AGE OF MAMMALS. 279 



brought down from wide regions by the Mississippi and 

 Missouri rivers, and the building up of the peninsula of 

 Florida by the coral animals, which are at the present 

 time building as busily as ever. 



389. Post-tertiary Period. This period extends from 

 the conclusion of th* Tertiary to the advent of man. 

 The building of all the continents was now, as I have just 

 said of North America, essentially completed that is, 

 the land had now reached its present bounds, and its 

 grand ranges of prominences were raised to their des- 

 tined heights. But there needed to be a greater diver- 

 sification of surface than had as yet been effected a di- 

 versification for the most part in detail. Hills and val- 

 leys were to be made ; stones of various sizes were to be 

 scattered over the surface ; rivers were to be extended 

 and multiplied ; and terrace-like formations were to be 

 made along rivers, and lakes, and seas. Moreover, it 

 was necessary that much rock should be broken and 

 ground up, to make a sufficient quantity of fertile earth 

 for man and the accompanying races of animals. All 

 this was done by the agency of water in its two forms, 

 liquid and solid, as you will see as I proceed. 



390. Divisions. This period has been variously di- 

 vided. As the system is developed in this country, it 

 is divided by Dana into three epochs. First is the Gla- 

 cial, when, in the higher latitudes, the land was raised 

 much above its present elevation, and arctic cold pre- 

 vailed over a large portion of the earth from either pole 

 toward the equator, producing glaciers and icebergs. 

 The second is the Champlain^ so named because the 

 beds of this epoch are well developed at Lake Cham- 

 plain. In this epoch there was, in strong contrast with 

 the Glacial, a depression of the surface below its present 

 level, and there were deposits of three kinds river-bor- 

 der, lacustrine, and marine, or sea-border. As the land 

 sank down, the glaciers and icebergs melted, and the wa- 

 ters swept over the varied surface of the land, producing 



