particularly when we take into consideration the small depth to which they would probably descend in the earth's crust.

We have also to regard the effects arising from the dislocation of the strata, as noticed by Sir John Lubbock. There are few geologists who are not now prepared to admit that the surface of the earth, since we may assume any solidity in that surface, has been in an unquiet state, some large areas moving upwards, some downwards, and these movements sometimes repeated in the same area; deposits crushed and folded against each other here and there in long lines, so that parts of them are thrust high up above the level of the sea, while masses of accumulations are forced asunder in other situations, and mineral matter raised from beneath occupies parts of the area over which they previously spread. Up to the present time mineral matter is here and there vomited forth in fusion, or blown out of vents by the discharge of vapours and gases, and large tracts of the solid surface of the earth are violently shaken, and portions of land raised or depressed. We also know that at the present, slow changes in the relative levels of sea and land are being effected. Thus from our own experience and from the study of what has formerly happened, we find that the surface of our planet is and has been, during the lapse of such geological time as we can trace, in an unquiet state. We of course know nothing of the height to which the crushing or elevating of rocks into mountain-chains may have forced mineral accumulations, though we may often infer that very great heights are but the remains of rocks, the removed portion of which rose still further into the atmosphere; but, taking the Himalayan chain as the highest land, we have nothing rising six miles above the sea-level. If we increase this height to ten miles, we should still have an insignificant fraction of the earth's radius.

The researches of Mr Hopkins lead him to infer that at present the solid crust of the earth cannot be less than 800 to 1000 miles thick. Supposing this to be so, the hypothesis of a cooling globe would give a less thickness in past geological times, one gradually diminishing to the early period when solid matter could be first formed. I need scarcely call your attention to the view which has been taking of the forcing-up of mountain-chains, and the unequal tilting and adjustment of masses of the surface to accommodate the. crust to the still fluid mass beneath, as cooling proceeded. Neither need I speak of the effects which would follow from the action of the heated and still fluid mass upon the portions of the fragments which may have descended different depths into its surface, or of the intrusion of the molten matter amid the broken masses; we have only to inquire how far these breakings-up and squeezings of the previously solid crust at different times is likely to have interfered materially with its general uniformity, so that any important change in the earth's axis, with its geological consequences, may have resulted.

As regards the mineral matter thrust up into the atmosphere, we

see that, as soon as this is effected, it is attacked along the sea-level by the breakers, and both on coasts and inland by atmospheric influences, all tending to lower the altitude of the mass so elevated, and to carry its component parts into the sea, filling up any inequalities which may have been formed beneath it, in consequence of this surface-movement of the rocks. It is during this removal of mineral matter and its spread in various directions, that the remains of the animal and vegetable life of succeeding geological times become entombed, adding, and in many instances most materially, to the masses accumulated in various ways upon the previously moved rocks. This action, therefore, tends to plane down the unequal surface above the sea and fill up inequalities in its bed. While this proceeded, we should expect that the heated matter beneath would also melt down any portions of the solid masses, squeezed and forced into it by these movements, to a distance from the surface corresponding with the general heat of the globe at the time, and therefore the deeper as geological time advanced and the earth gradually parted with its heat, by radiation, into surrounding space.

Under this view there would be a tendency over the face of the globe to retain a general crust upon it of a thickness increasing with the lapse of geological time, less uneven beneath, as a whole, than above, from the kind of action to which it would be subjected, and yet no part protruding so far as to cause any very material difference in the figure of the earth, or of density, in the parts of such crust. Viewing the subject on the large scale, it would not appear improbable, that notwithstanding the dislocation, unequal tilting, and squeezing together of masses, the adjustments were such as to keep a spheroidal coating of the mass beneath, which did not very materially differ as a whole in density. Should this not have been so, we have in our geological hypotheses to take into account the effects pointed out by Sir John Lubbock as resulting from the modification or absence of the general conditions above inferred, their amount or geological value necessarily depending upon the magnitude of the causes to which he adverts.

On the Downward Progress of the Glaciers of the Alps.
By ED. COLLOMB.

Glaciers being the definite result of meteorological and climatological phenomena, their secular encroachment upon the lower valleys of the Alps may serve as a term of comparison to determine the changes that have taken place in the climate of the country.

This encroachment may take place in two ways, either by the progression of their frontal portion, or by the swelling out of their lateral parts.

