110 GEOLOGICAL SUEVEY OF THE TERRITORIES. 



deposit that a very large i)ortion of the West is indebted for its misur- 

 passed fertihty and iirodiictiveness. It covers the country with such 

 uniformity that it conceals almost entirely the basis rocks from view. 

 Underlying this marl is a considerable deposit of drift material, as 

 rounded pebbles or boulders and coarse sand, often presenting the most 

 singular illustrations of oblique layers of deposit. The marl is usually 

 quite homogeneous in its composition, and almost or entirely destitute 

 of stratification, and the materials seemed to have been deposited in 

 very quiet waters, and to have settled to the bottom of a fresh-water 

 lake like gently-falling snow. The drift materials, as a rule, exhibit the 

 irregular laminae as if they had been deposited by currents of water. 

 The exceedingly great importance of this yellow marl deposit is not yet 

 well understood or appreciated, but it seems to me that the wonderful 

 fertility of the soil of the western States and Territories, and its perma- 

 nent productiveness for all time to come, is due to it. 



The eastern portion of ISTebraska is already quite thickly settled, and 

 is susceptible of cultivation, but the western part must be inhabited, if 

 settled at ail, by a pastoral people. 



These broad, level prairies are covered with a thick growth of short, 

 nutritious grass, but the scarcity of water for the purpose of irrigation, 

 and the almost entire absence of forest trees, must ever prevent settle- 

 ments to any great extent. In the autumn nearly all the smaller streams 

 dry up entirely, and several seasons the Platte has been known to be- 

 come so low as to have no continuous current. It is a peculiar feature 

 of these western streams, at times to be larger toward their sources than 

 at their mouths. The Platte in its various branches always has an 

 abundant supply of water, as their heads issue from the mountain sides, 

 but in traversing the plains there are few or no springs or branches en- 

 tering into it, or the water is entirely absorbed by the arid earth or thirsty 

 air, until the bed becomes as dry as the dusty road. Hence all over the 

 Eocky Mountain regions in the autumn are what are called dry creeks, 

 with beds which, when full in the spring time, form large rivers. 



The Platte Eiver flows, for a distance of over four hundred miles, 

 through the southern portion of what I have termed the White Eiver 

 tertiary basin, in contradistinction to the great lignite tertiary basin. 

 The former has been separated into two formations, the White Elver 

 group and the Loup Eiver beds, on account of the organic remains char- 

 acterizing each. The two former are entirely distinct, not a species pass- 

 ing from one to the other. I have supposed hitherto that the Platte 

 Eiver flowed through strata belonging to the Loup Eiver group. They 

 are certainly of quite recent age, but the i^liocene remains that I col- 

 lected on the Mobrara Eiver came from loose gray sands which rested 

 with a certain kind of unconformability on the eroded surface of the 

 White Eiver group. It is x^lain also that the valleys of the more import- 

 ant streams have been worn out, to some extent, prior to the deposition 

 of the pliocene sands. 



In the valley of the Mobrara and Loup Fork the pliocene sands are 

 quite thick, and the line of separation between them and the White Eiver 

 group is very irregidar, w^hile on the hills the sands occur in many places, 

 on and in, isolated hills. 



The details of the geology of this most interesting region still remain 

 to be worked out, and its geographical extent will be found to be much 

 larger than has hitherto been supi:)osed. The soil composed of the ero- 

 ded materials of this basin is of moderate fertility, but owing to a want 

 of water cannot be cultivated to any great extent. The greater portion 

 of the surface underlaid by these beds is covered with a fine growth of 



