AGE OF MAMMALS. 283 



two enormous icebergs float side by side slowly over 

 that region, dropping their freight of stones as they 

 melted away? This is the only supposition we can 

 make, but this is not at all satisfactory. 



394. Glacial Markings. In speaking of the effects of 

 glaciers in 190, I referred to various markings made 

 by them on the rocks of their beds. Similar markings 

 are found on the sides of mountains, and on the faces of 

 rocks where the glaciers and icebergs of the Post-ter- 

 tiary age moved along. We find them every where in 

 the tracks of boulders. There is great variety in them. 

 Sometimes there are deep furrows, and sometimes mere 

 scratches. Sometimes there are lines as delicate as if 

 made by the tool of an engraver, and sometimes the sur- 

 face of a stone susceptible of polish is as smooth as it 

 would be if it were artificially polished in the shop of 

 a manufacturer. Sometimes rocky ridges have been 

 rounded and smoothed, giving them an undulating ap- 

 pearance, like the roches montonnees, or embossed rocks 

 seen in the Alps, the result of the action of glaciers of 

 the present time, as represented in Fig. 42. It is evi- 

 dent that most of these markings must have been made 

 by glaciers, and not by icebergs. While icebergs can 

 plow up dirt, and sand, and gravel, and crush rocks, it 

 requires the firm, slow, steady movement of the glacier, 

 holding its rocky tools imbedded in the ice, to make the 

 regular parallel grooves and lines, and the polished sur- 

 faces. And, in proof of the correctness of this view, we 

 find a striking resemblance between the effects of mod- 

 ern glaciers on the rocks and those marks which appear 

 where the glaciers of the Post-tertiary age were supposed 

 to be. Still there is some coarse and irregular work, as 

 we may term it, which must have been done by icebergs, 

 as, for example, the making of furrows and valleys on 

 the summits of some of the lesser mountains. 



The glacial markings are found abundantly on high 

 mountains as well as in valleys and on plains. On the 



