384 HISTORICAL GEOLOGY. 



Eocene, came the Orohippus, about the same size, with 

 three toes behind and four in front the fifth splint being 

 dropped. Next, in the Miocene, came the Mesohippus 

 and the Miohippus (about the size of a sheep), with three 

 toes behind and in front, but the fourth toe of the Oro- 

 hippus still retained as a useless splint. In these the 

 horse family may be said to be fairly established. Then, 

 in the Lower Pliocene, came the Protohippus, about the 

 size of an ass, with three toes on all the feet, but the two 

 side-toes shorter, and the mid-toe larger, than before. 

 Then, lastly, in the uppermost Pliocene, come the Plio- 

 hippus and Equus, in which the side-toes are reduced to 

 useless splints, and the middle toe is greatly enlarged. 

 This is the case in the modern horse ; its side-splints attest 

 its three-toed ancestry. 



Crust-Movements during and closing the Tertiary Period. 



Remember that, during the Cretaceous, a great sea 

 covered the whole of the Plains and Plateau region, 

 dividing the continent into two continents. By the 

 gradual elevation of the region, this sea was obliterated 

 and replaced by great lakes. The formation of these 

 lakes inaugurated the Tertiary. The elevation of the 

 same region continuing, these Tertiary lakes were suc- 

 cessively obliterated, and the prodigious general erosion 

 and canon-cutting of this region commenced. 



On the Pacific border, at the end of the Miocene, the 

 Coast Range of California and Oregon was born. From 

 the beginning of the Cretaceous, the place of this range 

 had been marginal sea-bottom receiving sediment. At 

 the end of the Miocene, these yielded to horizontal pres- 

 sure, were crushed together, and swelled up into this 

 great range. Probably at the same time occurred the 

 great lava-flood of the northwest, described on page 218. 



On the Atlantic border the changes were far less 

 remarkable. There was, however, a gradual increase of 



