shown to belong to the canal-shaped glacier, with branches; so the upper glacier is an exact representative, in its lower part, of the oval glacier, for which I have taken that of the Rhone as a type; whilst many of the tributary glaciers of Grindelwald and the Jungfrau bear ample testimony to the general fact, that the structure of glaciers is developed during their progression, and after their primitive stratification has been annihilated, by their being projected in avalanches over appalling precipices.

To these brief notes, I have only to add one interesting discovery, though of a somewhat local importance, which I made at Chamouni. The ancient lateral moraine of the Glacier des Bois is acknowledged by De Saussure, and all subsequent writers, to be found in the barrier of debris which crosses the valley of Chamouni, at Les Tines; but very feeble traces have (I believe) been observed of the corresponding lateral moraine of the left bank of the glacier, excepting those between the Châlet of Montanvert, and the descent of La Filia. I have ascertained, however, that a good part of the ascent to the Montanvert, and especially near the châlets of Planaz, passes over a vast accumulation of debris, whose nature corresponds to that of the granites of the central chain, and which lies to an immense thickness against the rocky slopes of the valley, at the foot of the Aiguille de Blaitiere. The resistance offered by this mass of debris to the progress of the torrents, which descend from the glaciers of Grepon and Blaitiere towards the Arve, has diverted their course in a direction parallel to that of the valley of Chamouni, and it was the observation of this singularity which led me to the detection of the moraine first mentioned, which I could hardly believe had escaped me so long.

1844.]

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VII. SIXTH LETTER on GLACIERS, addressed to the Right Honourable EARL CATHCART.*

Analogies of Glaciers to Lava Streams Observations on Mount VesuviusMoraines of Lava Streams-Some Objections to the Plastic or Viscous Theory of Glaciers considered-Verticality of Crevasses accounted for.

ROME, February 5, 1844.

My Lord-In a letter which I addressed to you on the 29th ult., I gave some account of the few new observations which untoward circumstances permitted me to make, last autumn, upon the glaciers of Switzerland and Savoy. I have, however, had leisure to reflect maturely upon the theory of glaciers, which I have been occupied for two years in endeavouring to mature; and, without pretending to find in it a complete solution of every problem which might be proposed respecting these wonderful bodies, I am perfectly satisfied that it is fundamentally conformable to the laws by which they are governed. Some new analogies, to which your Lordship has referred in your last letter, such as that between glaciers and lava streams, may serve to render the subject more popularly intelligible; and in explaining them, I may have an opportunity of removing, in some degree, the difficulties which have arisen in the minds of candid and intelligent persons, who have studied this theory for the first time-difficulties which would probably disappear of themselves by a more prolonged attention.

I have not had the advantage of seeing the eruption of Etna, to which your Lordship alludes, which was indeed over before I arrived at Naples, and of which I did not even hear for a considerable time after; so small is the sensation which such events excite in the country. I have, however, had an opportunity-probably not less favourable, though far less imposing of studying the mechanism of plastic lava, in the small currents which, during the months of November and December,

*Edinburgh New Philosophical Journal, October 1844.

were very frequently flowing from mouths within the crater of Vesuvius. On the 30th November, in particular, I descended to the bottom of the crater, in order to examine a current of very liquid lava, fifteen or twenty feet wide, which issued from a cavity near the foot of the small cone which occupied the centre of the crater, and from whose top (in the shape of an inverted funnel, or of a blast furnace) there issued smoke and flames, occasionally accompanied by a discharge of volcanic projectiles. The lava issued in a very steady rapid stream, and spread itself over a gentle declivity with a velocity of not less, I think, than a foot per second.

