142 Professor Forbes's Fifteenth Letter on Glaciers. 



ance near H, — that the posterior parts of the mass over- 

 rode the anterior ones ; — in short, gave rise to the uprvard 

 and forward internal sliding motion, to which I ascribed, in 

 the glacier, the phenomenon of the frontal dip of the veined 

 strticture {Travels in the Alps, 2d edit., p. 164). This conden- 

 sation or swelling {boursoufflement) was noticed by M. Collin 

 as characterising earth-slides, and the actual " ascensional 

 movement" of the parts, due to the ^M«s2-hydrostatic pressure, 

 when a solid obstacle resisted the progress of the stream, is 

 also admitted by him.* Of course, a mass of the mud or 

 earth, of sufficient weight to produce an intense friction on 

 a level or on a small declivity, would produce the same ef- 

 fect. In these observations, the daily motion of the termi- 

 nal part of the land-slip varied from one-hundredth of a 

 metre (0*4 inch) to a metre and one-third (4^^ feet), but this 

 last motion seems to have been the result of a sudden con- 

 cussion ; the steadiest motion was from half an inch to four 

 inches daily. 1 



Having now attempted to do justice to M. Collin's inte- 

 resting observations, I pass on to the papers of Mr Milward. 

 This gentleman, being acquainted generally with the analo- 

 gies lately attempted to be established between viscous bo- 

 dies and glaciers, at once directed his attention to the pecu- 

 liarities of surface of the great mud-slide which he witnessed 

 at Malta. In the stream of the first slide he observed, un- 

 der a favourable light, curved bands, alternately dark and 

 light coloured, which, like their analogues, the dirt-hands of 

 glaciers, are best seen from a height, and when the light falls 

 obliquely. On close inspection, these bands were found to 

 be composed (superficially) of smooth, fine mud, and of rough, 

 coarse mud alternately, the latter being somewhat the higher 

 of the two. In a second case of a mud-slide, he found that the 

 smoothness of the mud was a superficial phenomenon due to 



* Collin, Glissements spoDtan^s, p. 47, and Plate XI. 



t My friend Mr John Thomson (of Glasgow), in a short note with which he 

 has favoured me, states that, in his experience of making railway embankments 

 (in Leicestershire), he has found concentric waves or wrinkles pressed out of 

 the soft clay of the embankment, in proportion as the load of earth increases. 



