684 NOTES TO BOOK XVIII. 



right angles to the crevasses, near the sides of the glacier, 

 while, near its central line, they dip towards the upper 

 part of the glacier. This structure appears to arise from 

 the difference of velocities of contiguous moving filaments 

 of the icy mass, as the crevasses themselves arise from 

 the tension of larger portions. Mr. Forbes has, in suc- 

 cessive publications, removed the objections which have 

 been urged against this theory. In the last of them, a 

 Memoir in the Phil. Trans., 1846, (Illustrations of the 

 Viscous Theory of Glacier Motion,) he very naturally ex- 

 presses astonishment at the opposition which has been 

 made to the theory on the ground of the rigidity of small 

 pieces of ice. He has himself shown that the ice of 

 glaciers has a plastic flexibility, by marking forty-five 

 points in a transverse straight line upon the Mer de 

 Glace, and observing them for several days. The straight 

 line in that time not only became oblique to the side, but 

 also became visibly curved. 



Both Mr. Forbes and other philosophers have made it 

 in the highest degree probable that glaciers have existed 

 in many places in which they now exist no longer, and 

 have exercised great powers in transporting large blocks of 

 rock, furrowing and polishing the rocks along which they 

 slide, and leaving lines and masses of detritus or moraine 

 which they had carried along with them or pushed before 

 them. It cannot be doubted that extinct glaciers have pro- 

 duced some of the effects which the geologist has to endea- 

 vour to explain. But this part of the machinery of nature 

 has been worked by some theorists into an exaggerated 

 form in which it cannot, as I conceive, have any place in 

 an account of geological dynamics which aims at being 

 permanent. 



