I think, have a real existence. It is a result to which the dirt-bands may be the indication, or it may have passed altogether unobserved. "We find at the heads of most glaciers, where the névé is passing into ice, and the body assuming its normal form and construction, that there are steeper elevations, from which the névé descends, and frequently ice-cascades. The glacier is in these parts, on account of the abrupt descent, very much broken up, and often impassable. Now it appears evident, that, at the foot of these slopes, the water which has passed more quickly down them will accumulate to a greater extent than if there had been no elevation behind, on account of the change of inclination. Now, during the summer months, the saturation thus taking place will be greatest, because of the large quantity of water then coming down. At this period of the year, likewise, the motion of the glacier is also greatest, and a large advance of the saturated body occurs. This, during the winter frosts, is consolidated, and formed, I imagine, into more compact ice than would have resulted from less saturated material.

"On the other hand, in the winter months, that part of the glacier at the foot of the upper slopes, or ice-cascades, will be less saturated, as the surface of the whole glacier is then in a state of comparative rest, in consequence of the diminished effect of the sun's rays in thawing the surface of the névé. At this time, also, the glacier moves with far less rapidity; and so the quantity of glacier in a less saturated state thus moving on, will be considerably less than that advancing during the summer. In consequence, also, of its being less saturated with water, it will, after consolidation, be less compact than that which moved forward during the summer. Viewed in this light, the foot of the upper slopes, or ice-cascades, may be considered as a kind of laboratory for the manufacture of alternate bands of compact and porous ice the former made during the summer, and the latter in the winter months. Thus, if my theory be correct, a wide band of comparatively compact ice, and a narrow band of porous ice, will be annually formed and added to the glacier.

"If these alternate bands be considered as identical with the porous and compact bands to which the dirt-bands belong, it follows, that the porous bands, during the progress down the glacier, become apparent by the absorption of the drift, which is washed over the surface, and their distinctness increases with the length of time during which they have been subjected to the drift.

"Thus the wide compact band answers to the interval between the

MR. MILWARD ON DIRT-BANDS IN GLACIERS.

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265

' dirt-bands,' and the narrow porous band to the dirt-band' itself. The one owes its formation to the summer, the other to the winter.

"It will result also from this theory, that the breadth of the dirtband' and interval, taken together, should equal the annual advance of the glacier. And this appears, from observation, to be the actual case. We might also expect the relative breadth of the dirt-bands and interval to approximate towards the proportion of the winter and summer mean glacial motion. There may, however, be causes, arising from the positions and proportions of the lower and upper slopes at the source of the glacier, which would disturb this proportion between the two bands, even more than they would alter the relation between the two bands taken together and the annual glacial motion.

"It is with the greatest diffidence that the writer would venture to submit, that a prima facia case has been made out for three subjects of inquiry.

"1st, Whether there are indications of the existence of wide structural bands (of which the bands on the surface of the mud-slide are the outcrop) in viscous fluids and glaciers.

"2d, Whether there are any traces in the upper parts of glaciers of ridges or waves answering to the ridges occurring on the mud-slide. And,

"3dly, Whether the saturation at the foot of the upper slope, which must theoretically exist, is practically effective, so as to cause the alternate bands of porous and compact ice, in the manner which I have endeavoured to describe."

Observations on the preceding communication, and especially on the cause of the Annual Rings of Glaciers.* By Professor Forbes.

"Professor Forbes stated that Mr. Milward's shrewd suspicion of the bands of ice of different consistence, being accompanied also by wrinkles or elevations, had been discovered by himself some years before at the very place and time pointed out as most likely; and he shewed that, while there is a tendency in a tenacious viscous fluid to produce wrinkles under pressure, capable of effecting detrusion even where the supply of the fluid is uniform, this quality is greatly increased when the supply of the fluid is by fits, as it is in fact at the head of the glacier, where the quasi-hydrostatic pressure from behind, combined with the frontal resistance, produces a thickening, or convex lip or wrinkle.

"He likewise mentioned the analogous instance of the production of Proceedings Royal Soc. Edin. 18th Dec. 1848.

equidistant wrinkles in [on] the sides of railway banks from mere pressure above; and more particularly in turnings of coarse malleable iron, where, though the detruding force is constantly equal, still detrusion takes place at intervals, forming in the shaving so many wrinkles, by which frontal resistance, too, it is thickened, and consequently shortened, similarly with the mechanism of the glacier."

APPENDIX, No. II.

MR. FARADAY ON THE PROPERTIES OF ICE.

Two

. . . Mr. Faraday then invited attention to the extraordinary property of ice in solidifying water which is in contact with it. pieces of moist ice will consolidate into one. Hence the property of damp snow to become compacted into a snow-ball-an effect which cannot be produced on dry, hard-frozen snow. Mr. Faraday suggested, and illustrated by a diagram, that a film of water must possess the property of freezing when placed between two sets of icy particles, though it will not be affected by a single set of particles. Certain solid substances, as flannel, will also freeze to an icy surface, though other substances, as gold leaf, cannot be made to do so. In this freezing action, latent heat becomes sensible heat, the contiguous particles must therefore be raised in temperature while the freezing water is between them. It follows from hence that, by virtue of the solidifying power at points of contact, the same mass may be freezing and thawing at the same moment, and even that the freezing process in the inside may be a thawing process on the outside. Mr. Faraday then referred to Mr. Thomson's memoirs on the effect of pressure on the freezing point. Mr. Thomson has shewn that immense pressure will prevent water from freezing. At 32° ice naturally occupies a greater volume than that of the water which forms it; and we may conceive that, when ice is pressed, the tendency is to give it both the water bulk and state. In conclusion, Mr. Faraday noticed briefly, and chiefly by way of suggestion, the molecular condition of ice as presenting many curious results, and called attention to the strangeness of striæ being formed in a body of such uniform composition as pure water frozen into ice."-From the Proceedings of the Royal Institu tion, reported in the Athenæum, 15th June 1850, p. 641.

