632 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



Corhicula ( Yeloritina) durJcei. 



Ifembranipora f 



Bhytophorus meeldi. 



Goniobasis cJirysallis. Twin Creek, about four miles above 



Goniobasis chrysaloidea. > its mouth, east of Bear EiverVal- 



Volsella {Brachydontes) sp. ? ( ley. 



Pyrgulifera Immerosa. 



Ostrea sp. ? • 



Neritina sp. % 



Corbioula Yeloritina durTcei. 1 



Corbula pyriformis. 

 Goniobasis chrysaloidea. 



Station 41, on Thomas Fork of Bear 



Goniobasis cleburni. > td • ' ^i, ^ o T,n ^^ -d 



Fyrgulifera Immerosa. f ^^^^^' ^^^^^ «^ ^^^1^**® ^^^S^' 



Unio vetustus. 

 Ostrea sp. ? 



Unio vetustus. » l^orth side of Smith's Fork of Bear 



) Eiver, 3 miles above the mouth. 



MESOZOIC AND POST-CEETACEOUS GEOGEAPHY. 



We have seen that with the beginning of the Trias there is a change 

 in the characters of the rocks. Instead of the massive limestones of the 

 Carboniferous, we have a series of interlaminated sandstones and then 

 limestones. Th^ beds are largely detrital, and their structure points to 

 their derivation from a land area not far distant. They indicate also 

 the probability of a progressive subsidence, interrupted by oscillations. 

 The continental area uplifted, according to King, at the end of the Car- 

 boniferous, is probably the source of the detrital material entering into 

 the comxjosition of the Triassic formation, as its eastern shore-line was 

 west of the j)resent Wahsatch Eange. That the Post-Carboniferous up- 

 lift did not affect the area embraced in our district is proved by the per- 

 fect conformity between the Carboniferous and the Trias everywhere 

 observed in our district. The only effect was indirect in the changing 

 of the character of the sediments. 



There is also no evidence of disturbance at the close of the Jurassic. 

 The latter is a limestone-making period at the beginning everywhere in 

 our district, and the change from the Trias is marked as though the pe- 

 riod had been ushered in by a considerable subsidence. Toward the 

 close the waters appear to have been shaEower, the sediment becoming 

 more and more arenaceous until in the Cretaceous we have shales and 

 sandstones almost exclusively. 



As already noted, the conformabiUty extends to the top of the Laramie 

 grouj), as exposed in our district. After the deposition of the latter the 

 region was subjected to the most intense orograi3hical disturbance. 

 The region from the Green Eiver Basin westward was upheaved. The 

 Wyoming Eange, the Salt Eiver Eange aud the i-solated mountains in 

 the western part of the district, all owe their origin to this folding, which 

 was accompanied by the faulting of the. strata that is seen along these two 

 ranges. The folding of Meridian Eidge was also synchronous. The Wah- 

 satch Lake was outlined, its eastern shore being the uplifted Wind Eiver 

 JMountains which had probably existed as an island in Palaeozoic and Me- 

 sozoic time, and its western shore the Wyoming Eange and Absaroka 

 Eidge. The Meridian Eidge and its northern extension probably at first 



