216 FIFTEENTH LETTER ON GLACIERS. [1848. 



bottom force out a crease, or two or three successive parallel 

 creases, on either side ; and, I daresay, you recollect that one 

 effect of the great land-slip at Lyme, some years ago, was to 

 elevate a ridge of shingle above the level of the sea at some dis- 

 tance from the shore. A succession of equidistant wrinkles 

 will be formed whenever different parts of a plastic body are 

 subjected in succession to a pressure violent enough to produce 

 detrusion, or sliding amongst the particles. Thus, if a very 

 viscid fluid is poured in a gradual stream upon a flat surface, so 

 that it may spread uniformly, a succession of circular creases 

 is formed in consequence of the hydrostatic, pressure from the 

 heaped-up centre becoming sufficient to overcome, for a moment, 

 the viscosity at a certain distance from the centre ; as the cir- 

 cumference rises in a crease, the centre falls, the central pres- 

 sure suddenly diminishes, and if the stream continue to be 

 poured uniformly upon the centre, although the circles will 

 expand slightly, it will not be until a sufficient head is again 

 raised to overcome suddenly the viscosity of the fluid that a 

 new wrinkle is formed in exactly the same relative position as 

 the first.* Treacle, mortar, tar, and similar bodies, usually 

 present such creases when poured out. What has now been 

 said of a viscid mass spreading uniformly from a centre applies 

 equally to one confined in a trough with parallel sides, if con- 

 stantly fed from one end. A succession of waves are thus 

 formed, as in Mr. Milward's mud-slides, or as 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 external 

 circumstances, perfectly uniform. The intervals of such waves 

 depend, in these cases, solely upon the physical qualities of 

 tenacity, specific gravity, etc., of the body, and the more or less 



* The steps of this process are attempted to be illustrated by the curves in Plate 

 IX. fig. 4. 



