The Theory of Glacier motion. 113 



so unequal, carried out on slopes of different inclination, on 

 ground studded with irregularities, and in channels whose 

 width varies at every moment. This is in my opinion the 

 least explicable of the phenomena of glaciers. Does it 

 advance in a mass like a block of marble on an inclined 

 plane? Does it advance in broken bits like the stones 

 which come down one after another in mountain gullies ? 

 Does it sink down upon itself to flow along the slopes^ as 

 lava would do, at once ductile and liquid ? Do the portions 

 which detach themselves at the edges of steep slopes 

 suffice to impress motion upon those which repose upon a 

 horizontal surface ? I know not. Perhaps, again, we might 

 say that in times of great cold the water which fills the 

 numerous transverse crevasses of the glacier becomingfrozen, 

 receives its accustomed movement of volume, gives a push 

 to its containing walls, and thus produces a motion towards 

 the bottom of the channel in which it flows " {id., 80 — 82). 

 Lastly, he says : " there is between the Glacier des Bois and 

 a river a resemblance so complete that it is impossible to 

 find in the glacier a circumstance which does not exist in 

 the river. In currents of water the velocity is not uniform 

 throughout their width, nor throughout their depth ; the 

 friction of the bottom, that of the sides, the action of 

 obstacles cause a variation in the velocity, which is undi- 

 minished only towards the middle of the surface. Now the 

 mere inspection of the glacier is sufficient to prove that the 

 velocity of the centre is greater than that of the sides. The 

 whole surface is cut by crevasses, which are in general 

 transverse to its direction. If the motion were the same 

 throughout the mass these crevasses which cut the surface 

 in parallel rifts would form a straight line which would 

 be always nearly perpendicular to the two banks, but this 

 is not so : the general line is a curve whose convexity advances 

 towards the bottom of the valley, a fact which can only be 

 attributed to the greater velocity of the ice at this poiitt" 

 {id., 85—86). 



