SUEVEY OF COLORADO AND NEW MEXICO. 39 



series of yellow and white sands and sandstones passing down into 

 brick-red sands. Among this series of variegated beds are two thin 

 beds of limestone. One of these is a very white rock, and on its weath- 

 ered snrface are small masses of chert, which appear to have the struc- 

 ture of corals. This bed is six or eight feet thick. Separated by eight 

 or ten feet of sandstones is another layer of bhiish limestone, which is 

 much nsed for lime. I have never been able to detect any well-defined 

 organic remains in these beds, but I believe a i^ortion of them, between 

 the lower cretaceons IsTo. 1 and the trne red beds, are of Jurassic age; 

 and it is even possible that a portion of the red beds are of that epoch. 



From the point where the Union Pacific railroad crosses the Laramie 

 Mountains to Colorado City, I have been unable to find any well-marked 

 carboniferous or Silurian rocks. The red sandstones, which I have been 

 accustomed to regard as triassic, jut up against the metamorphic rocks, 

 or are the only exposures that meet the eye of the geologist. I do not 

 believe that the carboniferous beds are altogether absent, for limestones 

 of considerable thickness, and containing characteristic fossils, occur at 

 Granite Caiion, on the Pacific railroad, high up on the margins of the 

 mountains ; and also at Colorado City, about two hundred miles to the 

 south. In this long interval I have been unable to discover any well- 

 defined carboniferous or silurian rocks, yet I am inclined to think that 

 the carboniferous beds, at least, exist underneath all the other sedimen- 

 tary rocks, but are not exposed by the upheaval. 



About five miles south of the Platte Caiion the upheaved ridges come 

 close up to the mountains, and are not worn away, but form the north- 

 ern side of the divide, so that the entire series of unchanged rocks known 

 in this region are exposed in regular continuity. A little further south 

 we come to a series of variegated beds of sands and arenaceous clays, 

 nearly horizontal, resting on the upturned edges of the older rocks. 

 These beds form the northern edge of an extensive tertiary basin of com- 

 l^aratively modern date, either late miocene or pliocene age. From the 

 point of their first appearance, about five miles south of the South Platte 

 Caiion to a i3oint about five miles north of Colorado City, these beds jut 

 up against the foot-hills of the mountains, inclining at a small angle, 

 never more than five to eight degrees, and entirely concealing all the 

 older sedimentary rocks. The upheaved ridge entirely disappears. Far 

 off to the eastward stretches this high tertiary divide, givin g rise to a large 

 number of streams, as Cherry Creek, Eunning Water, Kiowa, Bijou, and 

 other creeks. Through this basin also flows Monument Creek, which 

 has become so celebrated for its unique scenery. The beds of this forma- 

 tion are of various colors — reddish, yellow, and white — and of various 

 degrees of texture, from coarse pudding-stones to very fine-grained 

 sands or sandstones. There is very little lime in the entire series of 

 beds. There is much ferruginous matter in all the beds, to some of 

 which it gives a rusty brown color. The valley of Plum Creek is scooped 

 out of this basin. The high ridge to the eastward is capped with coarse 

 sandstones and pudding-stones. Along the immediate sides of the 

 mountains the rocks are mostly coarse pudding-stones, the water- worn 

 pebbles varying in size from a grain of quartz to a mass several inches 

 in diameter. But as we recede from the mountains, eastward, the sedi- 

 ments become finer and finer until the coarse pudding-stones disappear. 

 I am of the opinion that the materials composing tlie beds of this group 

 have been derived from the mountain ranges and vicinity. In their 

 general appearance the rocks of this group resemble the prevailing rocks 

 which cover the country from Fort Bridger to Weber Caiion, and also 

 a series of sands and sandstones along the Gallisteo Creek below Santa 



