WHITE ON THE LARAMIE GROUP. 869 



luscan fauDa should be noticed. All the branchiferous species of Mol- 

 lusca of the lower or brackish-water beds of the Laramie Group of Bear 

 Eiver Valley and the adjacent region are different from any of those yet 

 found in any other part of the Laramie Group. Besides this, there aro 

 two or three generic or subgeneric types among those mollusks that have 

 never been discovered elsewhere. This statement applies only to those 

 beds that have been so often called the "Bear Eiver Estuary Beds", and 

 not to the upper or coal-bearing beds of Bear Eiver Valley, as developed 

 near Evanston, Wyo. ; for, in the latter, a few species have been 

 recognized as identical with some that are found in other and distant 

 parts of the group.* Because of the general character of these Bear Eiver 

 brackish-water strata, and their relation to those both above and beneath 

 them, no reasonable doubt can be entertained that they form an integral 

 part of the great Laramie Group, notwithstanding the unique character 

 of a large part of their fossils. The existence of that remarkable local 

 fauna in the Laramie Group has a parallel in the similarly restricted and 

 unique fauna that is found in the Cretaceous series of Coalville, Utah, 

 and the region adjacent, extending as far northward as the valley of 

 Bear Eiver, where the Laramie beds before referred to are exposed. 

 The fauna! differences in both cases were probably due to a similar gen- 

 eral cause, and that cause probably had relation to the proximity of a 

 then existing western continental coast. 



Havingbrieflyconsidered the distinguishingcharacteristicsof the Lara- 

 mie Group, its relation to the other groups will be better understood by 

 a brief review of the physical conditions of that portion of the North 

 American continent which it occuijies, together with the portions adja- 

 cent. Much remains to be known upon this important subject, but the 

 facts hitherto ascertained seem to warrant the following statements and 

 conclusions : — 



East of west longitude 95°, North America is mainly occupied by 

 Paleozoic and Archaean rocks; as is also a large area which extends north- 

 ward and southward through Western North America; the eastern border 

 of the latter area being adjacent to the region here discussed and not far 

 from the one hundred and thirteenth meridian of west longitude. These 

 two great areas are taken to represent approximately the outline and 

 extent of the principal portions of the North American continent that 

 were above the level of the sea at the beginning of the Mesozoic time. 

 A broad expanse of Mesozoic sea then stretched between these two 

 continental factors, which were finally united by a general continental 

 elevation, and the consequent recedence of the sea. This elevation was 

 not, properly speaking, catastrophal, but gradual and oscillatory. That 

 intercontinental Mesozoic sea was narrower during the Jura-Trias period 

 than it was in the next epoch afterward, but it was always shallow, as 

 is shown by the lithological character of the strata of all the Mesozoic 



* In consequence of a misplaced label, I erroneously referred Macrccydis sjjcttiosa 

 Meek, to the Judith Eiver beds, in the table on p. 722, Bull. U. S. Geol. and Geog. Surv. 

 Terr. vol. iv. 