MR. DARWIN ON THE STRUCTURE OF LAVAS.

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APPENDIX, No. III.

EXTRACTS from LETTERS to PROFESSOR FORBES, on the Analogy of the Structure of some Volcanic Rocks with that of Glaciers. By C. Darwin, Esq., F.R.S. With Observations on the same subject, made by PROFESSOR FORBES.*

"I take the liberty of addressing you, knowing how much you are interested on the subject of your discovery of the veined structure of glacier ice. I have a specimen (from Mr. Stokes's collection) of Mexican obsidian, which, judging from your description, must resemble, to a considerable degree, the zoned ice. It is zoned with quite straight parallel lines, like an agate; and these zones, as far as I can see under the microscope, appear entirely due to the greater or lesser number of excessively minute, flattened air cavities. I cannot avoid suspecting that in this case, and in many others, in which lava of the trachytic series (generally of very imperfect fluidity) are laminated, that the structure is due to the stretching of the mass or stream during its movement, as in the ice-streams of glaciers.

"If the subject of the lamination of volcanic rocks should interest you, I would venture to ask you to refer to p. 65-72 of my small volume of 'Geological Observations on Volcanic Islands.'† I there

[Proceedings of the Royal Society of Edinburgh, 3d February 1845. There ought to have been a reference to this paper at page 92 of this volume.]

The laminated, volcanic rocks of Ascension, consist, as described by Mr. Darwin, of excessively thin, quite parallel layers of minute crystals of quartz (determined by Professor Miller) and diopside; of atoms of an oxide of iron, and of an amorphous, black angitic mineral; and, lastly, of a more or less pure feldspathic stone, with perfect crystals of feldspar placed lengthways. The following is a portion of the passage referred to:-" Several causes appear capable of producing zones of different tension in masses semi-liquified by heat. In a fragment of de-vitrified glass I have observed layers of sphærulites, which appeared, from the manner in which they were abruptly bent, to have been produced by the simple contraction of the mass in the vessel in which it cooled. In certain dykes on Mount Etna, described by M. Elie de Beaumont, as bordered by alternating bands of scoriaceous and compact rock, one is led to suppose that the stretching movement of the surrounding strata, which originally produced the fissures, continued, whilst the injected rock remained fluid. Guided, however, by Professor Forbes's clear description of the zoned structure of glacier ice, far the most probable explanation of the laminated structure of these feldspathic rocks appears to be, that they have been stretched, whilst slowly flowing onwards in a pasty condition, in precisely the same manner, as Professor Forbes believes, that the ice of moving glaciers is stretched and fissured. In both cases, the zones may be compared to those in the finest agates; in both,

throw out the idea, that the structure in question may perhaps be explained by your views on the zoned structure of glacier ice, the layers of less tension being, in the case of the Ascension obsidian-rocks, rendered apparent, chiefly by the crystalline and concretionary action superinduced in them, instead of, as in zoned ice, by the congelation of water.

"How singular it at first appears, that your discoveries in the structure of glacier ice should explain the structure, as I fully believe they will, of many volcanic masses. I, for one, have for years been quite confounded whenever I thought of the lamination of rocks which have flowed in a liquified state. Will your views throw any light on the primary laminated rocks? The laminæ certainly seem very generally parallel to the lines of disturbance and movement. Believe me, &c. C. DARWIN."

"To Professor FORBES."

Professor Forbes confirmed the previous remarks by others made. by himself on the specimens transmitted to him by Mr. Darwin, and on specimens from Lipari and Iceland in the collection of the Royal Society, as well as by direct observations made by himself on the lava streams of Etna.

they extend in the direction in which the mass has flowed, and those exposed on the surface are generally vertical. In the ice, the porous laminæ are rendered distinct by the subsequent congelation of infiltrated water; in the stony feldspathic lavas by subsequent crystalline and concretionary action. The fragment of glassy obsidian in Mr. Stokes's collection, which is zoned with minute air-cells, must strikingly resemble, judging from Professor Forbes's description, a fragment of the zoned ice; and if the rates of cooling and the nature of the mass had been favourable to its crystallisation, or to concretionary action, we should here have had the finest parallel zones of different composition and texture. In glaciers, the lines of porous ice and of minute crevices seem to be due to an incipient stretching, caused by the central parts of the frozen stream moving faster than the sides and bottom, which are retarded by friction. Hence, in glaciers of certain form, and towards the lower end of most glaciers, the zones become horizontal. May we venture to suppose that, in the feldspathic lavas with horizontal laminæ, we see an analogous case. All geologists who have examined trachytic regions have come to the conclusion, that the lavas of this series have possessed an exceedingly imperfect fluidity; and as it is evident that only matter thus characterised would be subject to become fissured, and to be formed into zones of different tensions, in the manner here supposed, we probably see the reason why augitic lavas, which appear generally to have possessed a higher degree of fluidity, are not, like the feldspathic lavas, divided into laminæ of different composition and texture. Moreover, in the augitic series, there never appears to be any tendency to that kind of concretionary action, which, we have seen, plays an important part in the lamination of rocks of the trachytic series, or, at least, in rendering that structure apparent."