GEOLOGICAL REPORT. 287 



ded with cedar and pine. Still higher up, above the region of the grasses, forests of 

 aspen and pine extend to the loftiest summits, to the region of nearly perpetual snow, 

 greatly enhancing the beauty of the landscape. 



In this district we have not found any igneous rocks, although the violent local 

 upheavals indicate their close proximity at various points, and they are prominently 

 developed at the eastern and western borders of the section, nor have we observed 

 any metamonilii'- ami paleozoic strata. 



TERTIARY FORMATIONS. 



We have observed several formations which we refer to the Tertiary period. 

 Most prominently developed is 



THE FORT BRIDGER SERIES, 



to which we give that name because Fort Bridger is in the center of the region where 

 it is most characteristically developed and best exposed. This series extends from the 

 South Pass to the divide between Bear and Weber Rivers, thus occupying the greatest 

 portion of this section. Although it consists of several subdivisions, well distin- 

 guished by the lithological character of the strata, these are all conformable to each 

 other, and unconformable to the older formations. They are the most recent for- 

 mations in this section, and we have not found them anywhere disturbed locally by 

 upheavals, but wherever they have been noticed, they exhibit a nearly horizontal posi- 

 tion, or rather a slight dip off the surrounding mountains toward the center of the basin. 

 They might, therefore, be supposed to have been deposited after the country had 

 attained its present configuration, but other observations show that this cannot be the 

 case. While they occupy the divides in the eastern ranges of the Wahsatch Mountains 

 and in the South Pass, seams of carbonaceous matter and numerous impressions of plants, 

 Ferns, Equisctu»i,&Q.,T>Yhu'h can only have grown on swampy landor in very shallow water, 

 were found many hundred feet lower down in their continuation. Along the valley of 

 Bear River an actual break or fault may be observed. It is evident, therefore, that during 

 and after their formation they have undergone dislocations, not however connected with 

 local outbursts of eruptive masses, and, undoubtedly, coinciding with the great conti- 

 nental upheaval at the close of the Tertiary period. This position of the strata proves 

 that the central and western portion of the continent has not only been raised as a 

 whole solid body, but that the mountain chains, which must have existed as such long 

 before that epoch, have, at the same time, been elevated more than the intervening 

 country. I compare it with the forming of a bubble. The subterranean forces gradu- 

 ally swelled the central part of the continent several thousand feet; the thinner portion 

 of the surface, corresponding to the lowest points far away from the mountains, seems 

 to have yielded most, and to have been raised high as the pressure began. Then those 

 deposits must have been formed. When the pressure again subsided, finding, perhaps, 

 vent in outbursts of igneous masses, and the elevation of mountain ranges at distant 

 points, the bubble collapsed ; the mountains, with their granitic centre and base, form- 

 ing immense solid bodies, retained the position which they had assumed, while the 



