CHAPTER XII.

TRANSPORT OF MINERAL MATTER BY ICE.-HEIGHT OF SNOW-LINE.—GLACIERS.-CAUSE OF THE MOVEMENT OF GLACIERS.-GLACIER MORAINES. MOTION OF GLACIERS.-GROOVING OF ROCKS BY GLACIERS.-ADVANCE AND RETREAT OF GLACIERS.-GLACIERS OF THE HIMALAYA.

VERY considerable attention has, of late years, been directed to the influence of ice in the distribution of detritus, both upon dry land and over the bottom of the sea, and to the mechanical effects ice may produce on hard rocks, or loose accumulations, on or against which it may move or be thrown, upon the land or beneath the sea.

We observe the influence of the sun's heat to be now such (whatever view may be taken of any supposed heat in the body of the earth itself, sufficient in previous times, to prevent the formation of ice on its surface), that the cold of the planetary space, as it has been termed, so acts upon the earth, that it is, as it were, encased in a comparatively thin warmer space, outside which, water remains permanently solid; this space having a spheroidal form somewhat more oblate than the sea-level, so that, at the equator, there is a difference of from 16,000 to 17,000 feet between the two, and that it joins that level in the Arctic and Antarctic regions. Above this, it is inferred that the temperature continues to decrease in the atmosphere until, finally, that of the planetary space alone prevails.*

Taking thus the heat derived from the sun as so influencing the present surface temperature of the earth, that the cold of the planetary space does not render the waters solid over the whole face of the world, we should, from the conditions under which this heat could prevail, anticipate many minor modifications in its

Fourier inferred that the temperature of the planetary space was -50° centigrade (58° Fahr.), and Svanberg held it was -49° 85 centigrade, employing another method. Observing this near approach to the result given by Fourier, the latter calculated the temperature according to Lambert's statements, and obtained -50° 35.

action.* These would arise from its different absorption and radiation according as it fell upon land or water, and in different latitudes; from the varied relief and character of the land, and its intermixture with surface waters; from the variation in the waters as to depths, and the motion of some portions of them from colder to hotter regions, or the reverse; from the movement of the atmosphere and its varied conditions; and from the periodical change in the position of portions of the earth's surface, according as one hemisphere or the other becomes most exposed to the influence of the sun.

Numerous observations have shown the exact regularity of the space, in which water commonly remains liquid, to be much disturbed by the modifications noticed; so that, for all the purposes required by the animals and vegetables of our planet, certain regions are rendered habitable which would otherwise scarcely support life. A very marked instance of this kind is found on the north flank of the Himalaya, where the perpetual snow-line, as it is termed, is, from a combination of physical conditions, more elevated by 1500 or 2000 feet than on the southern side of the same great range of mountains.† Minor modifications of the same kind are abundant, as also from the influence of great surfaces occupied by the sea, and from prevalent winds sweeping over it and reaching land; thus producing marked elevations of the general temperature above that at which ice would be common.

To obtain the snow reposing on the regions or elevated mountains, piercing through the space above noticed into those portions of our atmosphere where the temperature is such that snow more or less encrusts them during the whole of the various climatal changes of the year, we have to infer evaporation from the land and water, modified according to their various states, surfaces, and

* Respecting the temperature of our atmosphere, M. Arago has remarked, (Ann. de Phys. et de Chim., tom. 27,) that, "1st, in no part of earth on land will a thermometer, raised from two to three metres (6.5 to 10 English feet) above the ground, and protected from all reverberation, attain 46° centigrade (114°.8 Fahr.); 2ndly, in the open sea, the temperature of the air, whatever be the place and season, never attains 31° centigrade (87°.8 Fahr.); 3rdly, the greatest degree of cold which has ever been observed upon our globe, with the thermometer suspended in the air, does not descend 50° centigrade below zero (58° Fahr.)." To this he adds, " 4thly, the temperature of the water of the sea, in no latitude, and in no season, rises above 30° centigrade (86° Fahr.)."

With reference to the snow-line on the northern flank of the Himalaya, Dr. Hooker states (letter to Sir William Hooker, dated Tongu, N.E. Sikkim, altitude 13,500 feet, July 25, 1849), "that the snow-line, in Sikkim, lies on the Indian side of the Himalayan range at below 15,000 feet; on the Thibetan (northern) slope, at about 16,000 feet."

localities, sufficient to afford the needful falls of water in this form.*

From the polar regions, where we find such a great amount of climatal change, that the influence of the sun, as far as it can be there experienced, is uninterrupted, or nearly so, during one-half of the year, and unfelt during the remainder, to the tropical regions, where portions of mountain masses may rise so high into the atmosphere as to support a covering of snow, there are necessarily great variations of temperature, the latter becoming less changeable, as a whole, in the equatorial portions of the earth.

When attention is directed to the effects arising from these variations of temperature, it is found that the production of glaciers

* Experiments do not seem to give the temperature at which the evaporation of snow or water ceases, so that while a limit may be inferred for this evaporation at some height to which parts of a mountain-chain might be elevated, it might readily happen that there are none such on the face of our planet, the vapour of water always mixing with the gases of the atmosphere up to all the heights in it to which parts of the earth's surface have been protruded.

