GEOLOGICAL SURVEY OP THE TERRITORIES 161 



color, and are inclosed in a steel gray silicious paste ; but whether large or 

 small, all are angular. These might be called volcanic conglomerates, 

 for they are of igneous origin. They occur in the South Park and near 

 the sources of Lewis's Fork of the Columbia Eiver. 



CHAPTER XIII. 



GENERAL REVIEW OF THE GEOLOGY OF THE COUNTRY FROM OMAHA TO 



SALT LAKE VALLEY. 



In the preceding chapters I have endeavored to present a simple state- 

 ment of facts as I have been enabled to read them in nature. The 

 simplicity and unity of the Great Rocky Mountain system is such, that 

 when a sufficiently wide range of facts has been secured it will not be 

 difficult to derive from them some generalizations of permanent value. 

 In this chapter I desire to recapitulate somewhat briefly the principal 

 geological features of the country from the Missouri to Salt Lake Valley, 

 stopping here and there to discuss some obscure points. 



The upper coal measure limestones are seen at Omaha, near the water's 

 edge, and quarried all along the Platte nearly to the Elk Horn River. 

 The lower cretaceous rusty sandstones of No. 1 overlap the upper 

 carboniferous limestones about four miles above the mouth of the Platte, 

 and extend to the mouth of the Loup Fork ; but the yellow marl deposit, 

 or loess, conceals for the most part the underlying rocks. A fine yellow- 

 ish sand, of the same age, or a little less recent, overlaps the cretaceous 

 near Columbus. 



The chalky limestones of No. 3, with the characteristic Inoceramus 

 problematicus, here and there crop out, and some obscure exposures have 

 been detected in the Pawnee reservation, fifteen or twenty miles up the 

 Loup Fork. 



This fine y ello wish sand soon gives place to the pliocene beds of the Platte, 

 Loup Fork, and Niobrara Rivers, composed of indurated marls, sands, or 

 sandstones, which continue on as far as the margin of the Laramie range 

 of mountains, five hundred and thirty miles west of Omaha — that is, for 

 nearly four hundred and thirty miles along the line of the railroad. In 

 the grand anticlinal of the Laramie range, which I have already described, 

 they sometimes repose with a slight discordance on the older rocks; some- 

 times, as near the Laramie Peak, they rest directly on the granites, and 

 entirely conceal, for a distance of forty or fifty miles, all the unchanged 

 rocks of older date ; but a careful study of the eastern flank, from Red 

 Buttes to Long's Peak, will reveal all the „. u 



formations that are known to exist in this 

 part of the West, inclining from the sides 

 of the granitic nucleus at various angles. 



Figure 14 will illustrate the surface fea- j 

 tures of the Monument Creek Group. The ! 

 rocks are composed mostly of decomposed ! 

 granites, a feldspathic paste holding some 

 pebbles of quartz or feldspar. The columns 

 that are left standing over a large area, 

 are capped with a hard layer of rusty yellow 

 sandstone, as shown in the cut. This group 

 covers the divide between the South Platte 

 and the Arkansas Rivers, and is supposed i 



to be of Upper miocene Or pliocene age. Monument Creek, Colorado. 



11 G-. 



