But these organic remains are not the only evidence of the geological winter. There are a number of phenomena indicating that during this period two vast caps of ice stretched from the Northern pole southward and from the Southern pole northward, extending in each case far toward the Equator,—and that ice-fields, such as now spread over the Arctics, covered a great part of the Temperate Zones, while the line of perpetual ice and snow in the tropical mountain-ranges descended far below its present limits. As the explanation of these facts has been drawn from the study of glacial action, I shall devote this and subsequent articles to some account of glaciers and of the phenomena connected with them.
The first essential condition for the formation of glaciers in mountain-ranges is the shape of their valleys. Glaciers are by no means in proportion to the height and extent of mountains. There are many mountain-chains as high or higher than the Alps, which can boast of but few and small glaciers, if, indeed, they have any. In the Andes, the Rocky Mountains, the Pyrenees, the Caucasus, the few glaciers remaining from the great ice-period are insignificant in size. The volcanic, cone-like shape of the Andes gives, indeed, but little chance for the formation of glaciers, though their summits are capped with snow. The glaciers of the Rocky Mountains have been little explored, but it is known that they are by no means extensive. In the Pyrenees there is but one great glacier, though the height of these mountains is such, that, were the shape of their valleys favorable to the accumulation of snow, they might present beautiful glaciers. In the Tyrol, on the contrary, as well as in Norway and Sweden, we find glaciers almost as fine as those of Switzerland, in mountain-ranges much lower than either of the above-named chains. But they are of diversified forms, and have valleys widening upward on the slope of long crests. The glaciers on the Caucasus are very small in proportion to the height of the range; but on the northern side of the Himalaya there are large and beautiful ones, while the southern slope is almost destitute of them. Spitzbergen and Greenland are famous for their extensive glaciers, coming down to the sea-shore, where huge masses of ice, many hundred feet in thickness, break off and float away into the ocean as icebergs. At the Aletsch in Switzerland, where a little lake lies in a deep cup between the mountains, with the glacier coming down to its brink, we have these Arctic phenomena on a small scale; a miniature iceberg may often be seen to break off from the edge of the larger mass, and float out upon the surface of the water. Icebergs were first traced back to their true origin by the nature of the land-ice of which they are always composed, and which is quite distinct in structure and consistency from the marine ice produced by frozen sea-water, and called "ice-flow" by the Arctic explorers, as well as from the pond or river ice, resulting from the simple congelation of fresh water.
Water is changed to ice at a certain temperature under the same law of crystallization by which any inorganic bodies in a fluid state may assume a solid condition, taking the shape of perfectly regular crystals, which combine at certain angles with mathematical precision. The frost does not form a solid, continuous sheet of ice over an expanse of water, but produces crystals, little ice-blades, as it were, which shoot into each other at angles of thirty or sixty degrees, forming the closest net-work. Of course, under the process of alternate freezing and thawing, these crystals lose their regularity, and soon become merged in each other. But even then a mass of ice is not continuous or compact throughout, for it is rendered completely porous by air-bubbles, the presence of which is easily explained. Ice being in a measure transparent to heat, the water below any frozen surface is nearly as susceptible to the elevation of the temperature without as if it were in immediate contact with it. Such changes of temperature produce air-bubbles, which float upward against the lower surface of the ice and are stranded there. At night there may come a severe frost; new ice is then formed below the air-bubbles, and they are thus caught and imprisoned, a layer of air-bubbles between two layers of ice, and this process may be continued until we have a succession of such parallel layers, forming a body of ice more or less permeated with air. These air-bubbles have the power also of extending their own area, and thus rendering the whole mass still more porous; for, since the ice offers little or no obstacle to the passage of heat, such an air-bubble may easily become heated during the day; the moment it reaches a temperature above thirty-two degrees, it melts the ice around it, thus clearing a little space for itself, and rises through the water produced by the action of its own warmth. The spaces so formed are so many vertical tubes in the ice, filled with water, and having an air-bubble at the upper extremity.
Ice of this kind, resulting from the direct congelation of water, is easily recognized under all circumstances by its regular stratification, the alternate beds varying in thickness according to the intensity of the cold, and its continuance below the freezing-point during a longer or shorter period. Singly, these layers consist of irregular crystals confusedly blended together, as in large masses of crystalline rocks in which a crystalline structure prevails, though regular crystals occur but rarely. The appearance of stratification is the result of the circumstances under which the water congeals. The temperature varies much more rapidly in the atmosphere around the earth than in the waters upon its surface. When the atmosphere above any sheet of water sinks below the freezing-point, there stretches over its surface a stratum of cold air, determining by its intensity and duration the formation of the first stratum of ice. According to the alternations of temperature, this process goes on with varying activity until the sheet of ice is so thick that it becomes itself a shelter to the water below, and protects it, to a certain degree, from the cold without. Thus a given thickness of ice may cause a suspension of the freezing process, and the first ice-stratum may even be partially thawed before the cold is renewed with such intensity as to continue the thickening of the ice-sheet by the addition of fresh layers. The strata or beds of ice increase gradually in this manner, their separation being rendered still more distinct by the accumulation of air-bubbles, which, during a hot and clear day, may rise from a muddy bottom in great numbers. In consequence of these occasional collections of air-bubbles, the layers differ, not only in density and closeness, but also in color, the more compact strata being blue and transparent, while those containing a greater quantity of air-bubbles are opaque and whitish, like water beaten to froth.
