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.)
Thus the glacier records upon its surface its annual growth and progress, and registers also the inequality in the rate of advance between the axis and the sides.
But these are only surface-phenomena. Let us see what will be the effect upon the internal structure. We must not forget, in considering the changes taking place within glaciers, the shape of the valleys which contain them. A glacier lies in a deep trough, and the tendency of the mass will be to sink toward its deeper part, and to fold inward and downward, if subjected to a strong lateral pressure,—that is, to dip toward the centre and slope upward along the sides, following the scoop of the trough. If, now, we examine the face of a transverse cut in the glacier, we find it traversed by a number of lines, vertical in some places, more or less oblique in others, and frequently these lines are joined together at the lower ends, forming loops, some of which are close and vertical, while others are quite open. These lines are due to the folding of the strata in consequence of the lateral pressure they are subjected to, when crowded into the lower course of the valleys, and the difference in their dip is due to the greater or less force of that pressure. The wood-cut on the next page represents a transverse cut across the Lauter-Aar and the Finster-Aar, the two principal tributaries to the great Aar glacier, and includes also a number of small lateral glaciers which join them. The beds on the left, which dip least, and are only folded gently downward, forming very open loops, are those of the Lauter-Aar, where the lateral pressure is comparatively slight. Those which are almost vertical belong in part to the several small tributary glaciers, which have been crowded together and very strongly compressed, and partly to the Finster-Aar. The close uniform vertical lines in this wood-cut represent a different feature in the structure of the glacier, called blue bands, to which I shall refer presently. These loops or lines dipping into the internal mass of the glacier have been the subject of much discussion, and various theories have been recently proposed respecting them. I believe them to be caused, as I have said, by the snow-layers, originally deposited horizontally, but afterward folded into a more or less vertical position, in consequence of the lateral pressure brought to bear upon them. The sheets of dust and of ice alternating with the snow-strata are of course subjected to the same action, and are contorted, bent, and folded by the same lateral pressure.
Dr. Tyndall has advanced the view that the lines of apparent stratification, and especially the dirt-bands across the surface of the glacier, are due to ice-cascades: that is, the glacier, passing over a sharp angle, is cracked across transversely in consequence of the tension, and these rents, where the back of the glacier has been successively broken, when recompacted, cause the transverse lines, the dirt being collected in the furrow formed between the successive ridges. Unfortunately for his theory, the lines of stratification constantly occur in glaciers where no such ice-falls are found. His principal observations upon this subject were made on the Glacier du Géant, where the ice-cascade is very remarkable. The lines may perhaps be rendered more distinct on the Glacier du Géant by the cascade, and necessarily must be so, if the rents coincide with the limit at which the annual snow-line is nearly straight across the glacier. In the region of the Aar glacier, however, where my own investigations were made, all the tributaries entering into the larger glacier are ribbed across in this way, and most of them join the main trunk over uniform slopes, without the slightest cascade.
It must be remembered that these surface-phenomena of the glacier are not to be seen at all times, nor under all conditions. During the first year of my sojourn on the glacier of the Aar, I was not aware that the stratification of its tributaries was so universal as I afterward found it to be; the primitive lines of the strata are often so far erased that they are not perceptible, except under the most favorable circumstances. But when the glacier has been washed clean by rain, and the light strikes upon it in the right direction, these lines become perfectly distinct, where, under different conditions, they could not be discerned at all. After passing many summers on the same glacier, renewing my observations year after year over the same localities, I can confidently state that not only do the lines of stratification exist throughout the great glacier of the Aar, but in all its tributaries also. Of course, they are greatly modified in the lower part of the glacier by the intimate fusion of its tributaries, and by the circumstance that their movement, primarily independent, is merged in the movement of the main glacier embracing them all. We have seen that not only does the centre of a glacier move more rapidly than its sides, but that the deeper mass of the glacier also moves at a different rate from its more superficial portion. My own observations (for the details of which I would again refer the reader to my "Système Glaciaire ") show that in the higher part of the glacier, especially in the region of the névé, the bottom of the mass seems to move more rapidly than the surface, while lower down, toward the terminus of the glacier, the surface, on the contrary, moves faster than the bottom. The annexed wood-cut exhibits a longitudinal section of the glacier, in which this difference in the motion of the upper and lower portions of the mass is represented, the beds being almost horizontal in the upper snow-fields, while their lower portion slopes move rapidly downward in the névé region, and toward the lower end the upper portion takes the lead, and advances more rapidly than the lower.
