This accumulation of sand and dust goes on all the year round, but the amount accumulated over one and the same surface is greatest during the summer, when the largest expanse of rocky wall is bare of snow and its loose soil dried by the heat so as to be easily dislodged. This summer deposit of loose inorganic materials, light enough to be transported by the wind, forms the main line of division between the snow of one year and the next, though only that of the last year is visible for its whole extent. Those of the preceding years, as we shall see hereafter, exhibit only their edges cropping out lower down one beyond another, being brought successively to lower levels by the onward motion of the glacier.
Other observers of the glacier, Professor Forbes and Dr. Tyndall, have noticed only the edges of these seams, and called them dirt-bands. Looking upon them as merely superficial phenomena, they have given explanations of their appearance which I hold to be quite untenable. Indeed, to consider these successive lines of dirt on the glacier as limited only to its surface, and to explain them from that point of view, is much as if a geologist were to consider the lines presented by the strata on a cut through a sedimentary mass of rock as representing their whole extent, and to explain them as a superficial deposit due to external causes.
A few more details may help to make this statement clearer to my readers. Let us imagine that a fresh layer of snow has fallen in these mountain-regions, and that a deposit of dirt has been scattered over its surface, which, if any moisture arises from the melting of the snow or from the falling of rain or mist, will become more closely compacted with it. The next snow-storm deposits a fresh bed of snow, separated from the one below it by the sheet of dust just described, and this bed may, in its turn, receive a like deposit. For greater ease and simplicity of explanation, I speak here as if each successive snow-layer were thus indicated; of course this is not literally true, because snow-storms in the winter may follow each other so fast that there is no time for such a collection of foreign materials upon each newly formed surface. But whenever such a fresh snow-bed, or accumulation of beds, remains with its surface exposed for some time, such a deposit of dirt will inevitably be found upon it. This process may go on till we have a number of successive snow-layers divided from each other by thin sheets of dust. Of course, such seams, marking the stratification of snow, are as permanent and indelible as the seams of coarser materials alternating with the finest mud in a sedimentary rock.
The gradual progress of a glacier, which, though more rapid in summer than in winter, is never intermitted, must, of course, change the relation of these beds to each other. Their lower edge is annually cut off at a certain level, because the snow deposited every winter melts with the coming summer, up to a certain line, determined by the local climate of the place. But although the snow does not melt above this line, we have seen, in the preceding article, that it is prevented from accumulating indefinitely in the higher regions by its own tendency to move down to the lower valleys, and crowding itself between their walls, thus to force its way toward the outlet below. Now, as this movement is very gradual, it is evident that there must be a perceptible difference in the progress of the successive layers, the lower and older ones getting the advance of the upper and more recent ones: that is, when the snow that has covered the face of the country during one winter melts away from the glacier up to the so-called snow-line, there will be seen cropping out below and beyond that line the layers of the preceding years, which are already partially transformed into ice, and have become a part of the frozen mass of the glacier with which they are moving onward and downward. In the autumn, when the dust of a whole season has been accumulated upon the surface of the preceding winter's snow, the extent of the layer which year after year will henceforth crop out lower down, as a dirt-band, may best be appreciated.
Beside the snow-layers and the sheets of dust alternating with them, there is still another feature of the horizontal and parallel structure of the mass in immediate connection with those above considered. I allude to the layers of pure compact ice occurring at different intervals between the snow-layers. In July, when the snow of the preceding winter melts up to the line of perpetual snow, the masses above, which are to withstand the summer heat and become part of the glacier forever, or at least until they melt away at the lower end, begin to undergo the changes through which all snow passes before it acquires the character of glacial ice. It thaws at the surface, is rained upon, or condenses moisture, thus becoming gradually soaked, and after assuming the granular character of névé-ice, it ends in being transformed into pure compact ice. Toward the end of August, or early in September, when the nights are already very cold in the Alps, but prior to the first permanent autumnal snow-falls, the surface of these masses becomes frozen to a greater or less depth, varying, of course, according to temperature. These layers of ice become numerous and are parallel to each other, like the layers of ice formed from slosh. Such crusts of ice I have myself observed again and again upon the glacier. This stratified snowy ice is now the bottom on which the first autumnal snow-falls accumulate. These sheets of ice may be formed not only annually before the winter snows set in, but may recur at intervals whenever water accumulating upon an extensive snow-surface, either in consequence of melting or of rain, is frozen under a sharp frost before another deposit of snow takes place. Or suppose a fresh layer of light porous snow to have accumulated above one the surface of which has already been slightly glazed with frost; rain or dew, falling upon the upper one, will easily penetrate it; but when it reaches the lower one, it will be stopped by the film of ice already formed, and under a sufficiently low temperature, it will be frozen between the two. This result may be frequently noticed in winter, on the plains, where sudden changes of temperature take place.
There is still a third cause, to which the same result may possibly be due, and to which I shall refer at greater length hereafter; but as it has not, like the preceding ones, been the subject of direct observation, it must be considered as hypothetical. The admirable experiments of Dr. Tyndall have shown that water may be generated in ice by pressure, and it is therefore possible that at a lower depth in the glacier, where the incumbent weight of the mass above is sufficient to produce water, the water thus accumulated may be frozen into ice-layers. But this depends so much upon the internal temperature of the glacier, about which we know little beyond a comparatively superficial depth, that it cannot at present afford a sound basis even for conjecture.
There are, then, in the upper snow-fields three kinds of horizontal deposits: the beds of snow, the sheets of dust, and the layers of ice, alternating with each other. If, now, there were no modifying circumstances to change the outline and surface of the glacier,—if it moved on uninterruptedly through an open valley, the lower layers, forming the mass, getting by degrees the advance of the upper ones, our problem would be simple enough. We should then have a longitudinal mass of snow, inclosed between rocky walls, its surface crossed by straight transverse lines marking the annual additions to the glacier, as in the adjoining figure.
But that mass of snow, before it reaches the outlet of the valley, is to be compressed, contorted, folded, rent in a thousand directions. The beds of snow, which in the upper ranges of the mountain were spread out over broad, open surfaces, are to be crowded into comparatively circumscribed valleys, to force and press themselves through narrow passes, alternately melting and freezing, till they pass from the condition of snow into that of ice, to undergo, in short, constant transformations, by which the primitive stratification will be extensively modified. In the first place, the more rapid motion of the centre of the glacier, as compared with the margins, will draw the lines of stratification downward toward the middle faster than at the sides. Accurate measurements have shown that the axis of a glacier may move ten- or twenty-fold more rapidly than its margins. This is not the place to introduce a detailed account of the experiments made to ascertain this result; but I would refer those who are interested in the matter to the measurements given in my "Système Glaciaire," where it will be seen that the middle may move at a rate of two hundred feet a year, while the margins may not advance more than ten or fifteen or twenty feet. These observations of mine have the advantage over those of other observers, that, while they embrace the whole extent of the glacier, transversely as well as in its length, they cover a period of several successive years, instead of being limited to summer campaigns and a few winter observations. The consequence of this mode of progressing will be that the straight lines drawn transversely across the surface of the glacier above will be gradually changed to curved ones below. After a few years, such a line will appear on the surface of the glacier like a crescent, with the bow turned downward, within which, above, are other crescents, less and less sharply arched up to the last year's line, which may be again straight across the snow-field. (See the subjoined figure, which represents a part of the glacier of the Lauter-Aar.)
