104 GEOLOGY. 



are present there. Sometimes stones scale off from this 

 weathering process. In many cases this occurs in a very 

 regular manner. A beautiful example of this I found in 

 a boulder of greenstone nearly two feet in diameter. It 

 had a covering, in the form of a shell, of about a third of 

 an inch in thickness, of the same composition with the 

 boulder itself. Parts of this shell had been broken off. 

 but it was evident that it had been whole quite recently, 

 The first thought of any one that had no knowledge of 

 such results of weathering would be, that the boulder 

 had in some way received a coating of stone ; but this 

 could not in any way occur, and we are shut up to the 

 conclusion that this rind, as we may call it, of the boul- 

 der was separated from the body of the stone by the 

 weathering process. 



It is supposed that the famous Loggan, or rocking 

 stones, referred to in 100, Part I., have been shaped so 

 as to rock by this weathering. If a rock having a small 

 base rest upon another, the weathering will be apt to go 

 on quite rapidly on the under side, from the dampness 

 and the continued shade, and the base may after a while 

 become so narrow that the rock, though a large one, may 

 be easily moved back and forth upon it. 



188. Glaciers. Having spoken of the effects of water 

 in its liquid state, I pass now to consider those which 

 are produced by it in its solid condition. You will find 

 in another chapter that, great as these effects are at the 

 present time, they were vastly greater in ages of the 

 world long gone by. Both glaciers and icebergs had a 

 large agency in preparing some portions of the earth for 

 the use of man. 



A glacier is simply a river of ice. It flows down a 

 valley from the regions of eternal snow and frost above. 

 It does not merely slide, for the ice is somewhat plastic, 

 and so bends to accommodate itself to the different 

 widths of the valley and to uneven surfaces. The flow, 

 it is true, is very slow, but it is constant ; else the snow 



