10 NON-MARINE FOSSIL MOLLUSCA. 



" Bitter Creek Coal series," " Point of Rocks Group," and " Bear River 

 Estuary beds "* are all parts of the great Laramie Group. 



2. The Laramie Group is regarded as a transitional group between 

 the Cretaceous and Tertiary series, and therefore as representing a 

 period partaking of both the Mesozoic and Cenozoic ages. 



3. The " Wahsatch Group," " Vermilion Creek Group," and "Bitter 

 Creek Group " are regarded as at least approximately equivalent strata, 

 constituting the oldest member of the purely fresh-water Eocene Tertiary 

 series of deposits in the West. 



4. The Green River and Bridger Groups are respectively the second 

 and third members of that fresh-water Eocene series. 



5. The Wind River Group of Wyoming is regarded as of Eocene age. 

 G. The White River Group of Dakota is regarded as of Miocene age. 



7. The fresh-water deposit of the Kawsoh Mountains, in Northern 

 Nevada, and its equivalent in Southern Idaho, called by King the 

 Truckee Group, are regarded as of Miocene age. 



8. No strata of Pliocene age are referred to in this article except those 

 of Cache Valley, in Northern Utah, because with that exception no 

 non-marine mollusca are known to have been obtained from any North 

 American strata which may be referred to that epoch; unless certain 

 forms of Physa be also excepted, which have been found in the Brown's 

 Park Group of Powell, in Southern Wyoming. 



Our knowledge of the various geological formations which are found 

 within the limits of North America enables us to trace with a good de- 

 gree of satisfaction the history of the evolution of the continent or the 

 progressive steps by which it was elevated above the level of the sea. 

 A brief outline, or at least a statement of some of the phases of this 

 history, as it is understood by geologists, is necessary to a proper un- 

 derstanding of the facts which are presented in the following pages. 

 Without going into the details of investigations by which geologists 

 have arrived at their conclusions, it may be stated that the continent in 

 its present shape has been produced by the coalescence of t\\ o or more 

 principal portions which were elevated above the level of the sea in the 

 earlier geographical ages in consequence of the progressive elevation of 

 the continental area. The two principal portions of the continent pre- 

 vious to the Cretaceous period were an eastern and western one respect- 

 ively, and before the close of that period they were separated by a broad 

 stretch of open sea. By the continued slow rise of the whole continental 

 area this broad stretch of open sea became laud-locked at the close of the 



* It should be remarked here that the tnolluscau species of the Bear River beds 

 and their equivalents are all different from those of the Laramie Group olse where; 

 but those strata are referred to the Laramie Group because they hold the same strati- 

 graphic:;! relation to the Cretaceous below and the Tertiary above that the typical 

 Laramie strata do in other regions, and also because they contain a braokish-water 

 fauna. Whether this difference in the fanme is due to difference in age, contemporane- 

 ous isolation of waters as separate seas during the Laramie period, or to some other 

 cause, is not yet known. 



