AGE OF MAMMALS. 281 



is called drift. It is various in its composition, the ma- 

 terial being sand or gravel, or boulders of various sizes. 

 Sometimes the boulders are mingled with the sand and 

 gravel, and sometimes they are separate. When the ma- 

 terials of drift are in manifest layers, and to some extent 

 sorted, the finer and -coarser alternating, and the frag- 

 ments are rounded and smoothed, it is called modified 

 drift. Water has in this case acted upon the materials, 

 and laid them in beds. Now of all this material which 

 we call drift, none was produced where it lies, but it 

 was transported to its localities, and for the most part 

 from the north toward the south. There are two theo- 

 ries in regard to its transportation the one attributing 

 the result to glaciers, and the other to icebergs called 

 respectively the glacier and the iceberg theory. But the 

 facts show conclusively that neither could alone accom- 

 plish all the work, and that both must have been more or 

 less in operation. The drift must have produced great 

 changes on the surface, filling up valleys here and there, 

 making lakes to overflow, and altering the courses of riv- 

 ers. A marked instance we have of the latter change 

 in the case of the Niagara River. There is decisive evi- 

 dence that the bed which it flowed in, from the whirl- 

 pool onward, until the Glacial epoch was then filled up 

 with drift, and the water opened for itself a new gorge 

 through solid rock, through which it has run to the pres- 

 ent time. The drift is by no means confined to the val- 

 leys and plains, but extends high up on the sides of 

 mountains, and is found even on the tops of some of 

 them, as Mount Holyoke and Mount Tom, in Massachu- 

 setts. It is at the height of 2000 feet in the Green 

 Mountains, 3000 on Monadnock, and even 6000 on 

 Mount Washington. 



393. Boulders. These are angular, or more or less 

 rounded, according to the amount of friction to which 

 they have been subjected by water and other stones. 

 They vary much in size, and while they commonly do 



