Professor Forbes^s Fifteenth Letter on Glaciers. 141 



original declivity of forced earth, denoted by the dotted 

 line, remodelled by the slide over the surface DFE into 

 the bulged form FGH, vs^hich recalls at once the termi- 

 nal section of a glacier. In fig. 2, again, we have a simi- 

 lar phenomenon, observed at Cercey in Burgundy, where 

 the mass has been more solid, the swelling of the sur- 

 face less continuous, and transverse crevasses, exactly like 

 those of a glacier, have opened. The length of the talus or 

 declivity, before sliding, was about 26 metres, or 85 English 

 feet, in the first case, and 24 metres, or 79 English feet, in 

 the second. M. Collin also measured daily, for more than 

 two months, the horizontal advance of the lower extremity 

 of the earth-slide of Cercey, and likewise tlie perpendicular 

 fall of its upper extremity. These results, which are the 

 only ones of the kind which I have met with, are highly in- 

 teresting, as shewing the continuity and general regularity 

 of this very small motion in a mass which could not be called 

 fluid, in any ordinary sense of the word, since we are told 

 that there was not the slightest trace of any exudation 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 ob- 

 servations on a subsequent slip.t It follows, therefore, as a 

 mathematical necessity, that the central parts of the slip 

 being thus compressed, must either have discharged them- 

 selves laterally, or been heaped up vertically. An inspection 

 of the change of figure of the displaced matter FGH, in 

 ^g. 1, which originally had the section EADF, plainly shews 

 that the loosened earth was heaped up by the frontal resist- 



* Collin, Glissements spontanea, p. 50. f Ibid., p. 54. 



