THE FLORISSANT SHALES OF COLORADO. 163 



from the close resemblance between the marine fishes of 

 the Atlantic and Pacific coasts at the present time. 

 Also during Tertiary time was a period when what is 

 now Bering Strait was dry land, and animals were able 

 to cross from Asia to America, and vice versa. What 

 can these remote happenings have to do with Floris- 

 sant ? When Bering Strait was passable, there was a 

 migration of Old World animals into North America ; 

 we call it the Miocene migration. So again, later than 

 this, the Panama region was elevated and South Ameri- 

 can forms were able to pass into Central and North 

 America. With the first invasion came, for instance, 

 the elephant group ; with the second, the sloths. Now, 

 so far as we can judge, Florissant shows very distinct 

 evidence of the beginnings of the first invasion, but 

 none of the second. If we are right in this, it follows 

 that the Florissant shale was laid down in the interval 

 between the arrival during the Miocene of Asiatic ani- 

 mals, and that later on of South American ones. By 

 putting together various bits of evidence of this sort, 

 we may eventually obtain a relatively exact chronology 

 of the various deposits, and therefore types of life, which 

 are represented in the country. The actual number 

 of years represented is of course uncertain, but the 

 order in which the events occurred, and the nature of the 

 geographical and climatic changes, may be revealed 

 to us. Thus apparently insignificant fossils, which at 

 first seem to possess no general interest, may be the in- 

 dicators of the acts into which the great drama of the 

 earth is divided. 



References 



Popular Science Monthly, August, 1908, 'page 112; American Museum 

 Journal, November, 1916, page 443. 



