WHITE ON THE LARAMIE GROUP. 837 



In all places where the group is kuown, and from its base to the top, the 

 majority of its invertebrate fossils are brackish-water forms, and yet 

 in the same places and throughout the same vertical extent, a greater or 

 less number of mollusciin species occur that are referable to either a 

 fresh-water or land habitat. In many instances, the fresh-water species 

 occupy separate layers from those which contain the brackish-water 

 forms, and alternate with them, but it is very commonly the case that 

 both fresh- and brackish-water types are found to occupy the same 

 layers, the conation of the specimens of both categories being such as 

 to forbid the supposition that either of them was drifted from else- 

 where to their present places of deposit and association. For example, 

 numerous specimens of Unio, of many species, have been found asso- 

 ciated with equally numerous specimens of Corbula and Corbicula, a 

 large proportion of all of which still retain both valves together in their 

 natural jiosition. Associated with these, and in a similarly unmuti- 

 lated condition, there are other molluscan remains, the living repre- 

 sentatives of which are respectively of fresh- and brackish-water habitat ; 

 and all of them are in such condition as to force the conclusion that 

 they all lived together. The general prevalence of brackish-water 

 types throughout the group, including Ostrea in abundance, Anomia 

 quite plentiful, with occasional examples of Nuculana and Memhranacea 

 (or a closely related polyzoan), leaves no room for reasonable doubt that 

 the prevailing condition of the Laramie Sea was saline ; but the absence 

 of true marine species proves that its waters were cut off from the open 

 ocean. The conditions and association of species just explained show 

 also that there must have been in certain places and at different times 

 an alternation of greater and less saltness of its waters. 



It is well known that some species at least of certain genera of mol- 

 lusks are capable of living in both brackish and fresh waters, but the 

 evidence seems conclusive that certain forms found in the Laramie 

 Group, the living representatives of which are respectively confined to 

 either a fresh- or brackish-water habitat, then not only lived but thrived 

 together in the same waters ; and also that those waters were in some 

 degree saline. This commingling of brackish- and fresh-water types is 

 not exceptional in the Laramie Group, but quite common, yet there are 

 layers in some places, as for example near Black Buttes, in which all, 

 or nearly all, the Mollusca are ot fresh-water type. A statement of these 

 facts naturally suggests that this commingling of brackish- and fresh- 

 water forms took place in estuary waters, and that the strata containing 

 them are estuary deposits. But the character and condition of the 

 strata show that this is not the fact, or if so in any cases, they are rare 

 and at present unknown exceptions to the rule. While there were 

 necessarily tributary streams flowing into the Laramie Sea, and true 

 estuaries at the mouths of at least a part of them, I do not know of a 

 single deposit or part of one in any district or in any of the divisions ot 

 the great Laramie Group that presents the strati graphical characteris- 

 tics of an estuary deposit. 



