white.] GENEEAL DISCUSSION. 257 



It is easy to understand how the light shells, of land and palustral 

 gasteropoda might, after having been emptied of their decomposed 

 bodies, have been drifted to almost any distance unharmed, and finally 

 have found entombment with the shells of mollusks that lived and died 

 in the very sediments that now inclose them all. But there are cases of 

 equally heterogeneous association which cannot be accounted for in that 

 way. These cases consist of the presence in the same layers of the 

 shells of branchiferous mollusks, both conchifers and gasteropods, be- 

 longing to types that are respectively recognized as of brackish and fresh 

 water habitat. It is well known that the shells of fresh- water mollusks 

 are often carried down by the current of rivers and deposited in the 

 sediments of the brackish waters of estuaries along with those of such 

 mollusks as find a congenial habitat there. Where such is the case the 

 drifted shells suffer attrition, the effects of which are readily recog- 

 nized ; tlie opereula of gasteropods are separated from the shells, and 

 the valves of conchifers are separated from each other. Besides this 

 the sedimentary accumulations of an estuary contain inherent evidence 

 of their character as such aside from that which is afforded by the types 

 of its mollusca ; such as accumulation of river silt with its current-worn 

 fresh-water shells, and the peculiar stratification produced by floods and 

 changing currents. Although it has been not uncommon for geologists 

 to speak of the different brackish- water strata of the Laramie Group as 

 "estuary beds," or to refer to them as of estuary origin, I do not know 

 of a single deposit or part of one in any district, or in any of the divis- 

 ions of the great Laramie Group, to which the foregoing test of its estu- 

 ary origin can be applied. 



Although rivers of greater or less magnitude must necessarily have 

 flowed into the Laramie sea, in no part of the group at any of the numerous 

 localities where I have studied it have I found the character or condi- 

 tion of its strata in any way indicating that they were either influenced 

 or modified by fluvatilG influx. On the contrary, its sandstones, and 

 most of its other lithological features, are everywhere of the same 

 general character as those of the Fox Hills Group of Cretaceous strata, 

 which are plainly of marine origin. But notwithstanding this evident 

 uniformity of deposition, a large proportion of the fossiliferous Laramie 

 strata contain a commingling of brackish and fresh- water forms, the con- 

 dition and association of which show that those of neither category could 

 have been drifted to their present position from a different habitat. For 

 example, in the brackish-water beds of Bear Biver Valley great num- 

 bers of the shells of Corbicula, Corbula, and TJnio (two species of the 

 latter genus) are found associated together in the same layers, the major- 

 ity of the examples of all of which have both their valves together in 

 their natural position. Besides this, none of the numerous associated 

 shells of gasteropods show any evidence of attrition such as they would 

 have received if they had been drifted. These facts indicate that all 

 the mollusks referred to lived contemporaneously in the same waters, 

 and that the sediment upon which they lived is the same as that which 

 now incloses them. It is a well-known fact that some species of Corbi- 

 cula and Neritina may live in waters that are nearly or quite fresh, but 

 the presence among those shells of the Bear Biver strata of Corbula, 

 2Iembranipora, and a few scattered oyster-shells seems to make it certain 

 that the waters containing all of them were, at least in some degree, sa- 

 line. It also seems certain that there was some alternation of the degree 

 of saltness of those waters, because there has been found at least one 

 thin layer there which is composed almost wholly of a small Ostrea, with 

 no other associated shells. 



Again, at Black Buttes Station there is also evidence of alternation of 

 17 as 



