332 F. H. KNOWLTON CRETACEOUS-TERTIARY BOUNDARY 



few forms now known to have been incorrectly identified, we have a total 

 of only 21 or 22 species that are known to have cross'ed the line of the 

 unconformity. The full significance of this small number is brought out 

 when we aggregate the floras in the beds below and above the uncon- 

 formity. In the lower beds — that is, Vermejo, Laramie, Montana, etcet- 

 era — there are 350 species, and in the upper beds — Eaton, Dawson, Arap- 

 ahoe, Denver, Lance, etcetera — there are over 700 species. Twenty-one 

 or 22 species in common with 350 below and over 700 above is an insig- 

 nificant number. It shows that more than 90 per cent of the Cretaceous 

 flora was wiped out by the disturbances attending this diastrophic move- 

 ment. 



The length of this time interval is indicated by the flora in another way 

 other than in the destruction of the species. If the flora in the beds 

 above the unconformity was wholly or even largely of a different type 

 from that in the beds below, it might mean that it had come in suddenly, 

 "ready made," from an adjacent area, without great lapse of time ; but as 

 a niatter of fact it is in the main a continuation of the Cretaceous flora, 

 and its evolution from the Cretaceous remnants implies a considerable 

 length of time. 



We may now consider the paleobotanical evidence regarding the corre- 

 lation, age, etcetera, of the formations here involved, beginning with the 

 area where our knowledge is most complete and passing over the ground 

 in a reverse order from that in which the structural relations were dis- 

 cussed. 



By common consent the physical break is made the basis of the separa- 

 tion between the Cretaceous and Tertiary in the Atlantic and Gulf 

 coastal plain. In the Gulf region the Cretaceous Selma chalk and the 

 Eocene Midway formation are on opposite sides of the line, and both are 

 of marine origin. In a paper recently read before .the Geological Society 

 of Washington, Dr. L. W. Stephenson stated that the hiatus between the 

 Cretaceous and Tertiary represented a longer time, regarded as due to 

 evolutionary development, as measured by the change in life forms, than 

 is indicated by the full Upper Cretaceous section of the region. 



Above the Midway formation is the Wilcox formation, which is also 

 marine except in the Mississippi embayment. The Wilcox contains a 

 published flora of about 65 species, of which number about 25 are found 

 also in the Raton formation. But Mr. E. W. Berry is engaged in the 

 preparation of an. elaborate monograph of the Wilcox flora, in which he 

 will enumerate over 300 species, not one of which, by the way, has been 

 found in the Cretaceous anywhere. Less than 80 of its species have' been 

 found outside of this formation. Between 30 and 40 species are now 