I presented these results for the first time in two letters, dated October 9th, 1842, which were published in a German periodical, the Jahrbuch of Leonhard and Bronn. The last three wood-cuts introduced above, the transverse and longitudinal sections of the glacier as well as that representing the concentric lines of stratification on the surface, are the identical ones contained in those communications. These papers seem to have been overlooked by contemporary investigators, and I may be permitted to translate here a passage from one of them, since it sums up the results of the inequality of motion throughout the glacier and its influence on the primitive stratification of the mass in as few words and as correctly as I could give them to-day, twenty years later:—"Combining these views, it appears that the glacier may be represented as composed of concentric shells which arise from the parallel strata of the upper region by the following process. The primitively regular strata advance into gradually narrower and deeper valleys, in consequence of which the margins are raised, while the middle is bent not only downward, but, from its more rapid motion, forward also, so that they assume a trough-like form in the interior of the mass. Lower down, the glacier is worn by the surrounding air, and assumes the peculiar form characteristic of its lower course." The last clause alludes to another series of facts, which we shall examine in a future article, when we shall see that the heat of the walls in the lower part of its course melts the sides of the glacier, so that, instead of following the trough-like shape of the valley, it becomes convex, arching upward in the centre and sinking at the margins.
I have dwelt thus long, and perhaps my readers may think tediously, upon this part of my subject, because the stratification of the glacier has been constantly questioned by the more recent investigators of glacial phenomena, and has indeed been set aside as an exploded theory. They consider the lines of stratification, the dirt-bands, and the seams of ice alternating with the more porous snow, as disconnected surface-phenomena, while I believe them all to be intimately connected together as primary essential features of the original mass.
There is another feature of glacial structure, intimately connected, by similarity of position and aspect, with the stratification, which has greatly perplexed the students of glacial phenomena. I allude to the so-called blue bands, or bands of infiltration, also designated as veined structure, ribboned or laminated structure, marginal structure, and longitudinal structure. The difficulty lies, I believe, in the fact that two very distinct structures, that of the stratification and the blue bands, are frequently blended together in certain parts of the glacier in such a manner as to seem identical, while elsewhere the one is prominent and the other subordinate, and vice versâ. According to their various opportunities of investigation, observers have either confounded the two, believing them to be the same, or some have overlooked the one and insisted upon the other as the prevailing feature, while that very feature has been absolutely denied again by others who have seen its fellow only, and taken that to be the only prominent and important fact in this peculiar structural character of the ice.
We have already seen how the stratification of the glacier arises, accompanied by layers of dust and other material foreign to the glacier, and how blue bands of compact ice may be formed parallel to the surface of these strata. We have also seen how the horizontality of these strata may be modified by pressure till they assume a position within the mass of the glacier, varying from a slightly oblique inclination to a vertical one. Now, while the position of the strata becomes thus altered under pressure, other changes take place in the constitution of the ice itself.
Before attempting to explain how these changes take place, let us consider the facts themselves. The mass of the glacial ice is traversed by thin bands of compact blue ice, these bands being very numerous along the margins of the glacier, where they constitute what Dr. Tyndall calls marginal structure, and still more crowded along the line upon which two glaciers unite, where he has called it longitudinal structure. In the latter case, where the extreme pressure resulting from the junction of two glaciers has rendered the strata nearly vertical, these blue bands follow their trend so closely that it is difficult to distinguish one from the other. It will be seen, on referring to the wood-cut on page 758, where the close, uniform, vertical lines represent the true veined structure, that at several points of that section the lines of stratification run so nearly parallel with them, that, were the former not drawn more strongly, they could not be easily distinguished from the latter. Along the margins, also, in consequence of the retarded motion, the blue bands and the lines of stratification run nearly parallel with each other, both following the sides of the trough in which they move.
Undoubtedly, in both these instances, we have two kinds of blue bands, namely: those formed primitively in a horizontal position, indicating seams of stratification, and those which have arisen subsequently in connection with the movement of the whole mass, which I have occasionally called bands of infiltration, as they appeared to me to be formed by the infiltration and freezing of water. The fact that these blue bands are most numerous where two glaciers are crowded together into a common bed naturally suggests pressure as their cause. And since the beautiful experiments of Dr. Tyndall have illustrated the internal liquefaction of ice by pressure, it becomes highly probable that his theory of the origin of these secondary blue bands is the true one. He suggests that layers of water may be formed in the glacier at right angles with the pressure, and pass into a state of solid ice upon the removal of that pressure, the pressure being of course relieved in proportion to the diminution in the body of the ice by compression. The number of blue bands diminishes as we recede from the source of the pressure,—few only being formed, usually at right angles with the surfaces of stratification, in the middle of a glacier, half-way between its sides. If they are caused by pressure, this diminution of their number toward the middle of the glacier would be inevitable, since the intensity of the pressure naturally fades as we recede from the motive power.
Dr. Tyndall also alludes to another structure of the same kind, which he calls transverse structure, where the blue bands extend in crescent-shaped curves, more or less arched, across the surface of the glacier. Where these do not coincide with the stratification, they are probably formed by vertical pressure in connection with the unequal movement of the mass.
