The Glaciers of the Alps / Being a narrative of excursions and ascents, an account of the origin and phenomena of glaciers and an exposition of the physical principles to which they are related

by John Tyndall

FORBES'S RESEARCHES.

Previous observers had mistaken the lamination for stratification; but M. Guyot not only clearly saw that they were different, but in the comparison which he makes he touches, I believe, on the true cause of the glacier-structure. He did not hazard an explanation of the phenomenon, and I believe his memoir remained unprinted. In 1841 the structure was noticed by Professor Forbes during his visit to M. Agassiz on the lower Aar Glacier, and described in a communication presented by him to the Royal Society of Edinburgh. He subsequently devoted much time to the subject, and his great merit in connexion with it consists in the significance which he ascribed to the phenomenon when he first observed it, and in the fact of his having proved it to be a constitutional feature of glaciers in general.

FORBES'S THEORY.

The first explanation given of those veins by Professor Forbes was, that they were small fissures formed in the ice by its motion; that these were filled with the water of the melted ice in summer, which froze in winter so as to form the blue veins. This is the explanation given in his 'Travels,' page 377; and in a letter published in the 'Edinburgh New Philosophical Journal,' October, 1844, it is re-affirmed in these words:—"With the abundance of blue bands before us in the direction in which the differential motion must take place (in this case sensibly parallel to the sides of the glacier), it is impossible to doubt that these infiltrated crevices (for such they undoubtedly are) have this origin." This theory was examined by Mr. Huxley and myself in our joint paper; but it has been since alleged that ours was unnecessary labour, Prof. Forbes himself having in his Thirteenth Letter renounced the theory, and substituted another in its place. The latter theory differs, so far as I can understand it, from the former in this particular, that the freezing of the water in the fissures is discarded, their sides being now supposed to be united "by the simple effects of time and cohesion."[B] For a statement of the change which his opinions have undergone, I would refer to the Prefatory Note which precedes the volume of 'Occasional Papers' recently published by Prof. Forbes; but it would have diminished my difficulty had the author given, in connexion with his new volume, a more distinct statement of his present views regarding the veined structure. With many of his observations and remarks I should agree; with many others I cannot say whether I agree or not; and there are others still with which I do not think I should agree: but in hardly any case am I certain of his precise views, excepting, indeed, the cardinal one, wherein he and others agree in ascribing to the structure a different origin from stratification. Thus circumstanced, my proper course, I think, will be to state what I believe to be the cause of the structure, and leave it to the reader to decide how far our views harmonize; or to what extent either of them is a true interpretation of nature.

USUAL ASPECT OF BLUE VEINS.

Most of the earlier observers considered the structure to be due to the stratification of the mountain-snows—a view which has received later development at the hands of Mr. John Ball; and the practical difficulty of distinguishing the undoubted effects of stratification from the phenomena presented by structure, entitles this view to the fullest consideration. The blue veins of glaciers are, however, not always, nor even generally, such as we should expect to result from stratification. The latter would furnish us with distinct planes extending parallel to each other for considerable distances through the glacier; but this, though sometimes the case, is by no means the general character of the structure. We observe blue streaks, from a few inches to several feet in length, upon the walls of the same crevasse, and varying from the fraction of an inch to several inches in thickness. In some cases the streaks are definitely bounded, giving rise to an appearance resembling the section of a lens, and hence called the "lenticular structure" by Mr. Huxley and myself; but more usually they fade away in pale washy streaks through the general mass of the whitish ice. In [Fig. 39] I have given a representation of the structure as it is very commonly exhibited on the walls of crevasses. Its aspect is not that which we should expect from the consolidation of successive beds of mountain snow.

Further, at the bases of ice-cascades the structural laminæ are usually vertical: below the cascade of the Talèfre, of the Noire, of the Strahleck branch of the Lower Grindelwald Glacier, of the Rhone, and other ice-falls, this is the case; and it seems extremely difficult to conceive that a mass horizontally stratified at the summit of the fall, should, in its descent, contrive to turn its strata perfectly on end.

Again, we often find a very feebly-developed structure at the central portions of a glacier, while the lateral portions are very decidedly laminated. This is the case where the inclination of the glacier is nearly uniform throughout; and where no medial moraines occur to complicate the phenomenon. But if the veins mark the bedding, there seems to be no sufficient reason for their appearance at the lateral portions of the glacier, and their absence from the centre.