exmjch.1 SOUTHERN AREA POST-TEETIARY EROSIONS. 135 



Aery intimately associated with the basalts of the immediate surround- 

 ings. An analysis of a specimen from this locality is appended, which 

 shows a remarkable similarity of composition with some of the European 

 leueites.* 



Silica - 54.42 



Alumina 13. 37 



Ferric oxide 0. 61 



Ferrous oxide 3. 52 



Lime 4. 38 



Magnesia 6. 37 



Soda - - 1. 60 



Potassa - 10.73 



LitMa trace. 



Carbonic acid 1. 82 



Water 2. 76 



99.58 

 POST-TEETIAEY EEOSION. 



The extent and distribution of post-Tertiary erosion indicates, in a 

 measure, the condition of the surface after the close of Tertiary sedimenta- 

 tion. We find that south of the Wind Biver Mountains, enormous masses 

 of beds have been removed in a southwesterly direction. This is in con- 

 formity with the general dip of the strata,, as here exhibited. Deep, 

 wide valleys have been cut into the readily yielding material, and, indeed, 

 the majority of the higher beds have been entirely removed. South of the 

 Sweetwater Hills the drainage has carried away less material than farther 

 west, owing, no doubt, to the comparatively short course of the ancient 

 streams. While those flowing toward the Sweetwater found an opening 

 to permit their passage for an unlimited distance, the southward drain- 

 age was arrested by the existence of a shallow basin. Pouring into this 

 the water was retained in a series of lakes. From the western as well 

 as eastern rim of this depression the streams flowed into the Pacific and 

 Atlantic Oceans respectively. They cut away large portions of the strata 

 leaving isolated remnants of younger groups superimposed upon the 

 older ones. Within the Shoshone Basin the formerly existing streams 

 have formed narrow valleys, bordered by steep bluffs, and showing ex- 

 ceedingly gentle slopes. ]So doubt, a considerable number of square 

 miles were there at one time covered by bodies of standing water. The 

 " flats" we find to-day are ample evidence thereof. East of the basin the 

 drainage was essentially a north to south one, following the strike of the 

 strata and receiving tributaries from the east along the slope of the 

 anticlinal uplift. Although a very large quantity of material has here 

 1 >een carried away, the process of denudation was not so extensive as 

 farther west. To the eastward of the anticlinal ridge mainly two great 

 valleys, trending north and south, were produced by early erosion. One 

 of them, certainly, and probably both contained lakes of large dimensions. 



We observe from this that the younger Tertiary Groups probably 

 occupied a much greater area to the east than they do now. Westward 

 the younger beds showed a more decided development and rose to higher 

 absolute elevation. 



OrLACIATION. 



In the vicinity of Table Hills and Steamboat Buttes we find deposits 

 of erratic material that appears to have been transported by ice. Huge 

 blocks of granite and schists occupy the summits and higher ridges, 

 distributed in an irregular arrangement. Some of them have rolled 



*U. S. Geol. Surv. Fortieth Par., vol. ii, p. 237. 