With these facts before us, it seems to me plain that the primitive blue bands arise with the stratification of the snow in the very first formation of the glacier, while the secondary blue bands are formed subsequently, in consequence of the onward progress of the glacier and the pressure to which it is subjected. The secondary blue bands intersect the planes of stratification at every possible angle, and may therefore seem identical with the stratification in some places, while in others they cut it at right angles. It has been objected to my theory of glacial structure, that I have considered the so-called blue bands as a superficial feature when compared with the stratification. And in a certain sense this is true; since, if my views are correct, the glacier exists and is in full life and activity before the secondary blue bands arise in it, whereas the stratification is a feature of its embryo condition, already established in the accumulated snow before it begins its transformation into glacier-ice. In other words, the veined structure of the glacier is not a primary structural feature of its whole mass, but the result of various local influences acting upon the constitution of the ice: the marginal structure resulting from the resistance of the sides of the valley to the onward movement of the glacier, the longitudinal structure arising from the pressure caused by two glaciers uniting in one common bed, the transverse structure being produced by vertical pressure in consequence of the weight of the mass itself and the increased rate of motion at the centre.
In the névé fields, where the strata are still horizontal, the few blue bands observed are perpendicular to the strata of snow, and therefore also perpendicular to the blue seams of ice and the sheets of dust alternating with them. Upon the sides of the glacier they are more or less parallel to the slopes of the valley; along the line of junction of two glaciers they follow the vertical trend of the axis of the mass; while at intermediate positions they are more or less oblique. Along the outcropping edges of the strata, on the surface of the glacier, they follow more or less the dip of the strata themselves; that is to say, they are more or less parallel with the dirt-bands. In conclusion, I would recommend future investigators to examine the glaciers, with reference to the distribution of the blue bands, after heavy rains and during foggy days, when the surface is freed from the loose materials and decomposed fragments of ice resulting from the prolonged action of the sun.
The most important facts, then, to be considered with reference to the motion of the glacier are as follows. First that the rate of advance between the axis and the margins of a glacier differs in the ratio of about ten to one and even less; that is to say; when the centre is advancing at a rate of two hundred and fifty feet a year, the motion toward the sides may be gradually diminished to two hundred, one hundred and fifty, one hundred, fifty feet, and so on, till nearest the margin it becomes almost inappreciable. Secondly, the rate of motion is not the same throughout the length of the glacier, the advance being greatest about half-way down in the region of the névé, and diminishing in rapidity both above and below; thus the onward motion in the higher portion of a glacier may not exceed twenty to fifty feet a year, while it reaches its maximum of some two hundred and fifty feet annually in the névé region, and is retarded again toward the lower extremity, where it is reduced to about one-fourth of its maximum rate. Thirdly, the glacier moves at different rates throughout the thickness of its mass; toward the lower extremity of the glacier the bottom is retarded, and the surface portion moves faster, while in the upper region the bottom seems to advance more rapidly. I say seems, because upon this latter point there are no positive measurements, and it is only inferred from general appearances, while the former statement has been demonstrated by accurate experiments. Remembering the form of the troughs in which the glaciers arise, that they have their source in expansive, open fields of snow and névé, and that these immense accumulations move gradually down into ever narrowing channels, though at times widening again to contract anew, their surface wasting so little from external influences that they advance far below the line of perpetual snow without any sensible diminution in size, it is evident that an enormous pressure must have been brought to bear upon them before they could have been packed into the lower valleys through which they descend.
Physicists seem now to agree that pressure is the chief agency in the motion of glaciers. No doubt, all the facts point that way; but it now becomes a matter of philosophical interest to determine in what direction it acts most powerfully, and upon this point glacialists are by no means agreed. The latest conclusion seems to be, that the weight of the advancing mass is itself the efficient cause of the motion. But while this is probably true in the main, other elements tending to the same result, and generally overlooked by investigators, ought to be taken into consideration; and before leaving the subject, I would add a few words upon infiltration in this connection.