A cake of pond-ice, such as is daily left in summer at our doors, if held against the light and turned in different directions, will exhibit all these phenomena very distinctly, and we may learn still more of its structure by watching its gradual melting. The process of decomposition is as different in fresh-water ice and in land-or glacier-ice and that of their formation. Pond-ice, in contact with warm air, melts uniformly over its whole surface, the mass being thus gradually reduces from the exterior till it vanishes completely. If the process be slow, the temperature of the air-bubbles contained in it may be so raised as to form the vertical funnels or tubes alluded to above. By the anastomosing of these funnels, the whole mass may be reduced to a collection of angular pyramids, more or less closely united by cross-beams of ice, and it finally falls to pieces when the spaces in the interior have become for numerous as to render it completely cavernous. Such a breaking-up of ice is always caused by the enlargement of the open spaces produces by the elevated temperature of the air-bubbles, these spaces being necessarily more or less parallel with one another, and vertical in their position, owing to the natural tendency of the air-bubbles to work their way upward till they reach the surface, where they escape. A sheet of ice, of this kind, floating upon water, dissolves in the same manner, melting wholly from the surface, if the process be sufficiently rapid, or falling to pieces, if the air-bubbles are gradually raised in their temperature sufficiently to render the whole mass cavernous and incoherent. If we now compare these facts with what is known of the structure of land-ice, we shall see that the mode of formation in the two cases differs essentially.
Land-ice, of which both the ice-fields of the Arctics and glaciers consist, is produced by the slow and gradual transformation of snow into ice; and though the ice thus formed may eventually be as clear and transparent as the purest pond- or river-ice, its structure is nevertheless entirely distinct. We may trace these different processes during any moderately cold winter in the ponds and snow-meadows immediately about us. We need not join an Arctic exploring expedition, nor even undertake a more tempting trip to the Alps, in order to investigate these phenomena for ourselves, if we have any curiosity to do so. The first warm day after a thick fall of light, dry snow, such as occurs in the coldest of our winter weather, is sufficient to melt its surface. As this snow is porous, the water readily penetrates it, having also a tendency to sink by its own weight, so that the whole mass becomes more or less filled with moisture in the course of the day. Daring the lower temperature of the night, however, the water is frozen again, and the snow is now filled with new ice-particles. Let this process be continued long enough, and the mass of snow is changed to a kind of ice-gravel, or, if the grains adhere together, to something like what we call pudding-stone, allowing, of course, for the difference of material; the snow, which has been rendered cohesive by the process of partial melting and regelation, holding the ice-globules together, just as the loose materials of the pudding-stone are held together by the cement which unites them.
Within this mass, air is intercepted and held inclosed between the particles of ice. The process by which snow-flakes or snow-crystals are transformed into grains of ice, more or less compact, is easily understood. It is the result of a partial thawing, under a temperature maintained very nearly at thirty-two degrees, falling sometimes a little below, and then rising a little above the freezing-point, and thus producing constant alternations of freezing and thawing in the same mass of snow. This process amounts to a kind of kneading of the snow, and when combined with the cohesion among the particles more closely held together in one snow-flake, it produces granular ice. Of course, the change takes place gradually, and is unequal in its progress at different depths in the same bed of recently fallen snow. It depends greatly on the amount of moisture infiltrating the mass, whether derived from the melting of its own surface, or from the accumulation of dew or the falling of rain or mist upon it. The amount of water retained within the mass will also be greatly affected by the bottom on which it rests and by the state of the atmosphere. Under a certain temperature, the snow may only be glazed at the surface by the formation of a thin, icy crust, an outer membrane, as it were, protecting the mass below from a deeper transformation into ice; or it may be rapidly soaked throughout its whole bulk, the snow being thus changed into a kind of soft pulp, what we commonly call slosh, which, upon freezing, becomes at once compact ice; or, the water sinking rapidly, the lower layers only may be soaked, while the upper portion remains comparatively dry. But, under all these various circumstances, frost will transform the crystalline snow into more or less compact ice, the mass of which will be composed of an infinite number of aggregated snow-particles, very unequal in regularity of outline, and cemented by ice of another kind, derived from the freezing of the infiltrated moisture, the whole being interspersed with air. Let the temperature rise, and such a mass, rigid before, will resolve itself again into disconnected ice-particles, like grains more or less rounded. The process may be repeated till the whole mass is transformed into very compact, almost uniformly transparent and blue ice, broken only by the intervening air-bubbles. Such a mass of ice, when exposed to a temperature sufficiently high to dissolve it, does not melt from the surface and disappear by a gradual diminution of its bulk, like pond-ice, but crumbles into its original granular fragments, each one of which melts separately. This accounts for the sudden disappearance icebergs, which, instead of slowly dissolving into the ocean, are often seen to fall to pieces and vanish at once.
