AGE OF MAMMALS. 285 



changes in them. It is difficult, in many cases, to distin- 

 guish the deposits of this period from those of the suc- 

 ceeding one, but it may be remarked in the general that 

 the river-plains and sea-beaches of the Champlain epoch 

 are at the present day elevated plains and beaches that 

 is, existing at higher levels than the terraces made in the 

 succeeding period. 



396. Terrace Epoch. As you have seen, there have 

 been, in the course of the formation of the continents, 

 many extensive subsidences of land, letting the water 

 prevail for a time where it had been dry land before. 

 The last of these general submergences occurred in the 

 Champlain epoch. In the next epoch, the Terrace pe- 

 riod, the movement of the land was itpicard a move- 

 ment of emergence which went on until the land ac- 

 quired a comparatively settled condition that is, one 

 from which it has not varied, to any great degree, since 

 the advent of man. During this upward movement of 

 the Terrace epoch there was, to a considerable extent, a 

 rearrangement of the strata laid down in the Champlain 

 epoch. In this rearrangement the formation of terraces 

 was so common that this has given the name to the pe- 

 riod. The term terrace is applied to banks of loose ma- 

 terials skirting the sides of valleys about rivers and 

 lakes, having a level surface on the top, and fronting on 

 the river or lake with a more or less steep escarpment. 

 As they are often quite numerous, rising above each 

 other like the seats of an amphitheatre, but differing 

 much in width and in the lines of their edges, they give 

 great variety and beauty to the scenery, especially when 

 the habitations of men are built upon them, which is oft- 

 en done, or when, as is sometimes the case, they are used 

 as the dwelling-places of the dead. Not only do they 

 vary in width and in line of edge, but the variety is oft- 

 en increased by the action of rain with the currents 

 caused by it on their level-topped surfaces. You see in 

 Figs. 168 and 169 (p. 286) sections of two series of ter- 