The weight of the glacier, as a whole, is about the same all the year round. If, therefore, pressure, resulting from that weight, be the all-controlling agency, its progress should be uniform daring the whole year, or even greatest in winter, which is by no means the case. By a series of experiments, I have ascertained that the onward movement, whatever be its annual average, is accelerated in spring and early summer. The average annual advance of the glacier being, at a given point, at the rate of about two hundred feet, its average summer advance, at the same point, will be at a rate of two hundred and fifty feet, while its average rate of movement in winter will be about one hundred and fifty feet. This can be accounted for only by the increased pressure due to the large accession of water trickling in spring and early summer into the interior through the net-work of capillary fissures pervading the whole mass. The unusually large infiltration of water at that season is owing to the melting of the winter snow. Careful experiments made on the glacier of the Aar, respecting the water thus accumulating on the surface, penetrating its mass, and finally discharged in part at its lower extremity, fully confirm this view. Here, then, is a powerful cause of pressure and consequent motion, quite distinct from the permanent weight of the mass itself, since it operates only at certain seasons of the year. In midwinter, when the infiltration is reduced to a minimum, the motion is least. The water thus introduced into the glacier acts, as we have seen above, in various ways: by its weight, by loosening the particles of snow through which it trickles, and by freezing and consequent expansion, at least within the limits and during the season at which the temperature of the glacier sinks below 32° Fahrenheit. The simple fact, that in the spring the glacier swells on an average to about five feet more than its usual level, shows how important this infiltration must be. I can therefore only wonder that other glacialists have given so little weight to this fact. It is admitted by all, that the waste of a glacier at its surface, in consequence of evaporation and melting, amounts to about nine or ten feet in a year. At this rate of diminution, a glacier, even one thousand feet in thickness, could not advance during a single century without being exhausted. The water supplied by infiltration no doubt repairs the loss to a great degree. Indeed, the lower part of the glacier must be chiefly maintained from this source, since the annual increase from the fresh accumulations of snow is felt only above the snow-line, below which the yearly snow melts away and disappears. In a complete theory of the glaciers, the effect of so great an accession of plastic material cannot be overlooked.
I now come to some points in the structure of the glacier, the consideration of which is likely to have a decided influence in settling the conflicting views respecting their motion. The experiments of Faraday concerning regelation, and the application of the facts made known by the great English physicist to the theory of the glaciers, as first presented by Dr. Tyndall in his admirable work, show that fragments of ice with most surfaces are readily reunited under pressure into a solid mass. It follows from these experiments, that glacier-ice, at a temperature of 32° Fahrenheit, may change its form and preserve its continuity during its motion, in virtue of the pressure to which it is subjected. The statement is, that, when two pieces of ice with moistened surfaces are placed in contact, they become cemented together by the freezing of a film of water between them, while, when the ice is below 32° Fahrenheit, and therefore dry, no effect of the kind can be produced. The freezing was also found to take place under water; and the result was the same, even when the water into which the ice was plunged was as hot as the hand can bear.
The fact that ice becomes cemented under these circumstances is fully established, and my own experiments have confirmed it to the fullest extent. I question, however, the statement, that regelation takes place by the freezing of a film of water between the fragments. I never have been able to detect any indication of the presence of such a film, and am, therefore, inclined to consider this result as akin to what takes place when fragments of moist clay or marl are pressed together and thus reunited. When examining beds of clay and marl, or even of compact limestone, especially in large mountain-masses, I have frequently observed that the rock presents a net-work of minute fissures pervading the whole, without producing a distinct solution of continuity, though generally determining the lines according to which it breaks under sudden shocks. The net-work of capillary fissures pervading the glacier may fairly be compared to these rents in hard rocks,—with this difference, however, that in ice they are more permeable to water than in stone.
How this net-work of capillary fissures is formed has not been ascertained by direct observation. Following, however, the transformation of the snow and névé into compact ice, it is easily conceived that the porous mass of snow, as it falls in the upper regions of the Alps, and in the broad caldrons in which the glaciers properly originate, cannot pass into solid ice, by the process described in a former article, without retaining within itself larger or smaller quantities of air. This air is finally surrounded from all sides by the cementation of the granules of névé, through the freezing of the water that penetrates it. So inclosed, the bubbles of air are subject to the same compression as the ice itself, and become more flattened in proportion as the snow has been more fully transformed into compact ice. As long as the transformation of snow into ice is not complete, a rise of its temperature to 32° Fahrenheit, accompanied with thawing, reduces it at once again to the condition of loose grains of névé; but when more compact, it always presents the aspect of a mass composed of angular fragments, wedged and dove-tailed together, and separated by capillary fissures, the flattened air-bubbles trending in the same direction in each fragment, but varying in their trend from one fragment to another. There is, moreover, this important point to notice,—that, the older the névé, the larger are its composing granules; and where névé passes into porous ice, small angular fragments are mixed with rounded névé-granules, the angular fragments appearing larger and more numerous, and the névé-granules fewer, in proportion as the névé-ice has undergone most completely its transformation into compact glacier-ice. These facts show conclusively that the dimensions and form of the névé-granules, the size and shape of the angular fragments, the porosity of the ice, the arrangement of its capillary fissures, and the distribution and compression of the air-bubbles it contains, are all connected features, mutually dependent. Whether the transformation of snow into ice be the result of pressure only, or, as I believe, quite as much the result of successive thawings and freezings, these structural features can equally be produced, and exhibit these relations to one another. It may be, moreover, that, when the glacier is at a temperature below 32°, its motion produces extensive fissuration throughout the mass.
