200 Cincinnati Society of Natural History. 



Prof. R. P. Whitfield described, from near San Antonio, Texas^ 

 Paramithrax ( ?) walkeri. And Prof. 0. C. Marsh* described, from the 

 Cretaceous chalk of Kansas, Holosaurus abrwptus. 



To conclude this cursory review of the sjrowth of our knowledofe of 

 the Cretaceous formation of North America, I will add a few observa- 

 tions upon the present state of the science. The Cretaceous is found 

 either exposed upon the surface or covered by the Tertiary, forming a 

 border of variable width, on the eastern coast, from New York to 

 Florida. It constitutes the surface rocks, or is overlaid with the Ter- 

 tiary at all places south of the 33d parallel, with the exception of lim- 

 ited areas in the mountain regions. It extends up into Tennessee, 

 spreads over all Mississippi, and reaches southern Illinois. West of 

 the 97th meridian from the 33d parallel to the Arctic ocean, the whole 

 country is covered with this formation, with the exception of limited 

 areas in the mountain regions, or inconsiderable extensions of land, 

 where it has been swept awa}^, and an area of some magnitude north 

 and west of Hudson's Bay. This of course includes the area covered 

 by the Tertiar3\ It is found east of the 97th meridian, extending into 

 Iowa, Minnesota, and some parts of British America. Or approxi- 

 mately stated, the Cretaceous now forms the surface rock, or is over- 

 spread b}'' the Tertiary, over more than half the area of the North 

 American Continent, and from the extensive denudation which it has 

 evidently suffered, we may fairlj^ presume, that at the commencement 

 of this formation the continent was an island of less than one third 

 its present dimensions. 



In the east and south the formation is exclusively a marine deposit, 

 but in the west, over great areas, the marine Cretaceous is succeeded 

 by a brackish or fresh water Cretaceous deposit. In the east it never 

 exceeds half a mile in thickness, but in the west the marine Cretaceous 

 sometimes exceeds a mile in thickness, and is followed by the brackish 

 and fresh water deposits, which are also more than a mile and some- 

 times even two miles in thickness. This formation is, therefore, pre- 

 eminently the building deposit or land making deposit of the North 

 American Continent. 



The brackish and fresh water deposits were first named the Fort 

 Union or Lignitic Group, and there is no reason known to the author, 

 why these deposits, wherever found, should bear an}^ other geological 

 name. It is true that the name Bear River Group was given to a 

 group of rocks lower than those first named the Fort Union Group, but 



■•'' Am. Jour. Sci. and Arts, 8d Ser., Vol. xix. 



