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seed to that of a pigeon's egg, and often with destructive effect on the crops of the farmer. Neither hail nor hoarfrost, however, exercises any perceptible influence on the rocky surface; and it is chiefly in the condition of snow that ice in the atmosphere becomes of interest to the geologist. This snow, formed of frozen vapour, and composed of myriads of the most delicate geometrical crystals, occurs during winter in all the higher latitudes, and at great elevations in all latitudes, wherever the surrounding air falls below the average of 32°. As the atmosphere frequently consists of strata of different temperatures, snow may be formed in the higher regions, and yet in falling may pass through a warmer stratum, and be melted into rain before it reaches the earth. But in ordinary circumstances it falls on the land-surface in soft downy flakes; and if that surface be at or under 32°, it often accumulates in great thickness, and especially in all the higher and colder regions. In the lower grounds it is melted and carried off by the next thaw; in the higher mountains, where it falls in dry, needle-like crystals, and rarely or ever in flakes, it may endure, summer and winter, for generations. The sands of the burning desert are not so light nor so easily moved as this dry crystalline snowpowder of the loftier mountains. The slightest breath disturbs it; the storm-wind sweeps it from the exposed heights, and drifts it into the sheltered gorges in masses hundreds of feet in thickness. But even there it cannot remain unchanged; summer suns and the pressure of newer accumulations condense and urge it downwardsfirst as névé,* or snow-ice, and ultimately as the pure transparent ice of the glacier. Here, however, it becomes ice on land, and falls to be considered under a different section of our subject.

Névé-the name given to the stratified, slightly-compressed snow of the higher Alps before it is condensed into the crystalline ice of the glacier.

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Like ice in the atmosphere, ice on land occurs chiefly in the higher latitudes and at great elevations; though under a clear sky and extreme night-radiation, ice may be formed on the ground even in sub-tropical and tropical countries.* In our own islands, every one must have witnessed the effects of frost on the rocks and soils. The water held in the pores of all rocky substances is rapidly converted into ice; in this state it expands, pushes asunder the particles, and when thaw comes, the separated particles, having lost their cohesion, necessarily fall asunder, and are ready to be carried away by winds, rains, and runnels of water. The force with which water expands in freezing is tremendous. The strongest vessels are burst asunder; the hardest rocks are split into chips and fragments. Every winter we see its effects in the disintegration of our ploughed soils, and in the mounds of debris at the foot of our cliffs and precipices. But under our insular climate the effects are trifling compared with what takes place in higher latitudes and in more elevated regions. In the higher Himalayas Dr Hooker found the cliffs and precipices rent and rugged with its force, and the ravines choked with the ruptured blocks and fragments; in Norway every peasant can point to the mounds of angular blocks as the work of the "Bergrap;" and in Spitzbergen the Danish expedition found the sea-cliffs fresh with recent rupture-every winter severing

*The destructive effects of these night-frosts, under a clear, dry, and serene sky, are now unfortunately too well known to our Australian sheepfarmers. Even in the deserts of Africa, Arabia, and Persia, European travellers have felt their effects. In Bengal, where ice is never formed naturally, advantage is taken of the principle for its artificial production. Shallow pits are dug, which are partially filled with straw, and on the straw flat pans, containing water which has been boiled, are exposed to the clear, dry, and serene firmament. The water is a powerful radiant, and sends off its heat rapidly into space. The heat thus lost cannot be supplied from the earth, this source being cut off by the non-conducting straw; and before sunrise a cake of ice is formed in each vessel.

with its icy wedge, and every summer dissolving the connection. It is needless, however, to multiply instances; every intelligent mind must perceive the power of this recurring ice-force, and its universality, in all the colder and higher regions of the globe. And he has only to allow sufficient duration, and every peak and precipice would be rounded and worn down by its power.