Now that water pervades this net-work of fissures in the glacier to a depth not yet ascertained, my experiments upon the glacier of the Aar have abundantly proved; and that the fissures themselves exist at a depth of two hundred and fifty feet I also know, from actual observation. All this can, of course, take place, even if the internal temperature of the glacier never should fall below 32° Fahrenheit; and it has actually been assumed that the temperature within the glacier does not fall below this point, and that, therefore, no phenomena, dependent upon a greater degree of cold, can take place beyond a very superficial depth, to which the cold outside may be supposed to penetrate. I have, however, observed facts which seem to me irreconcilable with this assumption. In the first place, a thermometrograph indicating -2° Centigrade, (about 28° Fahrenheit,) at a depth of a little over two metres, that is, about six feet and a half, has been recovered from the interior of the glacier of the Aar, while all my attempts to thaw out other instruments placed in the ice at a greater depth utterly failed, owing to the circumstance, that, after being left for some time in the glacier, they were invariably frozen up in newly formed water-ice, entirely different in its structure from the surrounding glacier-ice. This freezing could not have taken place, did the mass of the glacier never fall below 32° Fahrenheit. And this is not the only evidence of hard frost in the interior of the glaciers. The innumerable large walls of water-ice, which may be seen intersecting their mass in every direction and to any depth thus far reached, show that water freezes in their interior. It cannot be objected, that this is merely the result of pressure; since the thin fluid seams, exhibited under pressure in the interesting experiments of Dr. Tyndall, and described in his work under the head of Crystallization and Internal Liquefaction, cannot be compared to the large, irregular masses of water-ice found in the interior of the glacier, to which I here allude.
In the absence of direct thermometric observations, from which the lowest internal temperature of the glacier could be determined with precision in all its parts, we are certainly justified in assuming that every particle of water-ice found in the glacier, the formation of which cannot be ascribed to the mere fact of pressure, is due to the influence of a temperature inferior to 32° Fahrenheit at the time of its consolidation. The fact that the temperature in winter has been proved by actual experimentation to fall as low as 28° Fahrenheit, that is, four degrees below the freezing-point at a depth of six feet below a thick covering of snow, though not absolutely conclusive as to the temperature at a greater depth, is certainly very significant.
Under these circumstances, it is not out of place to consider through what channels the low temperature of the air surrounding the glacier may penetrate into the interior. The heavy cold air may of course sink from the surface into every large open space, such as the crevasses, large fissures, and moulins or mill like holes to be described in a future article; it may also penetrate with the currents which ingulf themselves under the glacier, or it may enter through its terminal vault, or through the lateral openings between the walls of the valley and the ice. Indeed, if all the spaces in the mass of the glacier, not occupied by continuous ice, could be graphically represented, I believe it would be seen that cold air surrounds the glacier-ice itself in every direction, so that probably no masses of a greater thickness than that already known to be permeable to cold at the surface would escape this contact with the external temperature. If this be the case, it is evident that water may freeze in any part of the glacier.
To substantiate this position, which, if sustained, would prove that the dilatation of the mass of the glacier is an essential element of its motion, I may allude to several other well-known facts. The loose snow of the upper regions is gradually transformed into compact ice. The experiments of Dr. Tyndall prove that this may be the result of pressure; but in the region of the névé it is evidently owing to the transformation of the snow-flakes into ice by repeated melting and freezing, for it takes place in the uppermost layers of the snow, where pressure can have no such effect, as well as in its deeper beds. I take it for granted, also, that no one, familiar with the presence of the numerous ice-seams parallel to the layers of snow in these upper regions of the glacier, can doubt that they, as well as the névé, are the result of frost. But be this as it may, the difference between the porous ice of the upper region of the glacier and the compact blue ice of its lower track seems to me evidence direct that at times the whole mass must assume the rigidity imparted to it by a temperature inferior to the freezing-point. We know that at 32° Fahrenheit, regelation renders the mass continuous, and that it becomes brittle only at a temperature below this. In other words, the ice can break up into a mass of disconnected fragments, such as the capillary fissures and the infiltration-experiments described in my "Système Glaciaire," show to exist, only when it is below 32° Fahrenheit. If it be contended that ice at 32° does break, and that therefore the whole mass of the glacier may break at that temperature, setting aside the contradiction to the facts of regelation which such an assumption involves, I would refer to Dr. Tyndall's experiments concerning the vacuous spots in the ice.
