Whoever has truly felt the aërial perspective of Turner has attained a delicate sense of the pictorial significance of the bridge; for, as we look through his floating mists, we descry, amid Nature's most evanescent phenomena, the span, the arch, the connecting lines or masses whereby this familiar image seems to identify itself not less with Nature than with Art. Among the drawings which Arctic voyagers have brought home, many a bridge of ice, enormous and symmetrical, seems to tempt adventurous feet and to reflect a like form of fleecy cloud-land; daguerreotyped by the frost in miniature, the same structures may be traced on the window-pane; printed on the fossil and the strata of rock, in the veins of bark and the lips of shells, or floating in sunbeams, an identical design appears; and, on a summer morning, as the eye carefully roams over a lawn, how often do the most perfect little suspension-bridges hang from spear to spear of herbage, their filmy span embossed with glittering dew-drops!
INTERNAL STRUCTURE AND PROGRESSION OF THE GLACIER.
It is not my intention, in these articles, to discuss a general theory of the glaciers upon physical and mechanical principles. My special studies, always limited to Natural History, have but indifferently fitted me for such a task, and quite recently the subject has been admirably treated from this point of view by Dr. Tyndall, in his charming volume entitled "Glaciers of the Alps." I have worked upon the glaciers as an amateur, devoting my summer vacations, with friends desirous of sharing my leisure, to excursions in the Alps, for the sake of relaxation from the closer application of my professional studies, and have considered them especially in their connection with geological phenomena, with a view of obtaining, by means of a thorough acquaintance with glaciers as they exist now, some insight into the glacial phenomena of past times, the distribution of drift, the transportation of boulders, etc. It was, however, impossible to treat one series of facts without some reference to the other; but such explanations as I have given of the mechanism of the glacier, in connection with its structure, are presented in the language of the unprofessional observer, without any attempt at the technicalities of the physicist. I do not wonder, therefore, that those who have looked upon the glacier chiefly with reference to the physical and mechanical principles involved in its structure and movement should have found my Natural Philosophy defective. I am satisfied with their agreement as to my correct observation of the facts, and am the less inclined to quarrel with the doubts thrown on my theory since I see that the most eminent physicists of the day do not differ from me more sharply than they do from each other. The facts will eventually test all our theories, and they form, after all, the only impartial jury to which we can appeal. In the mean while, I am not sorry that just at this moment, when recent investigations and publications have aroused new interest in the glaciers, the course of these articles brings me naturally to a discussion of the subject in its bearing upon geological questions. I shall, however, address myself especially, as I have done throughout these papers, to my unprofessional readers, who, while they admire the glaciers, may also wish to form a general idea of their structure and mode of action, as well as to know something of the important part they have played in the later geological history of our earth. It would, indeed, be out of place, were I to undertake here a discussion of the different views entertained by the various students who have investigated the glacier itself, among whom Dr. Tyndall is especially distinguished, or those of the more theoretical writers, among whom Mr. Hopkins occupies a prominent position.
Removed, as I am, from all possibility of renewing my own observations, begun in 1836 and ended in 1845, I will take this opportunity to call the attention of those particularly interested in the matter to one essential point with reference to which all other observers differ from me. I mean the stratification of the glacier, which I do not believe to be rightly understood, even at this moment. It may seem presumptuous to dissent absolutely from the statements of one who has seen so much and so well as Dr. Tyndall, on a question for the solution of which, from the physicist's point of view, his special studies have been a far better preparation than mine; and yet I feel confident that I was correct in describing the stratification of the glacier as a fundamental feature of its structure, and the so-called dirt-bands as the margins of the snow-strata successively deposited, and in no way originating in the ice-cascades. I shall endeavor to make this plain to my readers in the course of the present article. I believe, also, that renewed observations will satisfy dissenting observers that there really exists a net-work of capillary fissures extending throughout the whole glacier, constantly closing and reopening, and constituting the channels by means of which water filtrates into its mass. This infiltration, also, has been denied, in consequence of the failure of some experiments in which an attempt was made to introduce colored fluids into the glacier. To this I can only answer, that I succeeded completely, myself, in the self-same experiments which a later investigator found impracticable, and that I see no reason why the failure of the latter attempt should cast a doubt upon the former. The explanation of the difference in the result may, perhaps, be found in the fact, that, as a sponge gorged with water can admit no more fluid than it already contains, so the glacier, under certain circumstances, and especially at noonday in summer, may be so soaked with water that all attempts to pour colored fluids into it would necessarily fail. I have stated, in my work upon glaciers, that my infiltration-experiments were chiefly made at night; and I chose that time, because I knew the glacier would most readily admit an additional supply of liquid from without when the water formed during the day at its surface and rushing over it in myriad rills had ceased to flow.
