118 KEPOET UNITED STATES GEOLOGICAL SUEVEY. 



first seen, the steady southerly dip of the sedimentary formations of the 

 Sweetwater and Seminole Hills ai^pears to be at total variance with their 

 anticMual structure. On account of the superincmnbency of Tertiary 

 strata the granitic outcrops are most frequently obscured for some dis- 

 tance, and the strata seem totally out of place. By tracing the con- 

 nection with the two antichnal folds, however, the necessity of such an 

 arrangement as exhibited becomes clear. Although I do not doubt that 

 a large portion of the sedimentary beds have been removed from the 

 region now occupied by the Sweetwater Group, I consider it highly i^roba- 

 ble that considerable masses still exist at some depth, which, if exposed, 

 would furnish us with the northern slopes of the anticlinal folds. An 

 interesting case is presented in Section XII, where the sedimentary 

 strata have twice been doubled by lateral x)ressure. In this instance the 

 granite, which had previously reached the surface by virtue of the west- 

 erly antichnal upheaval, formed an immovable barrier. Between this 

 and the portions elevated by the second fold lay a series of beds which 

 were thus plicated by the force which proijelled uj) ward the metamorphic 

 rock first rising into view. 



In the presence of the Tertiary groups of the Sweetwater region we 

 have before us a representative of one of the most interesting features of 

 the western country. The comparatively large number of basins which 

 have existed during previous periods, and which do exist at the present 

 time, is a characteristic which is productive of the most suri)rising va- 

 riation. Local accumulations of this kind sadly interfere with the ac- 

 cepted standards of geological succession of groui)s. Palseontological 

 evidence is generally the only resource, in such case, whereby their rela- 

 tive positious can be determined. In the succeeding chapter, I shall 

 have occasion to speak of a number of Tertiary grouj)s, ancl shall there 

 discuss, in connection with others, the relations of the Sweetwater series. 

 Interesting, as well as instructive, when viewed together with other 

 facts, is the nature and distribution of the Wyoming conglomerate. We 

 know that a number of the Tertiary groups of the west contain varying 

 masses of such material. This is the youngest one within the forma- 

 tion, one spread widely over the country. 



Evidence exists in this district that some of the glaciers starting from 

 the Wind Eiver Eange extended for very long distances, but as the 

 main portion of the jjroof is found beyond the Sweetwater drainage, this 

 shall be treated of in the succeeding chapter. In the Sweetwater Hills 

 we find the remains of " extinct" glaciers, which occupied a more or less 

 restricted area. I doubt not but that the surrounding conditions were 

 particularly favorable to their formation at this point, and that they re- 

 ceded, and finally disapi)eared, in a ratio proportionate to the destruction 

 of those causes that first led to their origin. So far as I have seen the 

 western country, I am persuaded that the glaciation of certain regions 

 was due more directly to local meteorological causes than to the existence 

 of a general glacial epoch. In consequence, I do not beUeve that the 

 glaciers of the Sweetwater Hills of Wyoming and those of the San Juan 

 mountains in Colorado were formed at the same time, or disappeared 

 synchronously. The general depression of temperature during a certain 

 epoch cannot be denied, but it does not necessarily include the factor of 

 very abundant precipitation, which is the first condition essential to the 

 formation and perj)etuation of glaciers. 



Eeferences have been given in the itinerary and in Chapter I to the 

 eUmatal conditions and agricultural pursuits of this region. It has often 

 been spoken of with especial reference to these subjects, and the re- 

 marks thereuj)on may prove to aftbrd some additional information. 



