436 JO. A. White — JR elation of the Laramie Group 



uppermost one received upon it the great brackish- and-fresh- 

 water Laramie formation. Therefore the first announcement 

 of Dr. Dawson's discovery was received with not a little sur- 

 prise by the geologists who had studied the formations re- 

 ferred to in more southern regions. 



I have never visited the Saskatchewan region and cannot 

 therefore speak of the formations there from personal observa- 

 tion. But after carefully reading the accounts which have been 

 published by the Canadian geologists, and having had grati- 

 fying personal interviews upon the subject with both Dr. Daw- 

 son and Mr. Whiteaves, I can now see no good reason to doubt 

 the correctness of their observations. Accepting their conclu- 

 sions, it appears that in the region referred to, the deposition 

 of marine Cretaceous strata was interrupted at the close of the 

 Niobrara epoch by such a change in physical conditions as 

 caused the introduction upon the area which had been occu- 

 pied by marine waters of a bracki.sh- and fresh-water forma- 

 tion similar to the Laramie. It also appears that upon the 

 completion of that brackish- and freshwater formation, marine 

 conditions, similar to the first, were resumed ; and the Pierre- 

 Fox Hills formation was then deposited. Furthermore, upon 

 the completion of the last named formation, brackish- and 

 fresh-water conditions were resumed, over the same area, when 

 the Laramie Group was deposited. 



The specific identity of a considerable part of the molluscan 

 fauna of the Belly River formation with Laramie forms makes 

 it necessary to assume that both faunas had a common origin. 

 This proposition being accepted, the stratigraphical relation of 

 the Belly River formation with the Laramie makes it further 

 necessary to assume that at least a large part of the fauna of 

 the Laramie was derived directly from that of the Belly River 

 formation. 



The introduction of a true marine formation between the 

 two which are of brackish and-fresh- water origin precludes 

 the supposition that the earlier fauna prevailed over the same 

 area which it first occupied during the deposition of that ma- 

 rine formation. The presence of certain identical species in 

 both the Belly River and Laramie formations is presumptive 

 proof that those species somehow and somewhere survived dur- 

 ing the time that the Pierre -Fox Hills formation was in course 

 of deposition. The absence of any equivalent of the Belly 

 River formation from the marine Cretaceous series which so 

 extensively prevails to the southward of the Missouri river 

 seems to indicate that the molluscan fauna of that formation 

 originated in that northern region, and that it did not then 

 extend far to the southward. 



