212 FIFTEENTH LETTER ON GLACIERS. [1848. 



of water from the reservoir, of which the mass in question 

 formed the embankment, and that " the absence of continued 

 rain during the period of observation, singularly favoured the 

 regularity of the descent." But the best proof of the solidity 

 of the material (a clayey soil, near the canal of Burgundy) is, 

 that it admitted of being cut to permanent slopes of 30, and 

 even of 45. The amount of horizontal movement of the lower 

 end of the land-slip increased gradually during the first three 

 weeks, and soon after ceased entirely ; but the top of the 

 slip continued to move during the whole continuance of the 

 observations. This fact was confirmed by independent observa- 

 tions on a subsequent slip.f It follows, therefore, as a mathe- 

 matical necessity, that the central parts of the slip being thus 

 compressed, must either have discharged themselves laterally, 

 or been heaped up vertically. An inspection of the change of 

 figure of the displaced matter FGH, in fig. 1, which originally 

 had the section EADF, plainly shews that the loosened earth 

 was heaped up by the frontal resistance near H. that the 

 posterior parts of the mass overrode, the anterior ones ; in short, 

 gave rise to the upward and forward internal sliding motion, to 

 which I ascribed, in the glacier, the phenomenon of the frontal 

 dip of the veined structure (Travels in the Alps, 2d edit. p. 164). 

 This condensation or swelling (boursoufflement] was noticed by 

 M. Collin as characterising earth-slides, and the actual " ascen- 

 sional movement" of the parts, due to the ^wm-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 

 effect. In these observations, the daily motion of the terminal 

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

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

 motion seems to have been the result of a sudden concussion ; 

 the steadiest motion was from half an inch to four inches daily. 



* Collin, Glissements spontane?, p. 50. f Ibid, p. 54. 



t Ibid, p. 47, and Plate XI. 



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



