Introduction to Life of the Lakes. 105 



the world received its stock of highly varied 

 land animals, the Mammalia. If, then, it was 

 in the Eocene period that the orders of mam- 

 malian life were so varied and spread over the 

 continent of North America, a glance at the 

 geography of the period will help to determine 

 the relation of this fact to our Shoshone 

 island. 



All through the Cretaceous period there 

 existed where the Rocky mountains now 

 stand, a deep Mediterranean sea. When, at 

 the close of this period, the Rocky mountains 

 were elevated, their mass was raised from the 

 bed of this sea. As part of the result of this 

 uplift the ocean waters were thenceforth shut 

 out from the interior of the continent and the 

 hollows that remained were filled with fresh 

 water and became lakes. Now into these lake 

 beds on both sides of the newly elevated 

 mountain range, were washed the skeletons of 

 the animals of the Eocene period and so pre- 

 served as fossils. These old lake beds are the 

 "Bad Lands" of to-day. 