Those who have read his startling investigations will remember that by sending a beam of sunlight through ice he brought to view the primitive crystalline forms to which it owes its solidity, and that he insisted that these star-shaped figures are always in the plane of crystallization. Without knowing what might be their origin, I had myself noticed these figures, and represented them in a diagram, part of which is reproduced in the annexed wood-cut. I had considered them to be compressed air-bubbles; and though I cannot, under my present circumstances, repeat the experiment of Dr. Tyndall upon glacier-ice, I conceive that the star-shaped figures represented upon Pl. VII. figs. 8 and 9, in my "Système Glaciaire," may refer to the same phenomenon as that observed by him in pond-ice. Yet while I make this concession, I still maintain, that besides these crystalline figures there exist compressed air-bubbles in the angular fragments of the glacier-ice, as shown in the above wood-cut; and that these bubbles are grouped in sets, trending in the same direction in one and the same fragment, and diverging under various angles in the different fragments. I have explained this fact concerning the position of the compressed air-bubbles, by assuming that ice, under various pressure, may take the appearance it presents in each fragment with every compressed air-bubble trending in the same direction, while their divergence in the different fragments is owing to a change in the respective position of the fragments resulting from the movement of the whole glacier. I have further assumed, that throughout the glacier the change of the snow and porous ice into compact ice is the result of successive freezing, alternating with melting, or at least with the resumption of a temperature of 32° Fahrenheit in consequence of the infiltration of liquid water, to which the effects of pressure must be added, the importance of which in this connection no one could have anticipated prior to the experiments of Dr. Tyndall. Of course, if the interior temperature of the glacier never falls below 32°, the changes here alluded to could not take place. But if the vacuous spaces observed by Dr. Tyndall are really identical with the spaces I have described as extremely flattened air-bubbles, I think the arrangement of these spaces as above described proves that it freezes in the interior of the glacier to the depth at which these crosswise fragments have been observed: that is, at a depth of two hundred feet. For, since the experiments of Dr. Tyndall show that the vacuous spaces are parallel to the surface of crystallization, and as no crystallization of water can take place unless the surrounding temperature fall below 32°, it follows that these vacuous spaces could not exist in such large continuous fragments, presenting throughout the fragments the same trend, if there had been no frost within the mass, affecting the whole of such a fragment while it remained in the same position.
The most striking evidence, in my opinion, that at times the whole mass of the glacier actually freezes, is drawn from the fact, already alluded to, that, while the surface of the glacier loses annually from nine to ten feet of its thickness by evaporation and melting, it swells, on the other hand, in the spring, to the amount of about five feet. Such a dilatation can hardly be the result of pressure and the packing of the snow and ice, since the difference in the bulk of the ice brought down, during one year, from a point above to that under observation, would not account for the swelling. It is more readily explained by the freezing of the water of infiltration during spring and early summer, when the infiltration is most copious and the winter cold has been accumulating for the longest time. This view of the case is sustained by Élie de Beaumont, who states his opinion upon this point as follows:—
"Pendant l'hiver, la température de la surface du glacier s'abaisse à un grand nombre de degrés au-dessous de zéro, et cette basse température pénètre, quoique avec un affaiblissement graduel, dans l'interieur de la masse. Le glacier se fendille par l'effet de la contraction résultant de ce refroidissement. Les fentes restent d'abord vides, et concourent an refroidissement des glaciers en favorisant l'introduction de l'air froid extérieur; mais an printemps, lorsque les rayons du soleil échaffent la surface de la neige qui couvre le glacier, ils la remènent d'abord à zéro, et ils produisent ensuite de l'eau à zéro qui tombe dans le glacier refroidi et fendillé. Cette eau s'y congèle à l'instant, en laissant dégager de la chaleur qui tend à ramener le glacier à zéro; et la phénomène se continue jusqu'à ce que la masse entière du glacier refroidi soit ramené à la température de zéro."[H]
But where direct observations are still so scanty, and the interpretations of the facts so conflicting, it is the part of wisdom to be circumspect in forming opinions. This much, however, I believe to be already settled: that any theory which ascribes the very complicated phenomena of the glacier to one cause must be defective and one-sided. It seems to me most probable, that, while pressure has the larger share in producing the onward movement of the glacier, as well as in the transformation of the snow into ice, a careful analysis of all the facts will show that this pressure is owing partly to the weight of the mass itself, partly to the pushing on of the accumulated snow from behind, partly to its sliding along the surface upon which it rests, partly to the weight of water pervading the whole, partly to the softening of the rigid ice by the infiltration of water, and partly, also, to the dilatation of the mass, requiting from the freezing of this water. These causes, of course, modify the ice itself, while they contribute to the motion. Further investigations are required to ascertain in what proportion these different influences contribute to the general result, and at what time and under what circumstances they modify most directly the motion of the glacier.
