26 GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 



low, or so liard, and the strata so covered with grass or debris, that they 

 elude the scrutiny of the geologist. By exploring with much care large 

 numbers of these natural cuts, a very true conception of the geological 

 structure of a mountain range may be obtained. It is usually quite 

 difficult to measure the thickness of the beds ; indeed, it is impossible ; 

 and we must therefore rely upon a judicious estimate, aided by good 

 barometrical observations. Neither are exact instrumental measure- 

 ments of strata of great importance in this country. Take, for example, 

 the limestones of the Carboniferous age ; they vary in thickness in differ- 

 ent localities, all the way from a few hundred feet to as many thousands, 

 and yet they being sea-formed rocks, are supposed to tend toward uni- 

 formity of thickness. In this narrow valley we find that the Pliocene 

 beds which form the foot-hills of all the ranges of mountains surround- 

 ing the great Snake-Eiver Basin are also under the great basalt floor. 

 These beds sometimes are found 400 or 500 feet above the level 

 of the plains, and so conceal the underlyiug rocks, upon which they re- 

 pose unconformably, that it is difficult to unravel their connections. 

 Then there is a thickness of several hundred feet of grayish-brown lime- 

 stones, more or less arenaceous, with intercalated layers of clay or lime- 

 stone, and full of Jurassic fossils. Underneath is a group of sand- 

 stones, varying in color from a dark to a light brick-red, resembling the 

 sandstones so well shown in Weber Caiion, and probably of the same 

 age, but entirely destitute of organic remains. This group is 300 to 500 

 feet thick, and inclines 15° to 25° ; underlying the red sandstones are 

 limestones, which are undoubtedly Carboniferous, and beneath them 

 quartzites, sandstones, pudding-stones, conglomerates, of unknown 

 age. The group thus enumerated forms the mass or bulk of the regu.- 

 larly stratified rocks, composing the ranges around this great basin. 



Before closing this chapter, I may enumerate some of the elevations 

 along our route, for the purpose of showing the relative heights of the 

 iiills surrounding the plains and valleys, as well as to indicate one of the 

 important conditions for successful agriculture. There seems to be no 

 want of fertility in the soil of our western plains, and when the two 

 most imi)ortant conditions are favorable, climate and moisture, or water 

 for the ]mrposes of irrigation, then agriculture will be a success. How- 

 ever abundant the water may be, either in the form of rain or in streams 

 for irrigation, if the elevation is 7,000 feet and upward, the climate is 

 liable to be too severe and uncertain for settlement. From barometrical 

 observations along the route of travel we found that the elevation of our 

 camp on Boss's Fork was 4,394 feet above the sea ; on the divide toward 

 Fort Hall, 5,072 feet ; Fort Hall, 4,724. These figures will serve to indi- 

 cate the general elevation of the plains and the immediate foot-hills, and 

 they show that the climate need not be more unfavorable for agriculture 

 than that of Salt Lake Yalley, in which the Mormons have been so suc- 

 cessful. How far the excessive dryness of the atmosphere may be an 

 obstacle it is hardly possible to decide. The past season was an 

 unusually dry one. The difference between the wet and dry bulb in 

 June on the Snake Eiver plains was 35°, which indicates an unusual 

 freedom from moisture in the air. The broad bottoms in the immediate 

 vicinity of Snake Eiver are at a somewhat lower level, and can be made 

 very productive; large quantities of hay are prepared every season. 

 Inasmuch as an Indian reservation has been made on Eoss Fork, we 

 may have some experiments in agriculture on these plains in a short 

 time. 



