114 DESCBIPTIVE GEOLOGY. 



Paleozoic and Mesozoic Foemations.— Witliin the area of the North 

 Park may be found strata representing all the sedimentary divisions, which 

 occur east of the Colorado Eange, from the Carboniferous limestone to the 

 top of the Colorado Cretaceous, but which, unlike the beds bordering the 

 Plains, are here associated with volcanic outbursts of trachytes, rhyolites, 

 and basalts. Still later Tertiary beds, essentially horizontal, are found cov- 

 ering the greater part of the basin. 



The upturned sedimentary ridges are all, from base to summit, con- 

 formable, and in their main features, as their mode of occurrence, their rela- 

 tion to the Archaean foundation, and topographical structure, closely resemble 

 the same beds east of the mountains. They rest unconformably upon the 

 Archaean, lying inclined at all angles from 50° to 10°, but usually occur 

 from 15° to 30° ; flattening out with less and less angle, away from the high 

 ranges, until concealed beneath the Tertiary strata. At different localities, 

 along the Archaean rim of the Park, beds of the Carboniferous, Triassic, 

 Dakota sandstone, and Colorado Cretaceous may all be observed resting 

 directly upon the older crystalline rocks, the underlying sedimentary beds 

 failing to reach the surface, but the upper members of the series to the top 

 of the Colorado group being usually well developed. 



Carboniferous limestones were recognized at only one locality, and 

 that upon the northeast side, where they lie up against the Medicine Bow 

 Range, inclined at a low angle. They occupy but a comparatively small 

 area, and derive their chief interest from being the single instance ob- 

 served of Palaeozoic strata within the Parks. No fossils were found ; but 

 there can be no doubt that the beds belong to the Carboniferous series, 

 inasmuch as thoy underlie a very considerable development of Red Beds, 

 and measure, at least, two or three hundred feet in thickness, which is far 

 too heavy to represent one of the limestone strata included within the 

 Triassic. Moreover, in their bedding and texture they resemble the upper 

 members of the Coal-Measure limestone wherever seen below the red sand- 

 stones. 



The Red Beds, although showing considerable variety in composition 

 and color, possess prevailing deep reddish tints, and are made up of fine 

 sands, with red clays and shales far less prominent than along the Colorado 



