exduch.] SWEETWATER DISTRICT CONCLUSION". 153 



itself to me while prosecuting the study of this district, and I believe 

 that eventually it will be found to be correct. 



Concerning the geological period during which the main range was 

 elevated, we have obtained some data, which, to me, appear conclusive. 

 The general easterly and northeasterly dip of the older sedimentary 

 formations at the base of the mountains is established. We have traced 

 the series, one conformable upon the other, from the Silurian to the Ter- 

 tiary, and have recognized the representative of the Eocene, the Wasatch 

 Group. Uncomformably overlying this last-named member we observe 

 the Sweetwater Group. Whatever may be the position in systematic 

 classification to which this is assigned, it is certainly considerably 

 younger than the Wasatch. This, therefore, is an instance, where an 

 Eocene Group, disturbed from its original, horizontal position by the ele- 

 vation of the main range and its accompanying upheavals, fails to remain 

 conformable with a younger Miocene Group. Consequently, we must re- 

 gard the period of elevation as falling between the Eocene and the termi- 

 nation of the Miocene. The exact chronological position it occupies is not 

 apparent, but so much can be said with certainty, that the Wind Eiver 

 Eange was elevated after the deposition of the Wasatch Series, and 

 reached high altitudes before the deposition of the Sweetwater Group. 

 A slight dip of the beds composing the latter, as well as of the Eliocene 

 beds covering them, could be observed throughout their exposure. This 

 indicates a general minor elevation, subsequent to the Miocene period, 

 along a curving line which essentially forms a continuation of the longi- 

 tudinal axis of the range. The vertical extent of this gradual rise was 

 comparatively inconsiderable, as the inclination of strata reaches a max- 

 imum of only 5°. 



From the evidence which could be obtained, I regard the formation of 

 the anticlinal folds east of the range as subsequent to the elevation of 

 the mountains. After they had come to a point of stability, the gentle 

 rise above mentioned took place. These observations confirm the views 

 held by Dr. A. C. Peale, presented in the American Journal of Science 

 and Arts, April, 1877. 



It will be remembered that the anticlinal uplift near Salt Wells (D in 

 Section XVII) trends west of south. At nearly right angles to this, 

 south of east, we observe the strike of the leucitic eruptions. Nearly 

 all of the lava has been poured out through fissures. We may readily, 

 therefore, assume a causal connection between the two phenomena. 



Sedimentary formations are amply represented in our district and are 

 studied with' great ease. Typical in every respect, they offer very little 

 difficulty as to identification. Although they take an active part in all 

 the dynamical disturbances of the region, their original position, as well 

 as present connection, can be traced without much trouble. Some hesi- 

 tancy is experienced in classifying the various groups of the Tertiary 

 formation, as but a portion of the entire series occurs. In the preced- 

 ing report this has been done in such a manner as is warranted by the 

 observations made in the field. Palseontological remains are compari- 

 tively rare, but enough were found to enable subdivision of formations 

 into their respective groups. 



An attempt has been made to classify the Archaean rocks with some 

 degree of accuracy. The region is favorable to this purpose, and the 

 relations of the groups, among themselves, are definite. In the appended 

 table they have been placed on a parallel with eastern systems, but 

 it is not intended that an absolute identity should thereby be expressed. 

 Eelatively, they occupy the same positions west as east, but are not pre- 

 cisely either the same in composition or arrangement of beds. 



