WHITE ON THE LAEAMIE GROUP. 875 



of the Lower Cretaceous of Europe, but that all North American strata 

 of the Cretaceous period are equivalent with certain portions of those of 

 the Upper Cretaceous of that part of the world. That the Fox Hills 

 Group is of Upper Cretaceous age no one disputes, the only ques- 

 tion being as to its place in the series. A comparison of its fossil 

 invertebrate types with those of the European Cretaceous rocks indi- 

 cates that it is at least as late as, if not later than, the latest known 

 Cretaceous strata of Europe. If, therefore, that parallelism is correctly 

 drawn, and the Laramie Group is really of Cretaceous age, we have a 

 great and important division of the Cretaceous represented in Amer- 

 ica which is yet unknown in any other part of the world. It is in view 

 of these facts that, for purposes of general grouping of the strata of the 

 Western Territories, the provisional designation of "Post-Cretaceous" 

 has been adopted for the Laramie Group in the reports of this Survey. 



It is well known that able American paleontologists regard the Lara- 

 mie Group as of Cretaceous age, and this opinion is understood to be 

 based upon the persistence of some vertebrate Cretaceous types up to 

 the close of the Laramie period and the first known appearance of Ter- 

 tiary types of mammals in North America, in the immediately superim- 

 posed Wasatch strata. It is not to be denied that these are important 

 considerations, but the following, as well as other relevant facts already 

 mentioned, ought to be duly considered in that connection. 



With rare and obscure exceptions, no mammalian remains are known in 

 North American strata of earlier date than those of the Wasatch Group 

 that were deposited immediately after the close of the Laramie period. 

 Immediately from and after the close of that period, as shown by abun- 

 dant remains in the fresh-water Tertiaries of the West, highly organ- 

 ized mammals existed in great variety and abundance. There is noth- 

 ing to forbid the supposition that all of these were constituents of a 

 Tertiary fauna, and many of them are, by accepted standards, of dis- 

 tinctively Tertiary types. If the presence of these forms in the strata 

 referred to, and their absence from the Laramie strata immediately 

 beneath them, together with the presence of Dinosaurians there, be 

 held to iDrove the Tertiary age of the former strata, tben was the 

 Tertiary period ushered in with most unnatural suddenness. Sed- 

 imentation W3,s, at least in part, unbroken between the Laramie Group 

 and the strata which contain the mammalian remains referred to, so 

 that the local conditions of the origin of all of them were substantially 

 the same, and yet, so far as any accumulated evidence shows, those 

 mammalia were not preceded in the Laramie period by any related 

 forms. Such suddenness of introduction makes it almost certain that 

 it was caused by the removal of some physical barrier, so that the 

 ground which was before potentially Tertiary, became so, of paleon- 

 tological record, by actual faunal occupancy. In other words, it seems 

 certain that those Tertiary mammalian types were evolved in some 

 other region before the close of the Laramie period, where they existed 

 Ball. iv. No. 4 9 



