erosion of portions of the strata. Prof. Stevenson does not 

 seem to remember that as we recede from the axis of elevation 

 the dip of the rocks diminishes, and that if the more steeply 

 inclined portion of the Mesozoic rocks be removed, as we 

 recede from the mountain- we meet onlv with the edges of the 

 strata where thev are I d, and that the older 



layers below also flatten < on in the same manner. I have already 

 shown the conformability in the interior. 



In the reports of Dr. Newberrv* and Prof. Cope.f I can hud 

 no evidences of the tin, f..rm v in the southern portion 



of the San Juan \i ;iln - „, 



Dr. Endlich says! that in hi? district, "although the Creta- 

 ceous beds dip off, an m the same directlon 

 (with Car/,,, ■ ties were noti* 

 instances." These he thinks were due to 

 that portion which was then land. I am inclined to 

 by the subsidence which, as I hope to show farther on, occurred 

 over so large a portion of what is now Colorado. 



The next statement of Prof. Stevens-.n - 

 B, that '• The second epoch of elevation began toward the close 

 of the Triassic." (p. 500.) , „. , 



This he bases on the unconformability between the Trias and 

 the Cretaceous, which he observed along the Eastern Range at 



* In Maeomb'a Report on the Exploring Expedition to the junction of Grand 



t In Report of Chief of Engineers, Part II, 1875. 

 t Report U. S. Geol. Survey, 1874, p. 215. 



