MORAINES. 1857.
TRIBUTARIES OF THE MER DE GLACE. 1857.
From the Aiguille du Géant and Les Périades a glacier descended, which was separated by the promontory of La Noire from the glacier proceeding from the Col du Géant. A small moraine was formed between them, which is marked a upon the diagram, Fig. 7. The great mass of the glacier descending from the Col du Géant came next, and this was bounded on the side nearest to Trélaporte by a small moraine b, the origin of which I could not see, its upper portion being shut out by a mountain promontory. Between the moraine b and the actual side of the valley was another little glacier, derived from some of the lateral tributaries. It was, however, between the moraines a and b that the great mass of the Glacier du Géant really lay. At the promontory of the Tacul the lateral moraines of the Glacier des Périades and of the Glacier de Léchaud united to form the medial moraine c of the Mer de Glace. Carrying the eye across the Léchaud, we had the moraine d formed by the union of the lateral moraines of the Léchaud and Talèfre; further to the left was the moraine e, which came from the Jardin, and beyond it was the second lateral moraine of the Talèfre. The Mer de Glace is formed by the confluence of the whole of the glaciers here named; being forced at Trélaporte through a passage, the width of which appears considerably less than that of the single tributary, the Glacier du Géant.
In the ice near Trélaporte the blue veins of the glacier are beautifully shown; but they vary in distinctness according to the manner in which they are looked at. When regarded obliquely their colour is not so pronounced as when the vision plunges deeply into them. The weathered ice of the surface near Trélaporte could be cloven with great facility; I could with ease obtain plates of it a quarter of an inch thick, and possessing two square feet of surface. On the 28th of July I followed the veins several times from side to side across the Géant portion of the Mer de Glace; starting from one side, and walking along the veins, my route was directed obliquely downwards towards the axis of the tributary. At the axis I was forced to turn, in order to keep along the veins, and now ascended along a line which formed nearly the same angle with the axis at the other side. Thus the veins led me as it were along the two sides of a triangle, the vertex of which was near the centre of the glacier. The vertex was, however, in reality rounded off, and the figure rather resembled a hyperbola, which tended to coincidence with its asymptotes. This observation corroborates those of Professor Forbes with regard to the position of the veins, and, like him, I found that at the centre the veining, whose normal direction would be transverse to the glacier, was contorted and confused.
WASTING OF ICE. 1857.
Near the side of the Glacier du Géant, above the promontory of Trélaporte, the ice is rent in a remarkable manner. Looking upwards from the lower portions of the glacier, a series of vertical walls, rising apparently one above the other, face the observer. I clambered up among these singular terraces, and now recognise, both from my sketch and memory, that their peculiar forms are due to the same action as that which has given their shape to the "billows" of the Mer de Glace. A series of profound crevasses is first formed. The Glacier du Géant deviates 14° from the meridian line, and hence the sun shines nearly down it during the middle portion of each day. The backs of the ridges between the crevasses are thus rounded off, one boundary of each fissure is destroyed, or at least becomes a mere steep declivity, while the other boundary being shaded from the sun preserves its verticality; and thus a very curious series of precipices is formed.
Through all this dislocation, the little moraine on which I have placed the letter b in the sketch maintains its right to existence, and under it the laminated structure of this portion of the glacier appears to reach its most perfect development. The moraine was generally a mere dirt track, but one or two immense blocks of granite were perched upon it. I examined the ice underneath one of these, being desirous of seeing whether the pressure resulting from its enormous weight would produce a veining, but the result was not satisfactory. Veins were certainly to be seen in directions different from the normal ones, but whether they were due to the bending of the latter, or were directly owing to the pressure of the block, I could not say. The sides of a stream which had cut a deep gorge in the clean ice of the Glacier du Géant afforded a fine opportunity of observing the structure. It was very remarkable—highly significant indeed in a theoretic point of view. Two long and remarkably deep blue veins traversed the bottom of the stream, and bending upwards at a place where the rivulet curved, drew themselves like a pair of parallel lines upon the clean white ice. But the general structure was of a totally different character; it did not consist of long bars, but approximated to the lenticular form, and was, moreover, of a washy paleness, which scarcely exceeded in depth of colouring the whitish ice around.
GROOVES ON THE SURFACE. 1857.
To the investigator of the structure nothing can be finer than the appearance of the glacier from one of the ice terraces cut in the Glacier du Géant by its passage round Trélaporte. As far as the vision extended the dirt upon the surface of the ice was arranged in striæ. These striæ were not always straight lines, nor were they unbroken curves. Within slight limits the various parts into which a glacier is cut up by its crevasses enjoy a kind of independent motion. The grooves, for example, on two ridges which have been separated by a small fissure, may one day have their striæ perfect continuations of each other, but in a short time this identity of direction may be destroyed by a difference of motion between the ridges. Thus it is that the grooves upon the surface above Trélaporte are bent hither and thither, a crack or seam always marking the point where their continuity is ruptured. This bending occurs, however, within limits sufficiently small to enable the striæ to preserve the same general direction.
SEAMS OF WHITE ICE. 1857.
My attention had often been attracted this day by projecting masses of what at first appeared to be pure white snow, rising in seams above the general surface of the glacier. On examination, however, I found them to be compact ice, filled with innumerable air-cells, and so resistant as to maintain itself in some places at a height of four feet above the general level. When amongst the ridges they appeared discontinuous and confused, being scattered apparently at random over the glacier; but when viewed from a sufficient distance, the detached parts showed themselves to belong to a system of white seams which swept quite across the Glacier du Géant, in a direction concentric with the structure. Unable to account for these singular seams, I climbed up among the tributary glaciers on the Rognon side of the Glacier du Géant, and remained there until the sun sank behind the neighbouring peaks, and the fading light warned me that it was time to return.
FOOTNOTES:
[A] "Le petit Balmat" my host always called him.
[B] To such towers the name Séracs is applied. In the chalets of Savoy, after the richer curd has been precipitated by rennet, a stronger acid is used to throw down what remains; an inferior kind of cheese called Sérac is thus formed, the shape and colour of which have suggested the application of the term to the cubical masses of ice.
(8.)
Early on the following day I was again upon the ice. I first confined myself to the right side of the Glacier du Géant, and found