144 Professor Forbes's Fifteenth Letter on Glaciers, 



eiFects of atmospheric and other action. . . It is, there- 

 fore, to the head of the glacier, where the true glacial struc- 

 ture commences, that we are to look for such ridges : the 

 best time, also, will be at the commencement of summer, 

 after the disappearance of the snow, and before the confusion 

 of the surface occasioned by the sun's influence."* In point 

 of fact, it will be found, from the reference above, that the 

 glacier wrinkles were observed on the Glacier du Geant, at 

 the upper part of the Mer de Glace ; — and that the season 

 was that of the disappearance of the winter snow, which lay 

 in wreaths between the ridges, thus perfectly defining their 

 contour. 



With respect to the origin of these wrinkles in the Mud 

 Slide, Mr Milward has, I think, very justly, rejected the ex- 

 planation of them formed on a supposed alternation of beds or 

 strata of difl^erent texture, the existence of such beds being 

 entirely hypothetical, and considering the manner in which 

 such a mud- slide is formed, utterly improbable. In looking 

 for an explanation of the analogous phenomena in glaciers, 

 it would, therefore, be wise to try to find one which should 

 hold good in a mass of uniform consistence, and rather to 

 look for the less compact structure of the ice beneath the 

 dirt-bands as an effect of the same cause which produces the 

 wrinkles, than as the cause itself. I believe that the pheno- 

 mena of ridges or wrinkles is a general one, depending on 

 the toughness of a semifluid or semisolid mass forcibly com- 

 pelled to advance or extend itself; that the periodicity, or 

 repetition, of the wrinkles at nearly regular intervals, is due 

 to mechanical causes alone, and to no variation of internal 

 consistence. 



Having successfully imitated these wrinkles in my experi- 

 ments, I think that I am able so far to account for them. 

 Although neither mud nor plaster is capable of retaining the 

 internal veined structure of the frontal dip, which bears 

 evidence of the direction of the slide being such as I have 

 stated. That evidence is to be found in the glaciers them- 

 selves, in certain cases of lava streams, which I have else- 



* Phil. Trans., 1846. 



