358 HATCHER MARINE AND NON-MARINE FORMATIONS. [A pril 7 



gradual, as is also that from the latter to the overlying marine 

 deposits known as the Pierre or Bearpaw shales. In the first 

 instance there is abundant evidence of the gradual replacement of 

 marine conditions by non-marine, commencing below with littoral 

 and brackish water deposits and terminating above with beds 

 indicative of typical fresh water and terrestrial conditions, while 

 in the second instance the evidence is equally conclusive of a grad- 

 ual replacement of terrestrial by true marine conditions. As with 

 the Eagle formation the Judith river beds continue for some 

 distance to the south and east, but decrease in importance and are 

 finally entirely replaced by marine deposits now universally referred 

 to the Pierre or Montana group of the Upper Cretaceous. It is 

 evident therefore that the Judith river beds in their typical devel- 

 opment represent deposits that were contemporaneous in origin 

 with these marine beds farther to the southeast and that they should 

 be correlated with them. 



Overlying the Judith river beds but partly contemporaneous with 

 them in origin is the series of shales already mentioned as the 

 Pierre or Bearpaw shales. These are true marine deposits and pass 

 upward through the Fox Hills sandstones into that great series of 

 brackish, fresh water and aeolian deposits aggregating from 2,000 

 to 3,000 feet in thickness and known as the Laramie forma- 

 tion. The passage from the Pierre to the Laramie is everywhere 

 very gradual, and the evidence is so strong as to be almost conclu- 

 sive that there was in this region a gradual advance of the land 

 upon the sea resulting finally in the recovery of the entire area 

 from the latter. Considering the enormous extent of the area 

 which marks the present distribution of the Laramie, a very great 

 length of time must have elapsed between that period when the first 

 portion of this region began to appear above the level of the sea 

 and that later period which marked the final recovery of the entire 

 region. The length of this period is also indicated to some extent 

 at least by the difference in the character of the brackish water 

 faunas that in various and widely separated localities are found in 

 beds immediately overlying the Pierre-Fox Hills^ and which would 

 have been contemporaneous had brackish water conditions been 



iThis sentence was written with the idea that the Bear River fauna of Western 

 Wyoming really belongs to the Laramie, as was once generally believed. It is 

 now known to belong to an entirely different horizon, and this statement was 

 agreed to by Mr. Hatcher at the time of our discussion. — T. W, S. 



