far the different glacial phenomena may be modified by their peculiar conditions, but it is also very possible that they might afford valuable tests, whether at rest or not, of the truth of particular theories of glacial motion.

The inclination of the surface to the horizon in large glaciers usually varies from 2° or 3° to 8° or 10°. As a general rule the surface is most rough and dislocated where the inclination is greatest and most irregular. In many glacial valleys there are also steep escarpments, over which the ice is precipitated, and broken into thousands of enormous fragments, forming one of the wonders of Alpine scenery. The re-cementing of these fragments into one continuous mass of glacial ice at the foot of an ice-fall was, till recently, one of the most mysterious of glacial phenomena.

When we look down on the surface of a glacier from a considerable height, the minor inequalities of its surface become scarcely sensible. We may generally observe, however, even on the smooth portions of the surface, certain transverse lines, rare in the centre of the glacier, but more numerous in its two marginal portions, in each of which these lines are respectively nearly parallel; and as they proceed from the flanks on either side towards the central portion, they incline towards the upper extremity of the glacier, instead of being perpendicular to its axis. These are the crevasses, gaping, vertical fissures, often large enough to present the most serious impediments to the progress of the traveller across them. They are rarely longitudinal in the elongated or canal-shaped glaciers, but in certain cases where the valley becomes suddenly divergent in its descent, the crevasses become also divergent, like the rays of a fan. The glacier of the Rhone, at its lower extremity, presents the best and most familiar example of crevasses of this latter kind. The theoretical explanation of all these phenomena belongs to the mechanics of glacial motion.

There is another group of objects, very striking in a bird's-eye view of the surface of a glacier. We allude to the long, dark, continuous lines of débris nearly parallel to the axis of the glacier, and stretching frequently from points near its upper extremity to its final termination. To the eye situated as above supposed, they appear free from all local asperities, following in graceful curves all the flexures of the valley. They consist of an aggregation of rocks and smaller detrital matter, the rocks varying from small pebbles to angular blocks of many tons in weight. These are the moraines. One is almost invariably found on each side of the glacier, and close to the bounding walls of the valley; they are the lateral moraines.

Vol. 114.-No. 227.

G

Another

moraine,

moraine, and usually the largest, is observed to coincide very nearly with the axis of the glacier, and is called the median moraine. In large glaciers there are frequently also other smaller moraines intermediate and parallel to those above mentioned. The glacier of the Aar furnishes, perhaps, the best examples of existing moraines with which we are well acquainted. Not far below the junction of its two great tributaries, as many as six or seven may be distinctly recognised. They are laid down with great accuracy in the map of this glacier, in Plate III. of the Atlas which accompanies the last work of M. Agassiz on glaciers, the Système Glaciaire. It should also be stated that aggregations of large blocks and smaller débris are usually found at the terminations of glaciers in front of the ice itself, and extending more or less completely across the valley. They are the terminal moraines.

The motion of a glacier is slow and persistent during all seasons, but slower in winter than in summer, and varying generally at different times and in different places, from a few inches to twenty or thirty inches a day. Moreover, in an elongated canal-shaped glacier, the axial portions move faster than what are termed the lateral or marginal portions. Also, the more superficial parts of the glacial mass move faster than the inferior parts. These inequalities of motion show that a glacier, in its aggregate mass, has a power of changing its form, so as to admit of these irregularities of motion, as well as to enable it to adapt itself to all the irregularities in the form and dimensions of the valley along which it descends. This property of the general glacier we call its pliability. It has been the subject of much earnest discussion.

The motion of the glacier enables us to account very clearly for the existence of central moraines. The lateral ones are manifestly due to the various blocks and débris which fall down the precipitous sides of the glacial valley on the glacier beneath, by the onward motion of which they are carried forward, sometimes the whole length of the glacier, and deposited in its terminal moraine. If, however, the lateral moraine belonging to one flank of a large tributary glacier meets the.corresponding flank of another tributary, with its moraine (as at the junction of the two great tributaries of the Aar glacier), the two moraines necessarily unite, and move forward along the central line of the resulting glacier as its central moraine. A similar explanation applies to the moraines which are intermediate to the median and either lateral moraine. They all arise from lateral and usually smaller tributaries to the general glacier or to its principal affluents. If a lateral moraine, for instance, be formed in

the

the upper portion of a large tributary, and a smaller tributary pour down its contents into the larger one, the lateral moraine of the latter, conjoined with one of the lateral moraines of the smaller tributary, will be thrust away from the side of the glacial valley, and will become one of the intermediate moraines above mentioned. A similar explanation applies to other moraines of this kind, the number of which is usually indicative of the number of minor tributaries which have helped to produce the general glacier. Many of these moraines extend to the lower extremity of the glacier and deposit their contents at the terminal one, which would thus grow incessantly, were it not that large portions of it are constantly removed by the current of water, frequently a powerful one, which issues from beneath the glacier at its extremity. Portions also of the lateral moraines work down to the bottom of the glacier, and are finally pushed forward to its extremity.

The powerful agency of glaciers, in transporting blocks of enormous magnitude from their original sites to points many miles distant, will be easily understood from what precedes. The recognition of this operation of transport as the daily employment, as it were, of nearly all glaciers, has led to some highly interesting conclusions in geology.

