306 W. T. LEE MORRISON A CRETACEOUS FORMATION 



PRINCIPLES 



The position that I assume on this question is largely due to the prin- 

 ciples enunciated several 3^ears ago by Prof. T. C. Chamberlin in a series 

 of lectures that it was my privilege to attend. Some of these principles 

 have been more recently amplified in his article on "The Shelf Seas of 

 the Paleozoic and their Eelations to Diastrophism" (7), and I take this 

 occasion to express my aj^preciation of the service that Professor Cham- 

 berlin has rendered to the stratigrapher. Tt is sometimes difficult to 

 adjust the time scale of one region to fit a standard previously established 

 for another. It needs no argument to convince the reader that a scale 

 appropriate^for any one continent is imperfect and must eventually give 

 place to a universal standard^ and it begins to look as if the principles 

 of diastrophism may eventually take a leading part in the adoption of 

 such a standard, in the establishment of which the laws of terrestrial 

 ph3'sics will play an important part; for I believe, with Schuchert (21, 

 page 586), that "diastrophic action is at the basis of chronogenesis." 



Without entering into a discussion of diastrophism, which, although 

 pertinent in this connection, is too large a subject to discuss here, I may 

 state that the basic principle that I shall use is this : A movement of land 

 or sea of sufficient magnitude to make an appropriate separation of sys- 

 tems on one continent must be recognizable on other continents and is 

 more likely to give exact time correlations than most groups of organisms. 

 In Europe, with which comparisons in this case are made, the Jurassic 

 closed with a retreat of the sea from the continent and the Lower Cre- 

 taceous was inaugurated by its return. It is not probable that the rela- 

 tions of land and sea would remain unchanged in America while such 

 movements were affecting the European continent. It seems altogether 

 probable that the withdrawal of the sea from Europe and from America 

 was due to some common cause and took place at the same time, and that 

 the ensuing early Cretaceous resubmergence began at the same time on 

 both continents. However, this postulate, sound as it may be, falls short 

 of complete solution of the Morrison problem, because the youngest marine 

 formation below the Morrison is of late Jurassic age, being correlated on 

 fossil evidence with the Oxfordian stage; and the oldest marine forma- 

 tion above it is of late Lower Cretaceous, of Washita age, leaving a possi- 

 bility that the Morrison might belong either in the Lower Cretaceous or 

 in the Jurassic. 



It might be appropriate at this point to inquire Avhat constitutes a 

 geologic system. There are fundamental differences of opinion that find 

 expression in the various schemes of classification, but I think that most 

 geologists will agree that a system miglit properly be terminated by a 