Humboldt (Fragmens Asiatiques, p. 549) has given the following table of the snowline on certain mountain ranges:

M. Durocher (Mémoire sur la limite des neiges perpétuelles. Voyage de la Recherche, 1845) places the line of perpetual snow in the Arctic Ocean in 78° N.; so that, at Spitzbergen (N.W. coast), it descends to the level of the sea.

ice

stands somewhat prominently forward among those which have a geological bearing. In the Alps, Europeans have been long familiar with the elongated masses of ice, so called, descending from the regions of snows, through ravines and rocky depressions of various forms, even into fertile valleys, where ripening crops and may be almost in contact under the heats of summer and autumn, in latitudes ranging from 44° to 47°. De Saussure, though not the first to examine them, by the charm of his writings, directed no little attention to glaciers, and to the effects produced by them. Other authors have, at various times, since described them; and, among those of late years, M. Charpentier* and M. Agassiz have written much in support of a particular hypothesis as to the mode in which these masses of ice moved outwards from the mountain heights whence they originated, and as to their former more considerable range and extension than at present, pointing to many circumstances connected with this subject, and of geological value, though the hypothesis itself may not be adopted.

The progress of researches respecting glaciers and their geological effects, affords a fair example of the necessity of careful observation in a right direction; so many assertions connected with the mode of occurrence and advance of these masses of ice, upon which hypotheses were based, having been found, upon actual investigation, unsupported by facts. Though this has been the case, many observations have, from time to time, been recorded, which have borne the test of careful investigation; and no one would appear more desirous of admitting the value and importance of real additions to our information on this subject, than Professor James Forbes, to whom so much of our present knowledge of the Alpine glaciers is due.‡

A glacier commences near the line of perpetual snow, but lower somewhat than that on the adjacent ground. "There is often a passage, nearly insensible, from perfect snow to perfect ice; at other times, the level of the superficial snow is well marked, and the ice occurs beneath it. No doubt the transition is effected in this way: -the summer's thaw percolates the snow to a great depth with water; the frost of the succeeding winter penetrates far enough to

* "Essai sur les Glaciers et sur le Terrain Erratique du Bassin du Rhone," 1841. Lausanne.

"Études sur les Glaciers," 1840. Neuchatel.

See his Travels through the Alps of Savoy and parts of the Pennine Chain, with observations on the Phenomena of Glaciers, 2nd edit., Edinburgh, 1845; and his papers printed in the Philosophical Transactions, for 1846.

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freeze it at least to the thickness of one year's fall, or, by being repeated in two or more years, consolidates it more effectually.' The part of a glacier, where the surface begins to be annually renewed by the unmelted accumulation of each winter, is commonly known as névé, and true stratification has been here recognized by De Saussure and other writers. Professor Forbes agrees with M. de Charpentiert in thinking that this stratification becomes entirely obliterated as the névé passes into complete ice. The crevasses, or great fissures, in the névé are considered to differ from those lower down the glacier in their greater width and irregularity, and the caverns in it to be more extensive and singular in their forms, from the greater facility with which the névé is thawed and water

worn.

Further down the valley, or ravine, in which the glacier finds its way, much will necessarily depend, as to the form and appearance of the latter, upon the general character of the ground traversed. The ice changes its character: it is not like that produced by the freezing of still water in the lake, but "laminæ, or thin plates of compact transparent blue ice, alternate, in most parts of every glacier, with lamina of ice not less hard and perfect, but filled with countless air-bubbles, which give it a frothy semi-opaque look." "The alternation of bands, then, is marked by blue and greenish-blue or white curves, which are seen to traverse the ice throughout its thickness whenever a section is made. It is, therefore, no external accident-it is the internal structure of a glacier, and the only one which it possesses, and may be expected to throw light upon the circumstances and formation of these masses."§

* Forbes' Travels through the Alps of Savoy, 2nd edit., p. 31.

For his views respecting glaciers, consult M. de Charpentier's Essai sur les Glaciers et sur le Terrain Erratique du Bassin du Rhone. Lausanne, 1841.

"The granulated structure of the névé is accompanied with the dull white of snow passing into a greenish tinge, but rarely, if ever, does it exhibit the transparency and hue of the proper glacier. The deeper parts are more perfectly congealed, and the bands of ice, which often alternate with the hardened snow, are probably due to the effect of thaw succeeding the winter coating, or any extraordinary, fall. On exposed summits, where the action of the sun and the elements is greater, the snow does not lie so long in a powdery state, and the exposed surface becomes completely frozen." Forbes' Travels through the Alps, &c., 2nd edit., p. 32.

§ Forbes' Travels through the Alps, &c., 2nd edit., p. 28. It is remarked, respecting this structure, that it is the consequence of the viscous condition of the mass and its movement. It is observed that it "has all the appearance of being due to the formation of fissures in the aërated ice or consolidated névé, which fissures having been filled with water drained from the glacier, and frozen during winter, have produced the compact blue bands" (p. 372). Professor Forbes considers, that, as the viscous mass moves onward, the central parts faster than the sides, these fissures, filled with ice, take a more horizontal position in the general mass, with such modifications as may be expected on the sides and bottom where the friction is greatest,