DIASTEOPHIC EVIDENCE 335 



inexpedient to discuss this at the present time. I may only say, there- 

 fore, that we are not concerned with the various theories as to why the 

 earth changes its fomi, but simply that it does do so, which is unques- 

 tioned. That these changes or crustal movements are periodic in their 

 action is also evident, and, moreover, it is possible to demonstrate a cer- 

 tain amount .of rhythm in these activities. That is to .say, while some of 

 these, movements have obviously been more or less local in their action, 

 O-thers have been not only continent -wide, but practically and simultane- 

 ously:, world-wide.. - These grand p.eriods of diastrophic activity, which 

 haye been variously called ^':revolutions,"^^critical periods," or "grand 

 cycles" in the history of. the earth, have long been recognized and ac- 

 cepted by most geologists. According to Ulrich, there have been at least 

 four such major periods of activity on the North American continent, the 

 latest important one being the one here involved, namely, at the close of 

 Cretaceous time. 



The Cretaceous was a period of maximum sea extension throughout the 

 world, and its close was marked by maximum sea exclusion. According 

 to Schuchert's paleographic maps of North America, the Cretaceous Sea 

 was spread over a wider continental area than had previously been, occu- 

 pied by marine waters since perhaps Silurian or Devonian time. The 

 Eocene witnessed the sea withdrawal in Europe as well as in North 

 America. 



The Eukopean Time Scale 



This is perhaps an opportune point at which to consider the so-called 

 European standard and to compare it with the American conditions and 

 requirements. In this connection I must express my indebtedness to 

 Mr. W. T. Lee, who has spent much time in consulting the literature. 



It is recognized that the subdivision of stratified rocks into systems is 

 imperfect. Probably no system is completely represented ' in any one 

 place. It is recognized that the type locality of a system may not contain 

 its fullest expression as to range and age. Hence a type standard — such, 

 for example, as the European standard — is at best only temporary, since 

 it was based on incomplete information and must eventually give place 

 to a standard of world-wide application. It is, therefore, obviously 

 unwise to set up a European standard for measuring American systems, 

 as if that standard were final. I say a European standard rather than the 

 European standard, for there are disagreements among Europen geolo- 

 gists as to the line of separation between Cretaceous and Tertiary just as 

 there are in America. •. - 



In order to answer the question "How does the European standard- aid 



