APPENDIX, No. I. AN ATTEMPT TO ILLUSTRATE THE ORIGIN OF "DIRTBANDS" IN GLACIERS. By A. MILWARD, Esq.* "It will be remembered that Professor Forbes, in his interesting work on the Glaciers of the Alps, describes the "dirt-bands to be nothing more than curved bands of porous ice, the surface of which affords a readier receptacle to the drift of the glacier. He found the dirt to be superficial, and merely an indication that the glacier is made up of two kinds of ice, the one more porous than the other; so that the dirt lodges in the one more readily than in the other. The question to be determined is, how we are to account for the existence of the different kinds of ice thus regularly alternating. The dirt being merely accidental to this subject of inquiry, it will be better to speak of the dirt-bands, and the intervals between them, as the alternating bands of porous and compact ice.† "The dirt-bands are found to follow the direction of the hyperbolic curves marked out by the outcrop of the structural planes, known by the name of the ribbon-structure. The ice forming the dirt-bands is made up of that structure, in the same way as the other ice; and depends, of course, upon those laws in obedience to which the ribbonstructure originates. For this reason, the curve of the bands is, like that of the structure, found to be elongated low down the glacier, and compressed as we approach its source. We have thus only to account for the existence of bands of different kinds of ice, the form of curve of those bands being explained in the same way as that of the ribbon-structure. "It may be observed, that the superior distinctness of the dirtbands, as we proceed lower down the glacier, is not necessarily an evidence that the bands of porous and compact ice are there more decidedly developed, but only that they are more distinctly apparent. And this, I imagine, arises from the fact, that the lower ice has been washed over for years; and thus the pores have become more discoloured by the deposit of drift than the pores of the corresponding porous ice above. * [From the Edinburgh New Philosophical Journal. Jan. 1849.] "It was the remarkable similarity of the alternating bands on the mud-slide already described, to the dirt-bands' on Professor Forbes' map of the Mer de Glace, that induced me to take an interest in the matter, and make a drawing of the phenomenon. In the first instance, the curved bands were a mystery to me; and I could not venture to found any argument on a mere analogy of appearance. The second mud-slide, however, seemed to shew me another step in the process; and, having explained the one from the other, I was led to ask myself whether the phenomena, to which one class of viscous fluids appeared to be subject, might not be common to another; in other words, to a glacier. "In our first mud-slide we observe, first, the occurrence of curved bands; and, secondly, a difference of consistency in those bands. Our second mud-slide shews the origin of those curved bands, as far as mud-slides are concerned, to be the previous existence of ridges or wrinkles. Turning, on the other hand, to the glacier, we find curved bands of different consistency and similar appearance, which I have called relatively bands of porous and compact ice. "We may then fairly ask,—If, in one species of viscous fluid, alternate bands are derived from pre-existing ridges, why should not analogous bands in another species of viscous fluid give rise to a prima facie presumption, that ridges are to be looked for in an earlier stage of that viscous fluid also? Is not the analogy just so far strong enough as to induce us to examine whether there is any trace of such ridges or waves; and, if so, whether their correspondence with the alternations of porous and compact ice is sufficient to account for the latter ? "It is evident that such ridges or waves, if they do exist, must be very slightly marked, or they would not have been overlooked; but then, it is to be remembered, that the difference between the two kinds of ice is also very slight-in fact, only barely apparent. It will be useless to look for them at the lower parts of glaciers, as they will have disappeared under the effects of atmospheric and other action through the lapse of many years, which will have degraded any existing ridges, just in the same way as in the case of outcropping strata. "There is also a tendency to the establishment of an equilibrium as to elevation, to say nothing of the disturbing effect of the lateral friction. It is, therefore, towards the head of the glacier, where the true glacial structure commences, that we are to look for such ridges: the best time, also, will be at the commencement of summer, after the MR. MILWARD ON DIRT-BANDS IN GLACIERS. 