Admitting the plastic or viscous theory of glaciers, the resemblance to lava fails (1.) In respect of the great liquidity of the lava near its source; (2.) From its very unequal rate of consolidation; a crust being very soon formed upon the surface, which becoming more and more massive, the principle of fluidity is not uniformly distributed throughout the mass, as in the glacier, but a tolerably perfect fluid struggles with the increasing load of its ponderous crust, which it tears and rends by the mighty energy of hydrostatic pressure; and here and there finding a freer exit far removed from its source, tosses high those mighty fragments of the stony arch which confined it into the wild shapes which strike the eye in crossing the wastes of a lava stream, and which seem at first incompatible with the fluid or semifluid principle of motion. This second circumstance, then, the very unequal and rapid superficial consolidation of the lava near its source, has no analogy in a glacier, nor even in a river, unless when breaking up a ponderous crust of ice after a sudden thaw. The regulated progression of the glacier, swiftest in its centre, and with a graduated retardation towards the sides, has a much more precise analogy to that of a river than the lava stream has, which is subdivided (when it

* I am able to add my distinct testimony to that of M. Pilla, as to the emission of flames by the crater of Vesuvius. I spent part of the evening of the 1st January on the top, and had not the least doubt that what I saw were actual flames, which issued from time to time from the orifices of the small cone, and which were of a pale colour, often inclining to blue.

1844.]

ANALOGY OF LAVA STREAMS TO GLACIERS.

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has any considerable breadth) into many little currents, each rolling past, and being retarded by its more sluggish or already consolidated neighbour; so that its surface resembles that of the bed of many torrents in the Alps, where the more solid matters, the rocks, stones, gravel, sand, and clay, trace out the form of a sluggish mass propelled downwards by gravity, whilst its surface is seamed by the trickling of innumerable rills of water, charged with the more portable materials which have been washed down, or squeezed from the general mass.

There are other circumstances, however, in which the analogy of the glacier with the lava stream is more complete ; and of these I will observe

I. That the cracks of the dark-coloured slag on the surface of the liquid lava, as it spreads itself abroad, on issuing from the fiery mouth, are radiated exactly as those of a glacier under similar circumstances, and which I have represented in the margin as I saw them on Vesuvius, the lines of fissure being

Fissures in the Crust of Lava during Crystallization.

marked by the liquid fire shining through. A perfect analogy here exists with the phenomena of radiating fissures in ice, which I first described in the glacier of the Rhone, and afterwards in the ice of the Glacier du Taléfre, where it joins the Glacier de Lechaud, in the Glacier of Arolla, and very many other instances.

II. That the slags, where solidified, presented stric or ripple-marks along their surface, parallel to the direction of the

"ribboned structure" of glacier ice, i. e., inclining slightly from the sides towards the centre of the current, in the direction in which the current is moving. These striæ, or ripple-marks, which have a striking analogy in certain cases of the retarded movement of rivers, are carefully to be distinguished, on the one hand, from the cracks or flaws, and, on the other, from the direction of motion of the fluid particles.*

III. When, at some distance from the source, the lava became viscid and tenaceous, and forced itself, in streamlets of a pasty consistence, through the interstices of its slag, thence it became streaky and drawn out, in the direction last mentioned, as molten glass does in the hands of the workman.

IV. But there is a more striking analogy to the ribboned structure of glacier ice, to be found in lava currents at a distance from their origin, and where by any circumstance their surface has been broken up, and their internal structure exposed. In the Fossa della Vetrana, for instance, and other places, I have found the lava divided into thin layers parallel to the interior of the surface of the channel through which it flowed, evidently produced by the adhesion or retardation which the soil exerted upon its adjoining film of lava, and the successive portions of lava upon one another, in proportion as the semifluid mass, rolling upon its own particles (or rather sliding imperfectly over them), produced a solution of continuity and a series of shells, parallel in direction to the bed upon which the whole rests. The thickness of these shells varies from one-third of an inch upwards. I have never, however, observed a structure in the interior of the lava except that parallel to the sides and bottom of the canal in which it moves; nothing, in short, corresponding to the frontal dip in glaciers. But this is quite natural and conformable to the very different constitution of a glacier; and, in particular, it corresponds to the fact so often urged as a difficulty to the semifluid theory of glaciers, namely, the want of

* A long accidental delay in the printing of this letter enables me to add, that I have found in the lavas of Etna a yet far more perfect analogy to the veined structure of glaciers than that described in the text. It is, indeed, so completely developed as to leave no doubt as to the identity of origin.