While we admit a number of causes as affecting the motion of a glacier, namely, the natural tendency of heavy bodies to slide down a sloping surface, the pressure to which the mass is subjected forcing it onward, the infiltration of moisture, its freezing and consequent expansion,—we must also remember that these various causes, by which the accumulated masses of snow and ice are brought down from higher to lower levels, are not all acting at all times with the same intensity, nor is their action always the same at every point of the moving mass. While the bulk of snow and ice moves from higher to lower levels, the whole mass of the snow, in consequence of its own downward tendency, is also under a strong vertical pressure, arising from its own incumbent weight, and that pressure is, of course, greater at its bottom than at its centre or surface. It is therefore plain, that, inasmuch as the snow can be compressed by its own weight, it will be more compact at the bottom of such an accumulation than at its surface, this cause acting most powerfully at the upper part of a glacier, where the snow has not yet been transformed into a more solid icy mass. To these two agencies, the downward tendency and the vertical pressure, must be added the pressure from behind, which is most-effective where the mass is largest and the amount of motion in a given time greatest. In the glacier, the mass is, of course, largest in the centre, where the trough which holds it is deepest, and least on the margins, where the trough slopes upward and becomes more shallow. Consequently, the middle of a glacier always advances more rapidly than the sides. Were the slope of the ground over which it passes, combined with the pressure to which the mass is subjected, the whole secret of the onward progress of a glacier, it is evident that the rate of advance would be gradually accelerated, reaching its maximum at its lower extremity, and losing its impetus by degrees on the higher levels nearer the point where the descent begins. This, however, is not the case. The glacier of the Aar, for instance, is about ten miles in length; its rate of annual motion is greatest near the point of junction of the two great branches by which it is formed, diminishing farther down, and reaching a minimum at its lower extremity. But in the upper regions, near their origin, the progress of these branches is again gradually less. Let us see whether the next cause of displacement, the infiltration of moisture, may not in some measure explain this retardation, at least of the lower part of the glacier. This agency, like that of the compression of the snow by its own weight and the pressure from behind, is most effective where the accumulation is largest. In the centre, where the body of the mass is greatest, it will imbibe the most moisture. But here a modifying influence comes in, not sufficiently considered by the investigators of glacial structure. We have already seen that snow and ice at different degrees of compactness are not equally permeable to moisture. Above the line at which the annual winter snow melts, there is, of course, little moisture; but below that point, as soon as the temperature rises in summer sufficiently to melt the surface, the water easily penetrates the mass, passing through it more readily where the snow is lightest and least compact,—in short, where it has not begun its transformation into ice. A summer's day sends countless rills of water trickling through such a mass of snow. If the snow be loose and porous throughout, the water will pass through its whole thickness, accumulating at the bottom, so that the lower portion of the mass will be damper, more completely soaked with water, than the upper part; if, on the contrary, in consequence of the process previously described, alternate melting and freezing combined with pressure, the mass has assumed the character of icy snow, it does not admit moisture so readily, and still farther down, where the snow is actually transformed into pure compact ice, the amount of surface-water admitted into its structure will, of course, be greatly diminished. There may, however, be conditions under which even the looser snow is comparatively impervious to water; as, for instance, when rain falls upon a snow-field which has been long under a low temperature, and an ice-crust is formed upon its surface, preventing the water from penetrating below. Admitting, as I believe we must, that the water thus introduced into the snow and ice is one of the most powerful agents to which its motion is due, we must suppose that it has a twofold influence, since its action when fluid and when frozen would be different. When fluid, it would contribute to the advance of the mass in proportion to its quantity; but when frozen, its expansion would produce a