296 H. F. OSBORN— JURASSIC-CRETACEOUS TIME 



strongly stated, we must take all our geologic time standards and de- 

 markations. In this connection I would like to repeat the main state- 

 ment in my address last year : "American events can be dated only by 

 comparison of American with European faunas and floras, unless simul- 

 taneous and world-wide diastrophic movements can be demonstrated to 

 have occurred." This statement does not refer to the general diastrophic 

 theory, which we are not now discussing, but to the attempt through 

 appeal to the diastrophic theory to determine such boundaries as the 

 Cretaceous-P^ocene and Jurassic-Cretaceous by reference to breaks in sedi- 

 mentation which may be local rather than world-wide. 



European Jurassic — Cretaceous Division Line 



In Europe the Jurassic-Cretaceous division line is by most geologists 

 drawn between the fresh-water series of clays and sands in England 

 known as the Wealdeu, and the imderlying Purbeckian ; in other words, 

 the demarkation may be expressed as follows: 



Base of the Cretaceous = AVealden 



Summit of the Jurassic = Portlandian-Purbeckian 



The problem before us in this symposium is, how can this Old World 

 stage of geologic time be most surely synchronized in the New AVorld? 



Having devoted many years to the special subject of correlation be- 

 tween the Tertiaries of Europe, IS^rtb America, and soutlieastern Asia, 

 I have formed tlie very strong personal o})ini()n that in correlation be- 

 tween such periods and stages as these we must rely chiefly on paleon- 

 tology. This is for the very important underlying reason that the most 

 stable, orderly, measurable, and coincident phenomena are those deep- 

 seated changes arising from the hereditary germ plasm which are out- 

 wardly and visibly expressed in the various forms of animal and plant 

 life. Paradoxical as it may sound, hereditary protoplasm is much more 

 stable than the surface of the earth. 



Diastrophic Axn paleontologic Evidence as Basis for 



Determinations 



Eising and falling coast or sedimentation lines in the pre-Tertiary and 

 Tertiary, or even the larger earth movements causing true unconformi- 

 ties, such as the birth of mountain systems and the earth changes incident 

 thereto, may or may not be coincident in time in two continents on op- 

 posite sides of the world. As a matter of fact, we know that the suc- 

 cessive erogenic movements or birthdays of many great mountain ranges, 