Ice of this kind may be seen forming every winter on our sidewalks, on the edge of the little ditches which drain them, or on the summits of broad gateposts when capped with snow. Of such ice glaciers are composed; but, in the glacier, another element comes in which we have not considered as yet,—that of immense pressure in consequence of the vast accumulations of snow within circumscribed spaces. We see the same effects produced on a small scale, when snow is transformed into a snowball between the hands. Every boy who balls a mass of snow in his hands illustrates one side of glacial phenomena. Loose snow, light and porous, and pure white from the amount of air contained in it, is in this way presently converted into hard, compact, almost transparent ice. This change will take place sooner, if the snow be damp at first,—but if dry, the action of the hand will presently produce moisture enough to complete the process. In this case, mere pressure produces the same effect which, in the cases we have been considering above, was brought about by alternate thawing and freezing,—only that in the latter the ice is distinctly granular, instead of being uniform throughout, as when formed under pressure. In the glaciers we have the two processes combined. But the investigators of glacial phenomena have considered too exclusively one or the other: some of them attributing glacial motion wholly to the dilatation produced by the freezing of infiltrated moisture in the mass of snow; others accounting for it entirely by weight and pressure. There is yet a third class, who, disregarding the real properties of ice, would have us believe, that, because tar, for instance, is viscid when it moves, therefore ice is viscid because it moves. We shall see hereafter that the phenomena exhibited in the onward movement of glaciers are far more diversified than has generally been supposed.
There is no chain of mountains in which the shape of the valleys is more favorable to the formation of glaciers than the Alps. Contracted at their lower extremity, these valleys widen upward, spreading into deep, broad, trough-like depressions. Take, for instance, the valley of Hassli, which is not more than half a mile wide where you enter it above Meyringen; it opens gradually upward, till, above the Grimsel, at the foot of the Finster-Aarhorn, it measures several miles across. These huge mountain-troughs form admirable cradles for the snow, which collects in immense quantities within them, and, as it moves slowly down from the upper ranges, is transformed into ice on its way, and compactly crowded into the narrower space below. At the lower extremity of the glacier the ice is pure, blue and transparent, but, as we ascend, it appears less compact, more porous and granular, assuming gradually the character of snow, till in the higher regions the snow is as light, as shifting, and incoherent, as the sand of the desert. A snow-storm on a mountain-summit is very different from a snow-storm on the plain, on account of the different degrees of moisture in the atmosphere. At great heights, there is never dampness enough to allow the fine snow-crystals to coalesce and form what are called "snow-flakes." I have even stood on the summit of the Jungfrau when a frozen cloud filled the air with ice-needles, while I could see the same cloud pouring down sheet of rain upon Lauterbrunnen below. I remember this spectacle as one of the most impressive I have witnessed in my long experience of Alpine scenery. The air immediately about me seemed filled with rainbow-dust, for the ice-needles glittered with a thousand hues under the decomposition of light upon them, while the dark storm in the valley below offered a strange contract to the brilliancy of the upper region in which I stood. One wonder where even so much vapor as may be transformed into the finest snow should come from at such heights. But the warm winds, creeping up the sides of the valleys, the walls of which become heated during the middle of the day, come laden with moisture which is changed to a dry snow like dust as soon as it comes into contact with the intense cold above.
Currents of warm air affect the extent of the glaciers, and influence also the line of perpetual snow, which is by no means at the same level even in neighboring localities. The size of glaciers, of course, determines to a great degree the height at which they terminate, simply because a small mass of ice will melt more rapidly, and at a lower temperature, than a larger one. Thus, the small glaciers, such as those of the Rothhorn or of Trift, above the Grimsel, terminate at a considerable height above the plain, while the Mer de Glace, fed from the great snow-caldrons of Mont Blanc, forces its way down to the bottom of the valley of Chamouni, and the glacier of Grindelwald, constantly renewed from the deep reservoirs where the Jungfrau hoards her vast supplies of snow, descends to about four thousand feet above the sea-level. But the glacier of the Aar, though also very large, comes to a pause at about six thousand feet above the level of the sea; for the south wind from the other side of the Alps, the warm sirocco of Italy, blows across it, and it consequently melts at a higher level than either the Mer de Glace or the Grindelwald. It is a curious fact, that in the valley of Hassli the temperature frequently rises instead of falling as you ascend; at the Grimsel, the temperature is at times higher than at Meyringen below, where the warmer winds are not felt so directly. The glacier of Aletsch, on the southern slope of the Jungfrau, and into which many other glaciers enter, terminates also at a considerable height, because it turns into the valley of the Rhone, through which the southern winds blow constantly.