wHim] GENERAL DISCUSSION. 263 



briefly some of the facts that bear upon the i>liysical conditions Avliicli 

 prevailed during the Laraiuie period within the region that was occupied 

 by its waters. 



After the facts presented and the remarks made upon preceding pages 

 of this report it is almost superfluous to say that the great Laramie Group 

 is regarded as having been deposited in a brackish-water sea, which, for 

 extent and character, has no existing piuallel. There are, liowever, cer- 

 tain characteristics of fossil fauna and strata that indicate some very 

 peculiar conditions of that sea then existing which deserve much inves- 

 tigation, but which, for obvious reasons, can receive only brief consider- 

 ation here. First, it is evident that at all times its waters had compara- 

 tively little depth, and that in many places it was repeatedly very shallow ; 

 and, furthermore, that the great thickness of the group, amounting to a 

 maximum of 4,000 feet, was accumulated by a gradual subsidence of the 

 bottom. As a rule, its molluscan fauna was composed of brackish-water 

 types, but often, and in many places, the waters were so far freshened as 

 to give congenial habitat to fresh-water forms. Judging from the char- 

 acteristics of existmg land-locked seas, it is difticult to understand clearly 

 how fresh and bracldsh waters could have existed in one and the same 

 sea in the absence of or at a distance from the mouths of tributary rivers. 

 But the character of the deposits of the Laramie sea, as well as its mol- 

 luscan fauna, warrants the suggestion that very large portions of its area 

 were at diiierent times and in (litt'erent places in the condition of marshes, 

 which were only slightly raised above the general water level, upon which 

 fresh waters from rains accumulated and gave congenial habitat to such 

 members of the molluscan fauna of the period as would preferably avoid 

 the brackish waters. This view is supported by the occasional presence 

 of land-shells among those of brauchiferous mollusks, the moi-e common 

 occurrence of palustral shells, the occurrence of deciduous leaves and 

 other fragments of vegetation, all in the same or associated strata; and 

 also the presence of numerous beds of lignite throughout the group. It 

 is also supported by the fact that the fossil moUusca are found not uni- 

 formly distributed throughout the group either verticallj' or geograph- 

 ically, but to occupy very small, distantly-separated areas, which are not 

 only locally restricted, but within which areas the vertical range of the 

 diderent species is limited. Admitting that such conditions prevailed, 

 it is easy to understand how it may have happened that certain layers, 

 containing the remains of mollusca, which could have flourished only in 

 salt or brackish waters, are found to alternate in close succession with 

 those containing fresh-water species. The conditions thus indicated 

 would also have brought the brackish and fresh water habitats of those 

 mollusca into such jiixtai)Osition that they must have frequently en- 

 croached upon each other. This frequent encroachment, or mingling of 

 habitats, and no doubt the frequent impracticabilitj' of retreat, Avould 

 have had a tendency to iniue at least a i)ortion of the mollusks of each 

 to an existence in the other. It is evident that many of the species were 

 capable of such interchange of habitat without disadvantage; and that 

 some of the species whose living representatives are regarded as strictly 

 fresh-water forms may have then lived in part in brackish waters, such 

 as Melania., Unio, &c., has already been suggested. 



In the foregoing report I haNC purposely avoided an expression of 

 opinion as to the true geological age of the Laramie Group, because, not- 

 withstanding the positive oi)inions that have been expressed by others 

 upon that subject, I regard it as still an open question. All i)aleoutolo- 

 gists agree that the Cretaceous period extended at least to the close of 

 the Fox Hills epoch, and that the Tertiary pei iod began at least as early 



