WTirrE.] GENERAL DISCUSSION. 259 



and ill all such cases their changed habitat seems to have been, at least 

 in some degree, forced uj^on them by environing conditions ; and the 

 individnal condition of those mollusks, when compared with that of the 

 same species in fresh waters, shows evidence of the nncongeniality of 

 their changed habitat. It seems impossible, however, to account for the 

 commingling of types which we find in the Laramie strata in any way 

 except by assuming that they lived together in the same waters ; and 

 their indi\idual condition in all cases suggests that they all thrived 

 equally. Furthermore, it seems to be unquestionable that the waters in 

 which the greater i)art of this commingling of types took place possessed 

 a considerable degree of saltness, and that the great Laramie sea was 

 essentially one of brackish waters. 



While very much remains to be known concerning the geological struct- 

 ure of the North American continent, the great array of facts that have 

 been already accumulated enables us to draw from them many legitimate 

 conclusions concerning the former physical conditions of certain x)ortions 

 of it, and to begin with some confidence to arrange them as materials 

 toward its i)hysical history. The following remarks upon this subject 

 are presented as supplementary to the foregoing report, bui; they are 

 based largely upon facts that have been previously accumulated and 

 X)ublished by various authors. They relate almost wholly to the Mesozoic 

 and Cenozoic Groups, and to the corresponding epochs in the geological 

 history of the continent. They are necessarily general in their character, 

 and are intended to apply especially to that portion of the national do- 

 main which may in a general way be designated as lying north of north 

 latitude 37° and between west longitude 95'^ and 113°. 



East of longitude 95°^ North America is mainly occupied by Paleozoic 

 and Archfean rocks, as is also a large area which extends northward and 

 southward through Western North America ; the eastern border of the 

 latter area being adjacent to the region here discussed, and not far from 

 the one hundred and thirteenth meridian of west longitude. These two 

 great areas are taken to represent approximately the outline and extent 

 of the principal portions of the present North American continent that 

 were above the level of the sea at the close of paleozoic time. % A broad 

 expanse of Mesozoic sea then stretched between these two continental 

 factors, which were finally united by a general continental elevation and 

 the consequent recedence of the sea. This elevation was not, j^roj)- 

 erly speaking, catastrophal, but gradual and oscillatory. That inter- 

 continental Mesozoic sea was narrower during the Jura-Trias period than 

 it was afterward, but it was always shallow as is shown bj^ the lithologi- 

 cal character of the strata of all the Mesozoic formations ; and as these ag- 

 gregate a great thickness there was, of course, for a long time, and over 

 a very large part of the space which it occupied, a gradual subsidence 

 of the bottom which allowed the successive deposition of shallow- water 

 formations. The following facts prove the occurrence of oscillations of 

 land surface and sea-bottom by which from time to time the eastern 

 border of the Mesozoic sea was shifted and the whole finally displaced. 



In Western Iowa, Eastern Nebraska, and Eastern Kansas the Creta- 

 ceous strata are known to rest directlj^ upon Carboniferous strata, the 

 Jura-Trias being absent. These last-named strata, however, are in full 

 force where the Mesozoic rocks are turned up against the eastern flanks 

 of the Rocky Mountains and Black Hills, as well as farther westward. 

 Their eastern border is certainly somewhere in the great plains beneath 

 later Mesozoic strata and the j^revailing sirrface debris, but its location 

 is not even approximately known. Cretaceous strata, continuous with 

 those of the West, are known to have been deposited as far eastward as 