We may have the case of a glacier with the frontal part alone advanced forward, without the parts situate towards the middle undergoing any dilatation. The reverse of this may, on the other hand, present itself; that is to say, the terminal talus may remain many years nearly stationary, and yet we may observe a sensible expansion of the surface in the middle region. Glaciers, therefore, exhibit two modes of proceeding in encroaching upon the land, one frontal, the other lateral.

These phenomena depend on three causes which act incessantly on the physiology of glaciers, if we may use such an expression. These are the alimentation, movement, and ablation.

The alimentation of glaciers, in other words the cause of their existence, is to be found in the quantity of snow which falls in the whole zone of the Alps situate above from 2800 to 3000 metres. At this altitude, the solar or ambient heat is insufficient to melt the snow that falls in the course of the year. In these high regions the alimentation exceeds the melting power, and if a movement did not take place from the first origin of the glaciers, at the end of a few ages such an accumulation of névé would ensue in these regions, that the conditions of existence in which the valleys of the Alps now are, would be completely changed.

At a height of 3000 metres, about 17 metres of snow fall annually; and its primitive density, according to M. Dollfus's experiments, is 85 kilogrammes* the cubic metre. By sinking,

* Kilogramme is equal to 2 pounds 4 ounces avoirdupois.

pressure, and evaporation, these 17 metres become reduced to 5 metres of névé, or ice of névé, the density of which is 250 kilogrammes the cubic metre. During the three warmest months of the year, or rather the three in which there is least cold, the ablation does not exceed one metre of the surface; there remain, therefore, four metres every year, which the sun cannot melt at that altitude. If no movement took place in these four metres, if the property they possess of moving onwards, and gradually reaching the lower regions, were taken away, and they were thus rendered immoveable, four new metres would be added every year to those of preceding years; and even during historical times, the névés accumulated in the upper regions would surpass the height of the most elevated summits of the chain.

Thus, by reference to facts, we obtain a proof that the movement alluded to takes place on all the cols or passes and most elevated summits of the Alps, wherever the heat of summer is not sufficient to melt the entire snows of winter.

This movement, therefore, has its point of departure in the circuses, passes, and summits; it commences at the upper limit of all the basins of the glaciers, propagates and develops itself throughout their whole mass, and the whole extent of their course; very feeble at first, it gradually acquires some degree of speed in the middle regions, and again becomes slower in the lower regions. All the snows above 3000 metres being destined by nature to be melted, that they may not accumulate indefinitely, they are made to descend to the lower regions where they are exposed to a warmer sun which allows them to dissolve into water.

The alimentation and movement are, therefore, two forces, one of which accumulates the snows of the higher regions in a vertical direction, and the other spreads them over a greater surface in a longitudinal direction. If these two forces existed alone in nature, nothing would arrest the glaciers in their downward progress, and they would invade all the valleys of the Alps; but a third important element is added to the two others, namely, ablation; it is proportionate to the surrounding temperature, and acts in the inverse ratio of altitude. In the upper regions there is scarcely any ablation;

it does not, as we have mentioned, carry off as an annual mean, much above one metre of the surface of the névés; in the middle region it carries off two to two-and-a-half metres of the ice; and in the lower regions of the glaciers, three to three-and-a-half metres; consequently, all the bodies of ice. which the movement carries from above downwards, are successively exposed to a stronger ablation.

To recapitulate what has been said regarding the three causes which concur in the mechanism of the formation of glaciers, we may state:

1st, The alimentation and movement of themselves would cause an extraordinary extension in the length.

2d, Alimentation and ablation alone would produce an indefinite extension in a vertical direction; the glaciers would not descend into the valleys, but continue in the higher regions.

3d, Movement and ablation, apart from alimentation, would put an end to their existence.

It is the combination and influence of these three causes which regulate the glaciers, and maintain them in their present state; this is the law of their existence, the condition of their being.

These preliminaries were necessary to enable us to perceive the force of the observations which follow; the more so, since we know that this equilibrium between the natural forces which confine glaciers within their present limits, has not always existed on the surface of the earth. At a period of its history comparatively recent, geologically speaking, the equilibrium has been disturbed, the alimentary force has prevailed over the melting force, and glaciers have acquired a considerable development.

In present times, since attention began to be paid to glaciers, we find that they have been subject to perpetual changes some years they are advancing, in others retreating; or one glacier may be advancing while a neighbouring one is receding. As a general rule, the front of glaciers advances in winter, and recedes more or less during the warm season; because, in the winter there is no ablation, alimentation and movement alone continuing to exert themselves. Yet we are about to make an endeavour to shew that glaciers,