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.