2G4 REPORT UNITED STATES GEOLOGICAL SURVEY. 



as the beginning of the Wasatch £poch; and the question is whether the 

 Cretaceous period closed with the close of the Fox Hills epoch or with 

 that of the Laramie period. The question might be extended so as to 

 embrace the inquiry whether the true chronological division between 

 Cretaceous and Tertiary did not really occur within the Laramie period; 

 but this, while not unreasonable, would perhaps be inconvenient and un- 

 profitable. The claim that Cretaceous types of vertebrates are found in 

 even the higher strata of the Laramie Group is freely conceded, and I have 

 no occasion to question the reference that has been m.ade of its fossil plants, 

 even those of the lowest strata, to Tertiary types. The invertebrate fos- 

 sils of the group itself, as I have elsewhere shown, are silent upon this 

 subject, because the types are either unique, are known to exist in both 

 Mesozoic and Tertiary strata, or pertain to living as well as fossil forms. 

 Every species found in the Laramie Group is no doubt extinct, but the 

 types have collectively an aspect so modern that one almost instinctively 

 regards them as Tertiary ; and yet some of these types are now known to 

 have existed in the Cretaceous and even in the Jurassic period. 



In view of the conflicting and silent character respectively of these pa- 

 leontological oracles the following suggestions are offered : It is a well- 

 known fact that we have in North America no strata which are, accord- 

 ing to European standards, equivalent with the Lower Cretaceous of 

 Europe, but that all North American strata of the Cretaceous period are 

 equivalent with 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 question being as to its place in the series. A comparison of its 

 fossil invertebrate types with those of the European Cretaceous indicates 

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

 strata in Europe. If, therefore, that parallelism is correctly drawn, and 

 the Laramie Group is of Cretaceous age, we have represented in America 

 a great and important period of that age which is yet unknown in any 

 other part of the world. Besides this, we may reasonably conclude that 

 the Fox Hills Group of the west is equivalent with the Upper Creta- 

 ceous strata of the Atlantic and Gulf coasts, between which and the 

 Eocene Tertiary of those regions there is no known equivalent of the 

 Laramie Group. 



If paleontologists should finally agree upon regarding the Laramie 

 Group as of Cretaceous age, it must be because of the continuance of 

 certain vertebrate Cretaceous types to the close of that period, and the 

 presence of mammalian Tertiary types in the strata immediately follow- 

 ing ; but the following facts, in addition to those which have been already 

 stated, should be carefully considered before any such agreement is made. 



With rare and obscure exceptions no mammalian remains are known 

 in North American strata of earlier date than that of those which were 

 deposited immediately after the close of the Laramie period and upon its 

 strata. Immediately from and after the close of the Laramie period 

 their abundant remains in the fresh-water Tertiaries of the West show 

 that highly organized mammals existed in great variety and abundance; 

 all of which may be properly regarded as constituents of a Tertiary fauna, 

 and many of which are, by accepted standards, of distinctively 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 prove the Tertiary 

 age of the former strata, then was the Tertiary period ushered in with 

 most unnatural suddenness. Sedimentation was, at least in part, un- 

 broken between the Laramie Group and the strata which contain the 

 mammahan remains referred to, so that the local conditions of the ori- 



