146 Professor Forbes's Fifteenth Letter on Glaciers, 



in a glacier. They become confounded or not at a distance 

 from the origin ; that depends entirely on the rate of motion 

 of the stream at different points, which again depends chiefly 

 upon the declivity of its bed. 



These wrinkles or creases, then, do occur at regular inter- 

 vals, even in bodies perfectly homogeneous, and, under ex- 

 ternal circumstances, perfectly uniform. The intervals of 

 such waves depend, in these cases, solely upon the physical 

 qualities of tenacity, specific gravity, &c., of the body, and 

 the more or less ample stream which furnishes it. We per- 

 ceive here nothing like an annual recurrence ; and this cir^ 

 cum stance at first puzzled me, the intervals between the 

 dirt-bands of the Mer de Glace (which are evidently the same 

 with the wrinkles) being, as observed by me at the very time 

 of their first discovery, so nearly consistent with what I sup- 

 posed to be the annual motion of the Ice Stream, and which 

 was afterwards confirmed by direct experiment, as scarcely 

 to allow us to suppose the coincidence fortuitous. 



But an easy experiment establishes the analogy perfectly. 

 If the stream of plastic matter already supposed be not uni- 

 formly supplied, but arrive in gushes, every such overflow, by 

 the rapid rise of the head, throws off" a wrinkle in the most 

 regular manner. So that, for example, on examining those 

 plaster models, formerly repeatedly described by me, in which 

 cupfuls of white and of blue plaster of Paris were alternately 

 poured down an inclined channel, each separate flow was 

 found to constitute a wave or crease. In a glacier, especially 

 in its higher regions, the diff*erence of summer and of winter 

 velocity is sufficient to produce what may be called (relative- 

 ly) a gush ; and I suppose that the wrinkles are formed in 

 most glaciers at the foot of the steeps of the neve (as Mr 

 Milward also believes), where a pressure a tergo is produced 

 by the heat of the short summer, suflicient to overcome the 

 incalculable resistance which a mass of half-fi*ozen snow, 

 hundreds of feet thick and hundreds of yards wide, presents, 

 to be squeezed and moulded, after the manner of a semi-fluid, 

 into a convex wrinkle. Of ihefact there is no doubt. Each 

 wrinkle, then, is nothing else than a local swelling, such as 

 those figured by M. Collin, taking place at the moment when 



