626 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



of which was then in progress, certain oscillatory movements, one of 

 which produced some unconformity of the Wahsatch upon the Laramie 

 group. At a distance from the mountains, this unconformity is very 

 slight, or perhaps wanting ; and it is not improbable that we may yet 

 discover localities where the strata of the Laramie group pass so insen- 

 sibly into those of the base of the Wahsatch group that the latter strata 

 must necessarily be included in the first-named group. The final change 

 from brackish to fresh waters took place during what is now understood 

 to be the early part of the Wahsatch epoch, and unaccompanied by any 

 disturbance of the already deposited strata, at least in the region where 

 these groups have been studied, and also with no perceptible change in 

 the character of the sedimentary material that continued to be deposited 

 in the freshened waters. The exact nature of the change in the physical 

 conditions of the region which caused the final banishment of salt from 

 the waters in which it had lingered so long after the cessation of true 

 marine conditions, without materially changing the character of the sedi- 

 mentation over so large an area, has not been investigated; but the 

 final freshening of the waters was doubtless accomplished through the 

 complete inclosure, by elevated land, of the large aqueous areas, save 

 some free outlets for the drainage of overflow. 



With the final freshening of the waters in the Wahsatch epoch ceased all 

 considerable saline-water deposits, at least in the region embracing 

 Southern Wyoming and the adjacent parts of Colorado and Utah. The 

 affinities of all the fossils found in those subsequently deposited strata 

 of the Wahsatch, Green Kiver, and Bridger groups, aggregating a mile in 

 thickness, are with such forms as are now found living only in fresh 

 waters. 



It is true that Professors Cope and Leidy have described two or three 

 species of Herring from the shales of the Green Eiver group; and Pro- 

 fessor Cope speaks of those strata, in connection with the Fish remains 

 as a brackish-water deposit. But it is not thought necessary to infer 

 the presence of salt in those waters in any proportion from the presence 

 of the Clupeid remains, because there appears to be no reason to suppose 

 that they were not land-locked species, permanently inhabiting the 

 Green Kiver Tertiary lake. Even if they were migratory species, they 

 doubtless went to that lake to spawn, as some species of Herrings now go 

 from marine to fresh waters for the same purpose. The known limits 

 of the deposits of that ancient lake, as well as those of the marine de- 

 posits of similar age, show that the shortest route from marine waters 

 to the lake by any outlet it may have had was too long to allow Fishes 

 so small as they were to reach the lake during the time the reproductive 

 impulse was upon them. These fossil Clupeids were therefore not proba- 

 bly migratory, but they doubtless permanently inhabited the fresh 

 waters of the Green Eiver lake. 



An examination of the fossils obtained from the Dakota, Colorado, 

 and Fox Hills groups, as they are developed in Southern Wyoming and 