That a glacier cannot be altogether compared to a river, although there is an unmistakable analogy between the flow of the one and the onward movement of the other, seems to me plain,—since the river, by the combination of its tributaries, goes on increasing in bulk in consequence of the incompressibility of water, while a glacier gradually thins out in consequence of the packing of its mass, however large and numerous may be its accessions. The analogy fails also in one important point, that of the acceleration of speed with the steepness of the slope. The motion of the glacier bears no such direct relation to the inclination of its bed. And though in a glacier, as in a river, the axis of swiftest motion is thrown alternately on one or the other side of the valley, according to its shape and slope, the very nature of ice makes it impossible that eddies should be formed in the glacier, and the impressive feature of whirlpools is altogether wanting in them. What have been called glacier-cascades bear only a remote resemblance to river-cascades, as in the former the surface only is thrown into confusion by breaking, without affecting the primitive structure;[I] and I reiterate my formerly expressed opinion that even the stratification of the upper regions is still recognizable at the lower end of the glacier of the Rhone.
The internal structure of the glacier has already led me beyond the limits I had proposed to myself in the present article. But I trust my readers will not be discouraged by this dry discussion of various theories concerning it, and will meet me again on the glacier, when we will examine together some of its more picturesque features, its crevasses, its rivulets and cascades, its moraines, its boulders, etc., and endeavor also to track its ancient course and boundaries in earlier geological times.
The preface of "Outre-Mer," Longfellow's first book, is dated 1833. The last poem in his last volume is published in 1863. In those thirty years what wide renown, what literary achievement, what love of friends in many lands, what abounding success and triumph, what profound sorrow, mark the poet's career! The young scholar, returning from that European tour which to the imaginative and educated American is the great romance, sits down in Bowdoin College in Maine, where he is Professor, and writes the "Epistle Dedicatory" to the "worthy and gentle reader." Those two phrases tell the tale. The instinct of genius and literary power stirring in the heart of the young man naturally takes the quaint, dainty expression of an experience fed, thus far, only upon good old books and his own imagination. The frolicking tone of mock humility, deprecating the intrusion upon the time of a busy world, does not conceal the conviction that the welcome so airily asked by the tyro will at last be commanded by the master.
Like the "Sketch-Book" of the other most popular of our authors, Irving, the "Outre-Mer" of Longfellow is a series of tales, reveries, descriptions, reminiscences, and character-pieces, suggested by European travel. But his beat lies in France, Spain, and Italy. It is the romance of the Continent, and not that of England, which inspires him. It is the ruddy light upon the vines and the scraps of old chansons which enliven and decorate his pilgrimage, and through all his literary life they have not lost their fascination. While Irving sketches "Rural Life in England," Longfellow paints "The Village of Auteuil"; Irving gives us "The Boar's Head Tavern," and Longfellow "The Golden Lion Inn" at Rouen; Irving draws "A Royal Poet," Longfellow discusses "The Trouvères," or "The Devotional Poetry of Spain." It is delightful to trace the charming resemblance between the books and the writers, widely different as they are. There is the same geniality, the same tender pathos, the same lambent humor, the same delicate observation of details, the same overpowering instinct of literary art. But Geoffrey Crayon is a humorist, while the Pilgrim beyond the Sea is a poet. The one looks at the broad aspects of English life with the shrewd, twinkling eye of a man of the world; the other haunts the valley of the Loire, the German street, the Spanish inn, with the kindling fancy of the scholar and poet. The moral and emotional elements are quite wanting in Irving; they are characteristic of Longfellow. But the sweetness of soul, the freedom from cynicism or stinging satire, which is most unusual in American, or in any humorous or descriptive literature, is remarkable in both. "I have no wife, nor children, good or bad, to provide for," begins Geoffrey Crayon, quoting from old Burton. But neither had he an enemy against whom to defend himself. It was true of Geoffrey Crayon, down to the soft autumn day on which he died, leaving a people to mourn for him. It is true of the Pilgrim of Outre-Mer, in all the thirty years since first he launched forth "into the uncertain current of public favor."
In this earliest book of Longfellow's the notable points are not power of invention, or vigorous creation, or profound thought, but a mellowness of observation, instinctively selecting the picturesque and characteristic details, a copious and rich scholarship, and that indefinable grace of the imagination which announces genius. The work, like the "Sketch-Book," was originally issued in parts, and it was hardly possible for any observer thirty years ago not to see that its peculiar character revealed a new strain in our literature. Longfellow's poems as yet were very few, printed in literary journals, and not yet signalizing his genius. It was the day when Percival Halleck, Sprague, Dana, Willis, Bryant, were the undisputed lords of the American Parnassus. But the school reading-books already contained "An April Day" and "Woods in Winter," and all the verses of the young author had a recognition in volumes of elegant extracts and commonplace-books. But the universal popularity of Longfellow was not established until the publication of "Hyperion" in 1839, followed by "The Voices of the Night" in the next year. With these two works his name arose to the highest popularity, both in America and England; and no living author has been more perpetually reproduced in all forms and with every decoration.
