250 Professor Forbes on the Leading Phenomena of Glacier^^ 



great portion of the icy mass is still plastic. It is, however,- ex- 

 tremely probable that the congelation extends to a considerar 

 ble depth, and produces the usual effects of expansion. I think, 

 however, that the explanation, though correct so far as it goes, 

 is inadequate, and that the main cause of the restoration of the 

 surface is the diminished fluidity of the glacier in cold weather, 

 which retards (as we know) the motion of all its parts, but es- 

 pecially of those parts which move most rapidly in summer. 

 The disproportion of velocity throughout the length and 

 breadth of the glacier is therefore less, the ice more pressed 

 together, and less drawn asunder ; the crevasses are consoli- 

 dated, while the increased friction and viscosity causes the whol^ 

 to swell, and especially the inferior parts, which are the most 

 wasted. Such a hydrostatic pressure, likewise, tending to 

 press the lower layers of ice upwards to the surface, may not 

 be without its influence upon the (so-called) rejection of blocks 

 and sand by the ice, and may even have some connection with 

 the recurrence of the " dirt bands" upon the surface of the 

 glacier. But I forbear to enlarge upon what is only as yet to 

 myself conjectural. 



I have no doubt, however, that the convex surface of the 

 glacier (which resembles that of mercury in a barometer tube,) 

 is due to this hydrostatic pressure acting upwards with most 

 energy near the centre. It is the " renflement" of Rendu, 

 the " surface bombee" of Agassiz. Exactly the contrary is 

 the case in a river, where the centre is always lowest ; but that 

 is on account of the extreme fluidity, so that the matter runs 

 off" faster than it can be supplied ; but in my plaster models, 

 this convexity, with its wrinkles and waves, was perfectly imi- 

 tated. 



In its bearing on the theory of the former extension of the 

 Swiss glaciers, we find, that the doctrine of semifluid mo- 

 tion leads us to this important conclusion, — that as large 

 and deep rivers flow along a far smaller inclination than small 

 and shallow ones (a circumstance depending mainly upon the 

 weight increasing with the section, and the friction, in this 

 particular case, with the line of contact with the channel), the 

 most certain analogy leads us to the same conclusion in the 