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In all the higher latitudes, snow, we have said, falls less or more during winter, and melts away during summer.+ But in all latitudes there is an elevation at which it lies perennially, and this elevation will differ, of course, with the latitude. This limit, above which snow lies at all seasons, is known as the snow-line, or line of perpetual congelation; and though it ascends a little higher during summer, and descends a little lower in winter, it is, on the whole, pretty stationary in every region of the globe. course it will come nearer the sea-level in high latitudes, and ascend higher and higher as we approach the equator; and thus it is that we have it at 1500 feet in Spitzbergen, 2400 at North Cape, 5000 in the Dovrefelds, 9000 in the Alps, 12,000 in the Atlas range, and on an average about 16,000 feet under the equator. In all the higher regions, therefore, this snow accumulates enormously, and would continue to accumulate were it not for three causes which tend as incessantly to prevent it. These are, first, atmospheric causes, such as summer's heat, warm winds, and

*The reader who takes interest in this matter will find marvellous illustrations of the power of frost in such works as Von Wrangell's 'Siberia,' Scoresby's 'Arctic Voyages,' Ross's 'Antarctic Voyages,' and Dr Hooker's 'Himalayan Journal.'

Though snow is the necessary product of cold, yet in all temperate and coldly-temperate latitudes a good heavy snowfall is beneficial in protecting vegetation from the severity of long-continued frosts. Such a covering is usually known as the snow-blanket, and in central and northern Europe its absence in early spring is often followed by most destructive results to the young growths of the farmer and gardener.

occasional rainfalls, which partially dissolve it; second, the mechanical pressure of the accumulating mass, which ever tends to urge it forward and downward to lower levels; and, third, the land-slopes, which afford greater or less facilities for its descent. As it descends by these means, so it melts away, and is carried by runnel and river to the ocean, again to be raised as vapour, again to be frozen and fall as snow, and again to be urged downward and melted to water. Occasionally its descent from the mountains is sudden and abrupt, as in the avalanche, which breaks away when the gravity of the mass becomes too great for the slope on which it rests, or when fresh weather destroys its adhesion to the surface. These snow-slips, or rather snow-and-ice slips, are frequently most destructive in their effects, and are the dread of the traveller and inhabitant on mountain regions like the higher Alps and Himalaya. These are usually distinguished as drift, or those caused by the action of the wind on the snow while loose and powdery; rolling, when a detached piece of snow rolls down the steep, licks up the snow over which it passes, and thus acquires bulk and impetus as it descends; sliding, when the mass loses its adhesion to the surface, and descends like a land-slip, carrying everything before it unable to resist its pressure; and glacial, when masses of frozen snow and ice are loosened by the heat of summer, and precipitated with crushing effects into the valleys below.

The great and persistent result, however, of this mountain-snow, is the glacier-the "ice-sheet" of the flatter heights, and the "ice-river" of the glens and ravines— which is ever pushing and grinding its downward way till it finally melts and becomes the gladdening stream of the lower valleys. The snow that falls on the higher peaks, being partially softened by the warmth and rains of summer, is converted into a sort of "snow-broth," or "slush,"

as the Scotch would call it, and has necessarily a tendency to move downward by its own gravity, however gentle the slope on which it rests. Pressed on summer after summer by newer masses, it gradually assumes greater consistence, loses the dull aspect of frozen snow, and passes into the hard transparent state of the glacier or ice-river. In its primary stage it is technically known as névé or ice-snow; and this névé, which stands intermediate between the pure unsunned snow of the winter heights and the moving glacier, is regarded as the fond or fountain of all true glaciers. Ever fed by new snowfalls from above, it is gradually pushed downwards by the force of gravitation, and in turn propels the glacier, whose own weight and partial mobility also assist the downward movement. The whole is, in fact, one great motion, just as it is part of the great circulation by which the water of the ocean is disseminated through the air and over the land, and the water of the land returned once more to the ocean. Acquiring volume and weight as it descends (and some of the Alpine glaciers are from 80 to 600 feet in thickness), the glacier grinds and smoothes the rocks over which it passes; and this it does by the earth, gravel, and rock-debris which become incorporated with its mass, and which act like so many rasps and chisels on the rocky surface. Slow in its motion, but persistent and irresistible, its course is ever downwards, and marked by abrasion, rounding, smoothing, and striation of the subjacent rocks. And as it descends, the blocks and debris, loosened by the frost from the adjacent cliffs, fall on its surface, and are borne along in long winding spits, till the mass finally melts away in the lower valleys, and then this rock-debris is left in mounds or moraines. These moraines-some of which are lateral, or on the sides; some medial, or in the middle; and some terminal, or at the melting end-bear ample testimony of the destruction that has taken place

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