58 BIOLOGICAL SOCIETY OF WASHINGTON. 



fresh waters in which those Uniones lived were not continued to 

 later periods, as some other ancient streams of fresh water were, 

 together with their molluscan faunas, and that the lines of descent 

 of those mollusks were consequently cut off and their types ex 

 tinguished. 



While many rivers have persistently held their ground through 

 several geological periods, despite even the elevation of mountain 

 ranges across their course ; the fact that great numbers of them have 

 been destroyed in past geological time by the physical changes which 

 have taken place in the regions they have occupied, is too evident 

 to be questioned. One of the many examples of the destruction of 

 bodies of fresh water which have become established upon the earth 

 in former geological times is suggested by the presence of a true 

 estuary deposit among the Cretaceous strata of Northern Utah. 

 This deposit, which is a rare one of the kind, was evidently formed 

 at the western border of the oceanic belt, which, it is understood, 

 then traversed the whole North American area in a northward and 

 southward direction, between two separate continental areas, and 

 at the mouth of a river which then drained part of the western 

 area. The region which that oceanic belt then occupied is now 

 the heart of the continent, and all traces of the ancient river 

 referred to are obliterated. Furthermore, the district which it 

 drained to the eastward is now drained by other channels running 

 in the opposite direction, into the Salt Lake Basin. I shall pres 

 ently have occasion to refer again to this estuary deposit and to 

 others which no doubt co-existed with it along the shores of the 

 same sea. 



The period which immediately succeeded that in which the last 

 of the marine Cretaceous deposits were made, and which contains 

 the estuary deposit that has just been referred to, namely, the Lara- 

 mie period, witnessed the production of one of the most remarka 

 ble features which has ever characterized any. continent. This fea 

 ture was a great inland sea, holding both brackish and fresh waters, 

 as the Caspian does now, but which was in other respects more like 



