characterise it. Mr J. D. Forbes studied them on the Mer de Glace of Chamonix; but it is on the glaciers of the Aar that the observations have been prosecuted with most care and perseverance. From 1842, MM. Agassiz and Desor, assisted by MM. Wyld, Otz, and Dollfus-Ausset, have been unceasingly occupied with this question; they have ascertained that, in its medium part, this glacier advances 71 metres a year. Toward the lower extremity, the rapidity of the progression decreases till it does not exceed 39 metres: it accelerates a little, on the contrary, towards the top, where the glacier annually traverses a space of 75 metres.*

The inclination of the slope on which the glacier descends does not appear to have any influence on the rapidity of its progress, but it is singularly modified by the walls of the hollow in which it moves. The friction of the ice against these walls considerably retards the advance of the lateral parts of the glacier. Besides this, if a promontory jut out towards the middle of the valley, the glacier, arrested on one of its sides, turns round the obstacle with extreme slowness, or rather this side remains behind, while the central portion and the opposite edge continue to advance with their relative quickness.

* The following is a short account of the method in which the advance of a glacier was measured. On the two banks, two rocks were chosen opposite each other; each of these rocks was marked by a white cross painted upon the stone. A series of stakes were then fixed in the ice in a line between these two points, so as to form a straight line perpendicular to the axis of the glacier. After some days, an observer placed himself before one of the crosses, and directed a telescope, bearing a level, towards the one opposite. The glacier having advanced, and the stakes along with it, the latter were no longer in their original line. Then a guide placed on the glacier and carrying a pole surmounted by a very visible object, placed the pole in the direction of the former line. This direction was indicated to him by the signal of the observer, whose eye was applied to the telescope. The latter caused the pole to be carried back till it was exactly at the point formerly occupied by the line of stakes. This done, the guide measured the distance on the ice from the foot of the pole to that of the stakes. This interval was exactly the length traversed by the glacier between the observations. This year, the process has been modified by MM. Dollfus, Otz, and myself, in such a manner as to permit us to follow the daily progress of the glacier of the Aar with such an exactness, that the error of observation cannot exceed two millimetres, or about a line.

2. Rocks Polished and Striated by existing Glaciers.

The friction of the glacier on its bottom and walls is too considerable not to leave traces on the rocks with which it comes in contact; but its action is different according to the mineralogical nature of these rocks, and the configuration of the bed it occupies. If we penetrate between the ground and the lower surface of the glacier, taking advantage of the caverns of ice which sometimes open on its sides and extremity, we creep on a bed of pebbles and fine sand mixed with water. If we remove this layer, we find that the subjacent rock is levelled, polished, worn by the friction, and covered with rectilinear striæ, sometimes resembling small furrows, more frequently perfectly straight rays which have been cut by a graving tool, or even a very fine needle. The mechanism. by which these striæ have been engraved is the same that human industry employs to polish stones and metals. By means of a fine powder called emery we rub the metallic surface, and give it a brilliancy which arises from the reflection of the light from an infinite number of excessively fine striæ. The layer of pebbles and mud interposed between the glacier and the subjacent rock, represents the emery. The rock is the metallic surface, and the mass of the glacier, which presses and displaces the bed of mud by continually descending towards the plain, represents the action of the polisher's hand. Accordingly the striae of which we speak are always in the same direction as the progress of the glacier; but as the latter is liable to small lateral deviation, the striæ sometimes cross each other, and form with each other very small angles. If we examine the rocks which border the glacier, we find the same striæ engraven on the parts which have been in contact with the congealed mass. I have often taken pleasure in breaking the ice which pressed upon the rock, and under the ice I found the surfaces polished and covered with striæ. The pebbles and grains of sand which had engraved them were still fixed in the glacier, as the diamond of the glazier is fixed at the extremity of the instrument which he uses to cut glass. The distinctness and depth of the striæ depend on many circumstances. If the fixed rock be calcareous, and the

emery composed of pebbles or sand derived from harder rocks, such as gneiss, granite, or protogine, the stria will be very strong marked. This may be observed at the foot of the glaciers of Rosenlaui and Grindelwald, in the Canton of Berne. On the contrary, if the rock be gneiss, granite, or serpentine, that is to say, very hard, the stria will be shallower and less marked, as may be observed in the glaciers of the Aar, Zermatt, and Chamonix. The polish will be the same in both cases; and it is often as perfect as that of the marbles which ornament our buildings.

