AGE OF MAMMALS. 287 



operation of the chief of these circumstances is stated by 

 Dana after this manner. Every river has its channel 

 and its flood-plain, the latter being overflowed whenever 

 a freshet occurs, as stated in 174. Now if, when the 

 land was rising, the rise was chiefly in the interior of a 

 continent, the coast being little changed, the rivers run- 

 ning toward the sea would have their slope increased, 

 and so greater force would be given to the descending 

 water. The result of this would be, that not only would 

 the channel be deepened, but a part of the flood-plain 

 would be at a lower level. This is indicated in Fig. 170, 



v^ 



Fig. 170. 



where is the bed of the river, and b the flood-plain, in 

 the Champlain period. The change thus effected by the 

 erosive power of water is represented by the dotted line. 

 This would leave the river as seen in Fig. 171, with b for 



Fig. 171. 



the flood-plain, and a terrace, c. As soon as the river by 

 this process attained its former slope, this process would 

 stop. Then, if the same rising went on, another similar 

 result would occur. And so with a succession of risings, 

 or perhaps with one continued rising, several terraces 

 might be made, as represented in Figs. 168 and 169. 

 There are many circumstances which vary the results of 

 this process, and make the terraces irregular in height 

 and form. There are also other processes that have pro- 

 duced terraces, but these I will not stop to describe. 



398. Sea-beaches. I have spoken of the beaches which 

 were made in the Champlain epoch. When these are 

 found in the neighborhood of terraces they lie at a high- 