When the traveller descends from the high point of view from which we have supposed him to survey the glacier, and begins to traverse its surface, he becomes sensible of the rugged route along which he has to make his way. He finds that many of the crevasses, which appeared to him like so many narrow wellmarked lines, are really deep yawning fissures, over which it is frequently impossible to pass without bridging them over by some artificial means. The large central moraines, also, which appeared like even dark longitudinal stripes on the glacier, he finds to be frequently elevated ridges of 20 or 30 feet in height. This elevation does not arise simply from the accumulation of the blocks and débris of the moraine, but partly also from an icy ridge which underlies them, and which has been formed by the protection against the wasting effects of sun and rain, afforded by the débris to the ice beneath it. Glacier tables, formed by large single blocks poised on pedestals of ice, are produced in a similar Also the less dislocated portions of the glacier surface present, especially on sunny days, a beautifully bright effect, arising from the innumerable rills of water produced by the superficial melting of the ice. These rills sometimes form, by their confluence, considerable rivulets, which, of course, precipitate themselves into the first crevasse that crosses their course, thus making their way to the bottom of the glacier, whence the

manner.

G 2

water

water is finally discharged from its lower extremity. The volume of water thus discharged in the winter is small, as might be expected; but in the warmer summer months is sufficient, in the case of a large glacier, to form at once a river of considerable magnitude.

It is impossible to overestimate the sublimity and beauty of these glacial masses, surrounded by their mountain accompaniments, whether we see them intruding themselves, as it were, at their lower extremities, into the fertile valleys of the lower Alps, and increasing by contrast the beauty of the summer verdure there, or whether we contemplate them in their solitary grandeur in the remoter portions of their higher regions. It was in 1841 that M. Agassiz may be said to have established himself on the glacier of the Aar, just below the junction of the two primary tributaries above described, for the purpose of observing the phenomena which the glacier might present to him. He there erected for himself, and two or three scientific friends who accompanied him, the tent which soon became so well known as the Hôtel Neuchâtelois, where, in that and two or three subsequent years, he received, with characteristic courtesy and hospitality, a large number of the philosophers of Europe. This glacier affords peculiar advantages for observations on glacial phenomena, and it was for this reason principally that M. Agassiz selected it. Nor should we conceive a continued summer residence on so accessible a glacier, and one which may be so easily traversed in any direction, as otherwise than very enjoyable. During the day-time, when the weather was fine, we have seen its whole surface alive, as it were, with innumerable gurgling rills of water, which, with the brightness of the snowy mountains, gave, even amidst the surrounding desolation, an animation to the scene which dissipated all feeling of loneliness. At sunset this scene is often suddenly and singularly changed. On the disappearance of the sun's rays, the surface-melting of the glacier, with every rill resulting from it, is immediately arrested, and, if the atmosphere is sufficiently serene, all is reduced at once to almost perfect stillness. The silence becomes imposing. Every little rill being hushed, there is sometimes literally not a sound to be heard, save that of the distant avalanche, occurring just often enough to make one the more sensible of the intensity of the silence. Such scenes offer, indeed, an adequate reward to every energetic traveller for all the effort he can make, and all the fatigue he may encounter, in seeking them.

We have no intention of entering into the earlier history of glacial science. We can do little more than mention the names of such glacialists as Simler, Scheuchzer, and Grüner, who, with

others

others of inferior note, collected a considerable number of facts respecting the phenomena and topography of glaciers. Scarcely any facts, however, were accurately observed, and a great part of their theories were formed with very little knowledge of physical and mechanical principles. But De Saussure's work, Voyages dans les Alpes,' was of a far higher order than any which had preceded it. The author was a Swiss philosopher fond of physical science, and a devoted admirer of his native mountains. He resided at Geneva, and availed himself of his proximity especially to Mont Blanc to make visits to that mountain, and also to the other Swiss mountains, almost every summer for upwards of twenty years. He commenced his observations in the year 1760. They were not restricted to glaciers, but were equally extended to all those numerous physical, geological, and topographical facts which that region presents to the notice of the philosophical traveller prepared to appreciate at once the true value of the principles and laws by which Nature works, and the beauty of those varied and magnificent scenes which, in a country like Switzerland, she always presents to us. The results of all the long-continued observations of this philosophical traveller are embodied in his work above mentioned, consisting of four quarto volumes published at different times, as additional matter was collected and arranged for each successive volume. The whole work consists of a happy combination of scientific observation and philosophical discussion, enlivened by the introduction of agreeable personal details, and charming descriptive touches of those magnificent scenes of beauty which characterise these Alpine regions, but which at that time were imperfectly known, even to the few secluded inhabitants of the lower and more accessible valleys of the district. There is something peculiarly national in this work, and the name of De Saussure is one of which his countrymen may reasonably be proud. Many of his more abstract scientific observations have been superseded, as might be expected, by more advanced and recent researches ; and the region which he was the first to describe in systematic detail is now popularly known from the large influx of travellers. But it must not be forgotten that his work remained for half a century the recognised and unrivalled receptacle of the best descriptions which existed of the scenery and physical phenomena of the Alps.

De Saussure did not devote his special attention to glaciers, and does not appear to have added to the then-existing knowledge of the subject much that was absolutely new, either in observed phenomena or in abstract reasoning. The great advantage which he conferred upon it seems to have been in methodising

and