218 Dr. Whe well's Additional Remarks on Glacier Theories. 



The parts which were most free to move would exert tensions 

 upon the rest, and would tend to produce fissures. At first, the 

 theorist would attend only to the direct cohesion of the mass. 

 If this were overcome, there would be transverse fissures; and 

 these would be the only results of the first form of the hypo- 

 thesis. But it is evident in glaciers, that the middle moves 

 faster than the sides. It would be necessary to make some 

 provision for this difference in the hypothesis. The first sup- 

 position might be, that the glacier is divided by arbitrary lon- 

 gitudinal separations into strips of finite breadth ; and that 

 these move past each other. But on this supposition, original 

 transverse lines on the glacier would be thrown e?i echellon in 

 definite portions. This evidently not being the case in fact, the 

 theorist might remodel his view. He might begin to see, that 

 besides the separation of a mass by overcoming the direct co- 

 hesion, there may be separation by sliding of one part past 

 another. He would then be led, by simple mechanical views, 

 to infer that the cohesive force of the mass will yield to such 

 sliding tendency, in directions perpendicular to the directions 

 of the transverse fissures; and as the mass is still coherent after 

 sliding, there will be in it bands, which are the traces of the 

 sliding lines. If this process take place over the whole breadth 

 of the glacier, we are brought very near the plastic view. On 

 this supposition, the differences of velocity of the different parts 

 of a transverse line will be much the greatest near the sides, 

 as they are in a river, and for the same reason. Hence any 

 attempt to calculate the relative motion of the parts of the 

 glacier would lead to the conclusion, that the flanks, at least, 

 must be plastic. If the conception of the change of a plastic 

 body, as requiring time to its performance, were new to the 

 theorist, and had been slowly arrived at, he might, with refer- 

 ence to these circumstances, term it secular plasticity : but this 

 epithet, as applied to the property itself, is superfluous, since 

 it merely expresses a necessary condition of all plasticity ; and 

 inappropriate, when applied to a process which takes place in 

 a few days in a conspicuous manner. 



When we have gone so far towards the plastic view of gla- 

 ciers, there still remains a point in which that view may be 

 resisted. It may be said, that though plastic when considered 

 horizontally, they are not plastic when considered vertically ; 

 that though the middle moves faster than the sides, the top 

 does not move faster than the bottom. And this assertion 

 would be difficult to disprove by any experiments made from 

 the upper surface of a glacier, because, just as the greatest dif- 

 ferences of velocity are greatest near the sides, so are they 

 greatest near the bottom ; and the upper and central portion 