If now we care to explain the eager and affectionate welcome which always hails his writings, it is easy to see to what general quality that greeting must be ascribed. As with Walter Scott, or Victor Hugo, or Béranger, or Dickens, or Addison in the "Spectator," or Washington Irving, it is a genial humanity. It is a quality, in all these instances, independent of literary art and of genius, but which is made known to others, and therefore becomes possible to be recognized, only through literary forms. The creative imagination, the airy fancy, the exquisite grace, harmony, and simplicity, the rhetorical brilliancy, the incisive force, all the intellectual powers and charms of style with which that feeling may be expressed, are informed and vitalized by the sympathy itself. But whether a man who writes verses has genius,—whether he be a poet according to arbitrary canons,—whether some of his lines resemble the lines of other writers,—and whether he be original, are questions which may be answered in every way of every poet in history. Who is a poet but he whom the heart of man permanently accepts as a singer of its own hopes, emotions, and thoughts? And what is poetry but that song? If words have a uniform meaning, it is useless to declare that Pope cannot be a poet, if Lord Byron is, or that Moore is counterfeit, if Wordsworth be genuine. For the art of poetry is like all other arts. The casket that Cellini worked is not less genuine and excellent than the dome of Michel Angelo. Is nobody but Shakspeare a poet? Is there no music but Beethoven's? Is there no mountain-peak but Dhawalaghiri? no cataract but Niagara?
Thirty years ago almost every critic in England exploded with laughter over the poetry of Tennyson. Yet his poetry has exactly the same characteristics now that it had then; and Tennyson has gone up to his place among English poets. It is not "Blackwood," nor any quarterly review or monthly magazine, (except, of course, the "North American" and the "Atlantic,") which can decree or deny fame. While the critics are busily proving that an author is a plagiarist or a pretender, the world is crowning him,—as the first ocean-steamer from England brought Dr. Lardner's essay to prove that steamers could not cross the ocean. Literary criticism, indeed, is a lost art, if it ever were an art. For there are no permanent acknowledged canons of literary excellence; and if there were any, there are none who can apply them. What critic shall decide if the song of a new singer be poetry, or the bard himself a poet? Consequently, modern criticism wisely contents itself with pointing out errors of fact or of inference, or the difference between the critic's and the author's philosophic or æsthetic view, and bitterly assaults or foolishly praises him. When Horace Binney Wallace, one of the most accomplished and subtile-minded of our writers, says of General Morris that he is "a great poet," and that "he who can understand Mr. Emerson may value Mr. Bancroft," we can feel only the more profoundly persuaded that fame is not the judgment of individuals, but of the mass of men, and that he whose song men love to hear is a poet.
But while the magnetism of Longfellow's touch lies in the broad humanity of his sympathy, which leads him neither to mysticism nor cynicism, and which commends his poetry to the universal heart, his artistic sense is so exquisite that each of his poems is a valuable literary study. In this he has now reached a perfection quite unrivalled among living poets, except sometimes by Tennyson. His literary career has been contemporary with the sensational school, but he has been entirely untainted by it, and in the present volume, "Tales of a Wayside Inn," his style has a tranquil lucidity which recalls Chaucer. The literary style of an intellectually introverted age or author will always be somewhat obscure, however gorgeous; but Longfellow's mind takes a simple, child-like hold of life, and his style never betrays the inadequate effort to describe thoughts or emotions that are but vaguely perceived, which is the characteristic of the best sensational writing. Indeed, there is little poetry by the eminent contemporary masters which is so ripe and racy as his. He does not make rhetoric stand for passion, nor vagueness for profundity; nor, on the other hand, is he such a voluntary and malicious "Bohemian" as to conceive that either in life or letters a man is released from the plain rules of morality. Indeed, he used to be accused of preaching in his poetry by gentle critics who held that Elysium was to be found in an oyster-cellar, and that intemperance was the royal prerogative of genius.
His literary scholarship, also, his delightful familiarity with the pure literature of all languages and times, must rank Longfellow among the learned poets. Yet he wears this various knowledge like a shining suit of chain-mail, to adorn and strengthen his gait, like Milton, instead of tripping and clumsily stumbling in it, as Ben Jonson sometimes did. He whips out an exquisitely pointed allusion that flashes like a Damascus rapier and strikes nimbly home, or he recounts some weird tradition, or enriches his line with some gorgeous illustration from hidden stores, or merely unrolls, as Milton loved to do, the vast perspective of romantic association by recounting in measured order names which themselves make music in the mind,—names not musical only, but fragrant:—