96 



a bottom bedding to the ice, and if this latter theory proved to be 

 true we might here have the origin of many of the rocks in the 

 boulder clay. 



We may perhaps take it for granted that an ice cap causes 

 immense denudation, but it may not be so easy to describe the 

 Jtiodus operandi. In what form is the material ground off? We 

 cannot examine the foot of the Greenland land ice, but we can do 

 this with the more accessible Swiss glaciers, from the end of which 

 usually rushes a thick turbid stream, holding in suspension a large 

 amount of mineral matter. We may perhaps suppose that as the 

 ice crushes over the rocks, grinding off infinitesimal portions, most 

 of the debris is carried away by the streams which it is thought are 

 at least not uncommon beneath the ice ; or if the theory were true 

 that stones rise in glaciers, why should not the impalpable powder, 

 which has not lost its specific gravity by grinding, be pressed by 

 the weight of the superincumbent ice into various interstices, and 

 rise slowly to the surface ; but this supposition is negatived by the 

 crystal clearness of the ice, wherever examined beneath the surface. 

 It might be that the worn off material accumulates between the ice 

 and the rock in the form of a fine mud. This mud might to a limited 

 extent be collected in the hollows or places where temporarily the 

 main pressure might not be, but would always be liable to be 

 carried forwards and downwards, never remaining permanently 

 stationary. It scarcely seems possible that this layer of mud could 

 accumulate to any thickness. If such were the case the grinding 

 action would cease, as the mud would protect the subjacent rock. 

 A greater difficulty even is to suppose conditions which would 

 allow of ice two thousand, or even two hundred, feet thick, resting 

 on soft clay and yet not scouring it out, so that the ice should once 

 more rest on its rocky bed. This last proposition may sound 

 plausible, but if we grant it, shall we not, of necessity, have to 

 reject the prevalent opinion that boulder clay was formed beneath 

 land ice, though it is very difficult to picture circumstances when 

 such a formation could take place. The boulder clay is, however, 

 an actual fact, and the more mysterious its origin, the more in the 

 nature of things shall we unceasingly attempt to solve its presence. 