displacement corresponding to the greater volume of ice as compared with water; add to this that while trickling through the mass it will loosen and displace the particles of already consolidated ice. I have already said that I did not intend to trespass on the ground of the physicist, and I will not enter here upon any discussion as to the probable action of the laws of hydrostatic pressure and dilatation in this connection. I will only state, that, so far as my own observation goes, the movement of the glacier is most rapid where the greatest amount of moisture is introduced into the mass, and that I believe there must be a direct relation between these two facts. If I am right in this, then the motion, so far as it is connected with infiltrated moisture or with the dilatation caused by the freezing of that moisture, will, of course, be most rapid where the glacier is most easily penetrated by water, namely, in the region of the névé and in the upper portion of the glacier-troughs, where the névé begins to be transformed into more or less porous ice. This cause also accounts, in part at least, for another singular fact in the motion of the glacier: that, in its higher levels, where its character is more porous and the water entering at the surface sinks readily to the bottom, there the bottom seems to move more rapidly than the superficial parts of the mass, whereas at the lower end of the glacier, in the region of the compact ice, where the infiltration of the water at the bottom is at its minimum, while the disintegrating influences at the surface admit of infiltration to a certain limited depth, there the motion is greater near the surface than toward the bottom. But, under all circumstances, it is plain that the various causes producing motion, gravitation, pressure, infiltration of water, frost, will combine to propel the mass at a greater rate along its axis than near its margins. For details concerning the facts of the case, I would refer to my work entitled "Système Glaciaire."
We will next consider the stratification of the glacier. I have stated in my introductory remarks, that I consider this to be one of its primary and fundamental features, and I confess, that, after a careful examination of the results obtained by my successors in the field of glacial phenomena, I still believe that the original stratification of the mass of snow from which the glacier arises gives us the key to many facts of its internal structure. The ultimate features resulting from this connection are so exceedingly intricate and entangled that their relation is not easily explained. Nevertheless, I trust my readers will follow me in this Alpine excursion, where I shall try to smooth the asperities of the road for them as much as possible.
Imparted to it, at the very beginning of its formation, by the manner in which snow accumulates, and retained through all its transformations, the stratification of a glacier, however distorted, and at times almost obliterated, remains, notwithstanding, as distinct to one who is acquainted with all its phases, as is the stratified character of metamorphic rocks to the skilful geologist, even though they may be readily mistaken for plutonic masses by the common observer. Indeed, even those secondary features, as the dirt-bands, for instance, which we shall see to be intimately connected with snow-strata, and which eventually become so prominent as to be mistaken for the cause of the lines of stratification, do nevertheless tend, when properly understood, to make the evidence of stratification more permanent, and to point out its primitive lines.
On the plains, in our latitude, we rarely have the accumulated layers of several successive snow-storms preserved one above another. We can, therefore, hardly imagine with what distinctness the sequence of such beds is marked in the upper Alpine regions. The first cause of this distinction between the layers is the quality of the snow when it falls, then the immediate changes it undergoes after its deposit, then the falling of mist or rain upon it, and lastly and most efficient of all, the accumulation of dust upon its surface. One who has not felt the violence of a storm in the high mountains, and seen the clouds of dust and sand carried along with the gusts of wind passing over a mountain-ridge and sweeping through the valley beyond, can hardly conceive that not only the superficial aspect of a glacier, but its internal structure also, can be materially affected by such a cause. Not only are dust and sand thus transported in large quantities to the higher mountain-regions, but leaves are frequently found strewn upon the upper glacier, and even pine-cones, and maple-seeds flying upward on their spread wings, are scattered thousands of feet above and many miles beyond the forests where they grew.