266. The same remarks apply to various portions of the Mer de Glace. At certain places the inclination. changes from a gentler to a steeper slope, and on crossing the brow between both the glacier breaks its back. Transverse crevasses are thus formed. There is such a change of inclination opposite to the Angle, and a still greater but similar change at the head of the Glacier des Bois. The consequence is that the Mer de Glace at the former point is impassable, and at the latter the rending and dislocation are such as we have seen and described. Below the Angle, and at the bottom of the Glacier des Bois, the steepness relaxes, the crevasses heal up, and the glacier becomes once more continuous and compact.

§ 40. Marginal Crevasses.

267. Supposing, then, that we had no changes of inclination, should we have no crevasses? We should certainly have less of them, but they would not wholly disappear. For other circumstances exist to throw the ice into a state of strain, and to determine its fracture. The principal of these is the more rapid movement of the centre of the glacier.

268. Helped by the labours of an eminent man, now dead, the late Mr. Wm. Hopkins, of Cambridge, let us master the explanation of this point together. But the pleasure of mastering it would be enhanced if we could see beforehand the perplexing and delusive appearances

Bit. Dropny dvi im dhe age to the bottom a dhe theks vẻ shoni breast the opposite mountain, Lī viù de fer Emestone peties of the Wetter11 l our ki, we should get ace a path which comTLDOS & THW off the glacier. Here we should see berfil examples of the opening of crevasses at the summa of a brow, and their dosing at the bottom. Bm the chief point of interest would be the crevasses imed at the side of this glacier-the marginal crevasses, as they may be called.

270. We should find the side copiously fissured, even at those places where the centre is compact; and we should particularly notice that the fissures would neither run in the direction of the glacier, nor straight across it, but that they would be oblique to it, enclosing an angle of about 45 degrees with the sides. Starting

from the side of the glacier the crevasses would be seen to point upwards; that is to say, the ends of the fissures abutting against the bounding mountain would appear to be dragged down. Were you less instructed than you now are, I might lay a wager that the aspect of these fissures would cause you to conclude that the centre of the glacier is left behind by the quicker motion of the sides.

271. This indeed was the conclusion drawn by M. Agassiz from this very appearance, before he had measured the motion of the sides and centre of the glacier of the Unteraar. Intimately versed with the treatment of mechanical problems, Mr. Hopkins immediately deduced the obliquity of the lateral crevasses from the quicker flow of the centre. Standing beside the glacier with pencil and note-book in hand, I would at once make the matter clear to you thus.

272. Let a c, in the annexed figure, be one side of the glacier, and B D the other; and let the direction of

transverse slice of the glacier, taken straight across it,

say to-day. A few days or weeks hence this slice will have been carried down, and because the centre moves more quickly than the sides it will not remain straight, but will bend into the form s′ T'.

273. Supposing Ti to be a small square of the original slice near the side of the glacier. In its new position the square will be distorted to the lozengeshaped figure r'". Fix your attention upon the diagonal Ti of the square; in the lower position this diagonal, if the ice could stretch, would be lengthened to T′ ï. But the ice does not stretch; it breaks, and we have a crevasse formed at right angles to ri. The mere inspection of the diagram will assure you that the crevasse will point obliquely upwards.

274. Along the whole side of the glacier the quicker movement of the centre produces a similar state of strain; and the consequence is that the sides are copiously cut by those oblique crevasses, even at places where the centre is free from them.

275. It is curious to see at other places the transverse fissures of the centre uniting with those at the sides, so as to form great curved crevasses which stretch across the glacier from side to side. The convexity of the curve is turned upwards, as mechanical principles declare it ought to be. (See sketch on opposite page.) But if you were ignorant of those principles, you would never infer from the aspect of these curves the quicker motion of the centre. In landslips, and in the motion

of partially indurated mud, you may sometimes notice appearances similar to those exhibited by the ice.

§ 41. Longitudinal Crevasses.

the origin of both But where a glacier

276. We have thus unravelled transverse and marginal crevasses. issues from a steep and narrow defile upon a comparatively level plain which allows it room to expand laterally, its motion is in part arrested, and the level portion has to bear the thrust of the steeper portions behind. Here the line of thrust is in the direction of the glacier, while the direction at right angles to this