876 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



contemporaueously with at least the later Dinosaurians, and that the 

 barrier which separated those faunse was removed by some one of the 

 various surface movements connected with the evolution of the conti- 

 nent. Tlie climate and other physical conditions which were essential 

 to the existence of the Dinosaurians of the Laramie period having evi- 

 dently been continued into the Tertiary epochs that are represented by the 

 Wasatch, Green Eiver, and Bridger Groups, they might, doubtless, have 

 continued their existence through those epochs as well as through the 

 Laramie period but for the irruption of the mammalian hordes to which 

 they probably soon succumbed in an unequal struggle for existence. 



According to the facts which I have here and elsewhere shown, we 

 have in the strata of the Western Territories an unbroken record from 

 the earlier Mesozoic far into Tertiary time, and consequently no com- 

 plete line or plane of demarkation between them exists. Therefore the 

 designation of any precise boundary between the Cretaceous and Ter- 

 tiary of that region must be a matter of conventional convenience 

 rather than of natural requirement. 



