white] GENERAL DISCUSSION. 77 



the fresh-water rnolluscau types that have been found in the various 

 groups of North American Mesozoic and Cenozoic strata, and that we 

 also find among living mollusca. That is, they have been preserved 

 through a continuity of habitat iu the congenial fresh waters of lakes 

 and rivers, flourishing in the lakes, when they existed, as well as in the 

 rivers, and escaping by the streams which were the former outlets and 

 inlets of the lakes, but which continued to flow after the obliteration of 

 the latter, as rivers or tributaries of river systems. 



Lakes are only parts of unfinished river systems which disappear by 

 being drained when the system is finished by the gradual weaving down 

 of its channel. A. lake consequently contains essentially the same 

 aqueous fauna that the fluvatile portion of the system does in case the 

 water of the lake is wholly fresh; or a modification of that fauna if the 

 waters of the lake are more or less saline. The great lakes which 

 existed in Western North America in the Tertiary and Laramie periods 

 successively became obliterated, but we may reasonably conclude that 

 at leas f a part of the river channels of to-day have existed as such from 

 earlier geological times; that the greater part of them were established 

 in epochs anterior to our own, and that those of some of the tributaries 

 of the present Mississippi River system are identical, at least in part, 

 with former outlets or inlets, or both, of the great ancient lakes which 

 have just been referred to. Consequently we may reasonably conclude 

 also that the rnolluscau fauna of the Mississippi Eiver system is lineally 

 descended from the faunae of those ancient lakes, and the river systems 

 of which they constituted lacustrine portions.* This view is confirmed 



*I here include the Laramie Sea in the use of the term "lacustrine," the term "sea" 

 being used simply to indicate that its waters were saline and not fresh ; just as the 

 Black and Caspian are called seas instead of lakes, and for the same reason. It may 

 seem to he the use of a misnomer to speak of the Laramie Sea as a part of a river sys- 

 tem, because it was so immensely large, and the continental area which was drained 

 into it was proportionally so small, but if these views concerning the conditions which 

 then existed are correct, that sea, with its tributaries and outlet, differed only in de- 

 gree and not in kind, from any river system which has a lake of any size in its prin- 

 cipal course. The waters of that sea having been saline, the Laramie hydrographic 

 system more nearly resembled that of the Black Sea than any other now existing that 

 is equally well known; and, although the ancient sea has long since disappeared from 

 the face of the earth, its "Hellespont " still flows as a part of the Missouri River, or of 

 some one of its tributaries. 



The commingling of brackish-water and fresh-water fossil forms in an estuary de- 

 posit is readily explained by the supposition that the river which debouched into the 

 estuary brought down the latter and mingled them with the former. But the com- 

 mingling of brackish-water and fresh-water forms occurs in some portions of the 

 Laramie deposits under such conditions as to compel the belief that some of them at 

 least lived and thrived together. There is evidence also that the fresh-water fauna 

 proper of the Laramie system not only inhabited the streams which emptied into its 

 sea, but (hat in great and shifting areas of the sea itself the waters were sufficiently 

 fresh to allow the existence iu them of such mollusks as Unto, Goniobagis, Viviparus, 

 Campeloma, &c., and saline enough in other parts for the existence of Ostrea, Anomia,. 

 Corbula, &c. This view of the conditions of the Laramie Sea being accepted it is 

 plainly seen to have been, what Ritter has aptly termed, an unfinished river system, 

 though an extreme example. 