The striæ engraved on the rocks containing these glaciers, are in general horizontal or parallel to the surface; but at the narrowing of the valleys these striæ rise and approach the vertical. We need not be surprised at this. Forced to make its way through a narrow pass, the glacier rises at the sides, and ascends along the flanks of the mountains which obstruct its progress. This is admirably witnessed near the Chalets of Stieregg, a narrow defile through which the lower glacier of Grindelwald is obliged to force its way, before it spreads itself in the valley of the same name. On the right bank of this glacier, the striæ are inclined 45° to the horizon; on the left bank, it rises sometimes to the neighbouring forests, and drags along large banks of earth covered with tufts of rhododendron, elder, birch, and pines. The soft and foliated rocks are bruised and broken in pieces by the prodigious force of the glacier. The hard rocks resist it; but these rocks also, by their flattened, worn, polished, and striated surfaces, bear testimony to the enormous pressure to which they have been subjected. It is in this way that, in the glacier of the Aar, the foot of the promontory on which M. Agassiz has erected his pavilion, is polished to a great height; and on the face turned up the valley I observed striæ incline 64°. The ice rising against this encampment seems as if it would have scaled it; but the granite rock resisted, and the glacier was obliged to turn slowly round it.

In short, the considerable pressure of a glacier, joined to its progressive movement, acts at the same time on the bottom and sides of the valley which it traverses. It polishes all the rocks which afford sufficient resistance not to be destroyed by

it, and often impresses on them a particular and characteristic form. By destroying all the asperities of these rocks, they level the surface, and give them a rounded form in front, while, behind, they sometimes preserve their abrupt, unequal, and rugged forms. It will be understood that the effect of the glacier is exerted principally on the side turned towards the amphitheatre from which it descends, in the same manner as the piers of a bridge are more damaged in front than behind by the ice carried down the river in winter. Seen at a distance, a group of rocks rounded in this manner reminds the spectator of the appearance of a flock of sheep; hence the name roches moutonnées given them by Saussure, and which has continued to be applied to them.

3. Moraines and Erratic Blocks of existing Glaciers.

There is another class of phenomena of great importance in the history of existing glaciers, and of those which formerly covered Switzerland. I speak of the fragments of rock of every size and nature which a glacier carries along with it. The appearance of the Alps seems to intimate to us that they are immense ruins. Everything conspires for their destruction; all the elements seem combined to abase their haughty peaks. The masses of snow which rest upon them in the winter, the rain which filters into their strata in summer, the sudden action of the waters of torrents, and the slower, but still more powerful, influence of chemical affinities, degrade, disintegrate, and decompose, the hardest rocks. Their debris fall from the summits into the amphitheatres occupied by glaciers, in considerable masses, accompanied by a frightful noise, and large clouds of dust. Even in the middle of summer, I have seen these avalanches of stones precipitated from the top of the peaks of the Schreckhorn, and form on the spotless snow a long black track, composed of enormous blocks, and an immense number of smaller fragments. In spring, the rapid melting of the winter's snow often gives rise to accidental torrents of extreme violence. If the melting be slow, the water insinuates itself into the smallest fissures of the rock, becomes frozen there, and splits the most refractory masses.

The blocks detached from mountains are sometimes of gigantic dimensions, some are found 20 metres in length, and such as measure 10 metres in every direction are not rare in the Alps.

If the glacier were immoveable, these debris would remain fixed in it without any order, but its movement causes a certain arrangement in the distribution of these materials, which shews a very remarkable degree of regularity. These blocks are disposed on the glacier in long tracks parallel to its sides, or they accumulate at the extremity in the form of large transverse mounds. Both of these accumulations have been distinguished by the name of moraines.

The following is the mechanism employed in the formation of moraines. The debris of the surrounding mountains falling on the sides of the glacier, they share in its motion, and advance along with it; but, additional quantities of materials falling upon it, it may be said daily, are deposited in a line with the first, and the whole united form those long convoys of substances which lie along the two sides of the glacier; these are the lateral moraines. A glacier often presents many lateral moraines, because the debris from above fall at points unequally distant from the centre, and the degree of their motion is consequently different. The greater part of tourists who have visited the great glaciers of Switzerland are acquainted with these lateral moraines, and more than one of them can recall the fatigue they had to undergo before they could scale these accumulations of gigantic blocks. They might be called ramparts raised by giants to prevent access to these fields of eternal snow, where nature has concealed the secret of the last revolutions of our globe. After passing the lateral moraine, the traveller almost always discovers a mound still more considerable, disposed longitudinally towards the centre of the glacier, and which is named the median moraine. This results from the junction of two glaciers of nearly equal magnitude. At the extremity of the projection which separates them, the left lateral moraine of the one rests against the back of the right lateral moraine of the other. These two lateral moraines are soon confounded in one, and form the median moraine of the new glacier, itself VOL. XLIII, NO. LXXXV.—JULY 1847.

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