WCP4977

[1] [p. 247]

HOLLY HOUSE, BARKING,

25th September 1870.

James Croll, Esq.

DEAR SIR, — Thanks for your paper on "The Cause of Motion in Glaciers,"¹ which I have read with much interest, though I cannot say I am quite satisfied your explanation is the correct one. The fact that glaciers do move in winter when thickly covered with snow, seems entirely to upset your theory of molecular changes caused by heat, as it does that of Canon Moseley¹. It seemed to me when I read Canon Moseley's paper² that his first error was in considering that the fractures of ice in a glacier are [2] [p. 248] shearing at all, in his sense of the word. If I take a piece of ice A, [a sketch of a glacier with fracture lines appears here] and by contrary and parallel pressure at a and b cause it to fracture all along the line a b simultaneously, so that one half of it is moved into the position a¹ b¹, I exert the shearing force as understood by Canon Moseley; but if I take the same or a similar piece of ice B, [another sketch of a glacier with fracture lines appears here] and by pressure at c with b as a fulcrum cause a fracture on the line a b, bringing the outer half into the position a¹ b¹, will it not require a very much less than the shearing force? Now the uneven bottom and sides of a glacier must supply a number of fulcra, and the constant motion of the glacier in an uneven bed must cause unequal tensions and compressions, producing numerous points and lines of least resistance, and to cause a fracture at some point a line of least resistance extending over a few inches or feet, a mass of ice many hundreds of feet thick may act by its weight. Thus a number of small fractures and dislocations may be produced in succession at the ever-varying lines of least resistance by a weight of ice which would be quite sufficient to fracture by shearing or in any other way the whole mass at once. If molecular changes caused the motion of glaciers, then why should there be so many fractures as there are? If the weight of the glacier is insufficient to produce fracture, whence come crevasses? If it is sufficient under favourable conditions to produce the great fissures which sometimes run for a mile and widen into crevasses, surely it can produce the smaller cracks and fissures, which, continually occurring at every changing line of least resistance, and being soon obliterated by regelation, will suffice for the slow and viscous-like motion of a glacier. It would be very important to watch the interior of a plank of ice while bending as in Mr. Matthews' experiment, and see whether this occurred by minute cracks or by molecular change. This could be done by means of [3] [p. 249] a beam of light passed through it and condensed on a screen.

The cause of the greater motion by day than night, and in summer than in winter, seems simple enough, the action of running water beneath the glacier and in its fissures forming a buoyant cushion for it to move on, and keeping its parts to some extent free.

Can it be proved mathematically that the force of gravity is not sufficient for the continual readjustment of the equilibrium of the parts of a glacier by repeated small fractures along lines of least resistance, the weight of large portions of the glacier acting successively to effect these fractures and readjustments of small portions of it? The parallel and simultaneous fracture of a mass of ice termed shearing can never, I think, occur in nature, but angular fracture, produced by unequal pressures on two sides of a fixed point, must be continually occurring. The fact that glaciers are most crevassed and fissured where the greatest change of form occurs, whether by passing through a narrow gorge or down a steep incline, plainly indicates that it is by means of fractures, and not by molecular changes, that the viscous-like flow of a glacier is produced.

I was sorry you were not at the British Association meeting³. I exhibited⁴ a large diagram drawn from your tables of eccentricity, and explained its bearing on climate and the rate of organic change. It excited some interest and caused a pretty good discussion, but a Russian mathematician⁵ maintained that all calculations of eccentricity and precession for more than a few 10,000 years were so uncertain as to be of no value whatever, and I wanted you to answer him, as I am not sufficiently acquainted with the subject. — Believe me, yours very faithfully,

ALFRED R. WALLACE.