242 Professor Forbes' s Sixth Letter on Glaciers. 



for reasons which I have elsewhere stated, but which need 

 not now be adverted to. My present object is to shew, that 

 the rigidity of ice, as a physical fact, cannot contradict the 

 mathematical evidence of the manner in which glaciers do 

 move, and that the seeming contradiction is reconciled by 

 shewing, that the ice bears permanent traces of the violent 

 strain to which it is subjected, and of the actual bruising and 

 disseverment of its parts, producing a phenomenon otherwise 

 impossible to be explained. 



I believe that it is during the progress of the glacier thus 

 subjected to a new and peculiar set of forces depending upon 

 gravity, and which remodel its internal constitution, by sub- 

 stituting hard blue ice, in the form of veins, for its previous 

 snowy texture, that the horizontal stratification observed in 

 the higher part of the glacier or neve^ is gradually obliter- 

 ated. 



If, as we cannot doubt, the slower motion of the glacier 

 near its sides be owing to the retardation which their exces- 

 sive friction occasions, there must necessarily be a retardation 

 at the bottom in a similar manner, and the surface of the 

 glacier will move faster than the strata in contact with the 

 ground ; to which it is even supposable, that, in some cases, 

 they may be entirely frozen. This retardation may, perhaps, 

 be less than the lateral retardation, because the slope of the 

 valley in which the glacier lies is probably more even, gene- 

 rally speaking, than its breadth is regular. In fact, so great 

 is the irregularity of the ground-plan of any compound valley, 

 — so frequent the interfering ridges or promontories, the bays 

 formed by adjoining tributary valleys, — and so numerous the 

 gorges or contractions, — that we cannot so properly call the 

 lateral resistance to the onward motion of a glacier, friction, 

 but rather a direct opposition to the exit of a solid body, 

 which renders its plasticity absolutely essential to its progres- 

 sion. Nevertheless, the inferior slope of the glacier bed being 

 also irregular, and its friction great, must cause a retardation 

 in the lower strata of ice, which must be continually over- 

 taken by the superior ones : and this appears to me to be so 

 plain and necessary a consequence of the combination of facts 

 which we have to consider, that perhaps the direct proof of it 