263 disappearance of the snow, and before the confusion of the surface occasioned by the sun's influence. แ "1. It has been suggested to me, that in the case of the mud-slide, there may be an original difference of consistency in the bands of mud, which is only increased by the action of the drainage water; and that the ridges and intervals are the outcropping of these beds which compose the mud-stream. If there be such a tendency in viscous fluids to separate into beds of different consistency, such a difference of consistency may exist in glaciers, although the actual ridges may never be sufficiently developed to be apparent. Such a peculiarity of viscous structure would account for the bands of porous and compact ice, whether the ridges be found or not. "At present, however, we have no proof of such an internal structure, and may therefore dismiss it. "2. If, however, ridges or waves are found to exist, the case becomes precisely analogous to that of the mud-slide: The lower extremity of each ridge will be more broken up than the other parts, just as we see the ridges and lower extremities of the several mud-streams to be broken up and more porous. From these high and broken parts, the water will drain and saturate, to a greater extent, the surfaces below. The ice thus saturated will become, by the action of frost, more compact than the rougher ridges. This fact, that saturated ice produces the most compact glacier, appears to have been already assumed (either from experience or otherwise), in the explanation which is given of the formation of the transparent blue bands of the ribbon-structure. An objection to which this explanation is open, will be found in the very different width of the porous and compact bands on the glacier; whereas in the mud-slide they are nearly equal. "In any case, however, it seems that there is sufficient ground why we should look for such ridges or waves; but at the same time, it is quite possible that they may be proper to a peculiar condition of viscous matter, to which class the glacier does not belong, or it may be that special resistance and obstruction prevents the development of such peculiarity in the case of glaciers. If this be so, the direct analogy between the glacier and the mud-slide in this respect vanishes. "3. From the manner in which the second mud-slide explains the first, depending, it will be remembered on the drainage of water altering the character of the mud-another deduction may, I think, be drawn as applicable to glaciers, although not exactly in the same The result to which I am about to draw your attention must, way. I think, have a real existence. It is a result to which the dirt-bands may be the indication, or it may have passed altogether unobserved. "We find at the heads of most glaciers, where the névé is passing into ice, and the body assuming its normal form and construction, that there are steeper elevations, from which the névé descends, and frequently ice-cascades. The glacier is in these parts, on account of the abrupt descent, very much broken up, and often impassable. Now it appears evident, that, at the foot of these slopes, the water which has passed more quickly down them will accumulate to a greater extent than if there had been no elevation behind, on account of the change of inclination. Now, during the summer months, the saturation thus taking place will be greatest, because of the large quantity of water then coming down. At this period of the year, likewise, the motion of the glacier is also greatest, and a large advance of the saturated body occurs. This, during the winter frosts, is consolidated, and formed, I imagine, into more compact ice than would have resulted from less saturated material. "On the other hand, in the winter months, that part of the glacier at the foot of the upper slopes, or ice-cascades, will be less saturated, as the surface of the whole glacier is then in a state of comparative rest, in consequence of the diminished effect of the sun's rays in thawing the surface of the névé. At this time, also, the glacier moves with far less rapidity; and so the quantity of glacier in a less saturated state thus moving on, will be considerably less than that advancing during the summer. In consequence, also, of its being less saturated with water, it will, after consolidation, be less compact than that which moved forward during the summer. Viewed in this light, the foot of the upper slopes, or ice-cascades, may be considered as a kind of laboratory for the manufacture of alternate bands of compact and porous ice the former made during the summer, and the latter in the winter months. Thus, if my theory be correct, a wide band of comparatively compact ice, and a narrow band of porous ice, will be annually formed and added to the glacier. "If these alternate bands be considered as identical with the porous and compact bands to which the dirt-bands belong, it follows, that the porous bands, during the progress down the glacier, become apparent by the absorption of the drift, which is washed over the surface, and their distinctness increases with the length of time during which they have been subjected to the drift. "Thus the wide compact band answers to the interval between the |