Mr. Hopkins on the Motion of Glaciers. 1 3 



face of the bed to be smooth, in the sense in which the term 

 may be applied to a slab rough-hewn for a pavement. I have 

 already explained (art. 4.) that this action is extremely small 

 when the velocity = 0, but increases rapidly with the velocity 

 till it becomes equal to the moving force of gravity along the 

 inclined plane; and thus it is that while the action of the bed 

 cannot prevent motion altogether, it never allows the velocity 

 to exceed a very small value. If, however, the bed of the gla- 

 cier should present sharp asperities of surface, such as would 

 penetrate into the solid mass, the action of these penetrating 

 points would no longer be like that here contemplated, and 

 might be much greater than the action of a more even surface. 

 Its eifectiveness in arresting the motion would depend almost 

 entirely on the weight of the mass and inclination of the plane. 

 If the weight were small, as in an experiment, comparatively 

 few projecting points, penetrating but little into the mass, might 

 be sufficient to destroy the motion, while, if the mass were in- 

 creased so as to bear any analogy in this respect to the case 

 of a glacier, the retarding effect might be entirely inappreci- 

 able. In estimating the effect of local obstacles, acting on the 

 sides or bottom of a glacier, to destroy its motion, the enor- 

 mous weight of the glacier must always be borne in mind. 



With respect to the bottoms of glacial valleys, however, I 

 conceive it to be impossible that they can present those local 

 asperities of surface which, if they existed generally, would be 

 most likely to prevent the sliding motion of a glacier; for how 

 could the hardest rocks resist for thousands of years the un- 

 ceasing efforts of infinitely the most powerful polisher that 

 nature has put in action ? If it be said that this argument as- 

 sumes the point at issue — the sliding of glaciers — I reply that, 

 of their sliding more or less, we have the most incontrovertible 

 evidence, as every one must allow who has examined the roches 

 polies at the end of a glacier, or along the continuations of ex- 

 isting glacial valleys. Nor must we limit the grinding and 

 polishing power of former glaciers by that of existing ones, 

 for the glaciers of the Alps must, at some former epoch, have 

 been, probably, three or four times as thick as they are at pre- 

 sent, varying perhaps from 1000 to 3000 feet for great por- 

 tions of their length. I venture therefoi'e to assert, without 

 fear of contradiction, that the surfaces of the rocks forming the 

 beds of glaciers must necessarily be free from such asperities 

 of surface as are here contemplated as most likely to impede 



the slidinfj of a glacier. 



... 

 Again, we may observe that it is not necessary, accordmg 



to this view of the subject, that the surface of a glacial valley 



should either be a plane surface, or should consist of a series 



of plane surfaces. If a chain of given length be placed along 



