1848.]

ANALOGIES DERIVED FROM LAND-SLIPS.

211

years ago, I obtained a French work, entitled, "Recherches Expérimentales sur les Glissements spontanés des terrains argileux, par Alexandre Collin, Ingénieur des Ponts et Chaussées." Paris, 1846, 4to. This interesting work, illustrated by plates, contains no allusion to the subject of glaciers;—the phenomena of mud-slides being considered solely in an engineering point of view. The principal object of the work is to investigate the form of the surface of sliding, which separates the solid from the moving soil of railway cuttings, embankments, and the like. That subject is not particularly connected with the one before us; but M. Collin has, at the same time, presented us with excellent and detailed sections of land-slips, the mere inspection of which recalls forcibly the outline of glaciers, and, although evidently unaware of my theory of the latter, his remarks confirm, in a very satisfactory way, several of my anticipations respecting the internal movements of viscous bodies. Thus, in the transverse section of an embankment of the Paris and Versailles Railway (Rive gauche) [copied in Plate IX. fig. 1., of the present volume], we find the original declivity of forced earth, denoted by the dotted line, remodelled by the slide over the surface DFE into the bulged form FGH, which recalls at once the terminal section of a glacier. In fig. 2, again, we have a similar phenomenon, observed at Cercey in Burgundy, where the mass has been more solid, the swelling of the surface 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 the perpendicular fall of its upper extremity. These results, which are the only ones of the kind which I have met with, are highly interesting, 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 observations on a subsequent slip. It follows, therefore, as a mathematical 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 "ascensional movement" of the parts, due to the quasi-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 (4 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 spontanés, p. 50.

Ibid, p. 47, and Plate XI.

† Ibid, p. 54.

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

1848.]

MR. MILWARD'S OBSERVATIONS.

213

Having now attempted to do justice to M. Collin's interesting observations, I pass on to the papers of Mr. Milward.* This gentleman, being acquainted generally with the analogies lately attempted to be established between viscous bodies and glaciers, at once directed his attention to the peculiarities of surface of the great mud-slide which he witnessed at Malta. In the stream of the first slide he observed, under a favourable light, curved bands, alternately dark and light coloured, which, like their analogues, the dirt-bands 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 the settling of the more fluid part in slight depressions existing between "the rough bands, which were raised from a foot-and-a-half to two feet, so as to form ridges, or waves, or wrinkles, swelling and falling over." The sketch given [by Mr. M.] of these "wrinkles" is shewn in fig. 3 of Plate IX.

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. [In the course of the last ten years I have continued to receive numerous additional confirmations from several correspondents.]

* [In the Edinburgh Philosophical Journal for January 1849.]