INTRODUCTORY REMARKS. XXXV 



it would seem, however, thai this rock is not extensively developed there, 

 being probably only a thin bed, not differing in composition from those of the 

 Niobrara and Fori Benton groups. Indeed, they usually think it- scarcely 

 separable there from what seems to be extensive developments of the older 

 members of the" Cretaceous series of the North. 



Along the eastern base of the Rocky Mountains, the Fort Pierre group 

 is met with through Colorado, and on northward to and far beyond the 

 Black Hills in Dakota and Wyoming. In Southern Wyoming, some of 

 its fossils have been brought from the mountains on the eastern margin 

 of the Laramie Plains, and farther westward along the Union Pacific Rail- 

 road from near Miser station, Medicine Bow, and the Salt Wells in Wyoming. 

 The latter locality is the farthest western point at which its fossils have 

 been observed in this internal portion of the continent, and corresponds also 

 nearly in longitude with the localities at which it occurs in Western Colo- 

 rado. We have reason, however, to helieve that it is represented on Van- 

 couver and Sucia Islands, west coast of British America, as well as in Cali- 

 fornia. * 



Fox Hills gkoup. — This division is much more arenaceous than the 

 Fort Pierre group, and also differs in presenting a yellowish, or more or less 

 ferruginous tinge. Toward the base it consists of sandy clays ; but as we 

 ascend to the higher beds the arenaceous matter is found to increase, so that 

 at some places the whole passes into a ferruginous sandstone. It is not 

 separated by any strongly-defined line of demarkation from the Fort Pierre 

 group below, the change from the fine clays of the latter to the more sandy 

 beds above being generally gradual. Nor are these rocks distinguished by 

 any very abrupt or strongly-marked change in their organic remains, since a 

 part of the fossils occurring in the upper fossiliferous beds of the Fort Pierre 

 group also pass up into the Fox Hills group ; while at one locality already 

 mentioned, on the Yellowstone, there is a complete mingling of the fossils of 

 t hese two rocks in the same beds. Indeed, it has sometimes been thought that 

 we might, with almost equal propriety, on palseontological grounds carry the line 

 separating these two groups down so as to include in the Fox Hills group 

 the upper fossiliferous beds of the Fort Pierre group. Most of the known 



* Sec paper by the writer describing Cretaceous fossils from Vancouver Island, in Trans. Albany 

 Inst., December, 185G ; also, another by same on Collections X. W. Boundary Survey, Proceed. Acad. 

 Nat. Sei. Philad., XIII. October, 1861. 



