vapour will materially promote the operation. As the frost continues the ice thickens, but not indefinitely, for in water of sufficient depth this thickening acts as a barrier to its further increase, and even in the coldest regions it is only the shallower waters that are ever completely converted into ice.

Besides the ice that forms on the surface of fresh water, there is occasionally witnessed the rarer phenomenon of ground-ice, or that which gathers in thin sheets along the pebbly beds of shallow lakes and streams. These pebbles, losing their heat by radiation quicker than the water—for heat radiates through as it does from the surface of the transparent water-act as points for the formation of icecrystals, and these passing from stone to stone, shortly convert the entire pebbly bed into a crust of ice. We have seen in a shallow ford of a Scottish stream groundice fully an inch in thickness when the stream itself was still flowing and unfrozen. This ground-ice, when broken up by freshets or other causes, floats down the stream, bearing with it its burden of encrusted pebbles, and thus becomes in nature a curious means of geological transport. Pebbly gravels may thus be laid down in situations where no current of water could carry them; just as Deas and Simpson found the ice-cake of the arctic shores driven forty or fifty feet above the sea-level, and as it melted away, leaving long ridges of beach-gravel at heights to which no ordinary wind-wave could ever transport it.

But important as the formation of ice on fresh-water lakes and streams may be, it is as nothing compared with the masses that accumulate on the surface of the arctic and antarctic seas. Instead of forming at 32°, ice does not appear on salt water till the temperature has sunk to 28°; and then it goes on increasing, according to

Sir Edward Belcher,* at the rate of about half an inch per day, during the long polar winters, often attaining a thickness of 10, 15, and 20 feet. This enormous crust, as it stretches unbroken over the ocean, is the "ice-field" of the sailor, which, when broken up, becomes his "ice-brash," and either floats 66 in away floes and "patches," or is drifted by winds and currents in "packs" and "streams."+ The conservative effect of this ice-crust on the warmth of the ocean is one of the most providential arrangements in nature-maintaining for the water beneath the mean of 39°, when in the air above the cold is often sufficiently intense to solidify mercury, whose freezing-point is 39°, or 39° below zero. Gradually as the ice thickens, it protects more and more the subjacent water; gradually as the water in contact with the under surface of the ice is chilled, it becomes heavier, and sinks,

* Sir Edward's observations were made in Wellington Channel (185254) when in search of the missing Franklin Expedition. See his Narrative for some curious and instructive facts respecting the formation, character, and deportment of polar ice.

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The names by which the different conditions of sea-ice are known to our whalers and navigators. The "ice-field" or "field of ice" is the unbroken ice of the polar oceans; when broken up by thaws and storms it becomes "brash-ice;" when drifted into dense masses it is "packice;" and when floated away by winds and currents it passes either into solitary "floes," into "patches" of several floes, or into streams," having a determinate direction. A solitary fragment floating with a considerable portion of its bulk above water is a "hummock ;" and when loaded with debris and chiefly under water it is a "calf." The young ice that is rapidly formed, on the approach of winter, between floes and patches is "pancake-ice;" when of greater thickness, and formed in creeks and inlets, it is "bay-ice." These different conditions are also known at a distance by their "blink" or reflection—this being clear for field-ice, white for packed, grey for newly formed, and deep yellow for snow.

As ice slightly contracts at temperatures under 32°, the intense cold of the polar regions only tends to render it more homogeneous and compact, and thus to increase still further its powers of protection.

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its place being taken by a warmer film; and gradually as the water is converted into ice (it freezes fresh, or only with such brine as may be entangled in its interstices), the upper film, being salter and denser, descends, and lighter and warmer particles ascend to take its place. In freezing, water, of course, gives out heat, and the heated air-bubbles may often be seen clustering beneath the ice and struggling as it were to escape upwards. Every bubble in this way melts its modicum of ice, and one by one, as they ascend in the same direction, they gradually pierce the thickest sheet, cutting rounded holes as clean and straight (in the words of Sir E. Belcher) as if they had been bored by an auger. In general, however, the heat is retained and diffused throughout the water, while all above is stark and lifeless at temperatures 50°, 60°, and even 70° below zero. How wonderful the provision by which the density and temperature of the ocean are preserved for the wants of its animal life! how perfect the scheme of compensation by which the most powerful agents are held in check, and the balance and equipoise of nature sustained!

It is at this stage, when the thaws and currents of a brief summer have broken up the polar ice into "floes" and "packs" and "streams," that we find it associated with the land-formed "berg;" the whole drifting to warmer latitudes, there to be dissolved, and to lose themselves once more in the liquid mass of the ocean. Purely sea-formed ice has no perceptible geological effect, but much of it is accumulated along shore and under cliffs and precipices (the "ice-foot" of the sailor), and this, along with the true iceberg, is generally laden with soil, sand, gravel, bouldery blocks, and other spoils of the land, and these, as the icemasses melt away, are dropped broadcast over the floor of the ocean. All that is, or has ever been, ground and worn from the surface of Greenland by the ice-sheet that envelops

it, has been spread by the iceberg on the bottom of the North Atlantic. Water in the solid state is as much a wearer and transporter of the land as water in a liquid state. The ice-stream grinds and degrades as surely as the waterstream; and the burden of both ultimately finds its way to the depths of the ocean. Nay, ice is the more potent of the two-the "berg" bearing blocks and boulders which no current of water could move, and scattering its burden over the outer depths of the ocean, while the river-sediment is merely fringing the inner shores.

Such is a brief review of the various aspects in which ice occurs, and the more prominent functions it appears to perform in the economy of nature. As snow in the lower grounds, it acts as a protecting blanket against the severity of long-continued frosts; as snow in the higher regions, it passes into névé and glacier to grind and round the rocky surface in its descent, and to smooth into gentler outlines the asperities over which it passes in its slow but irresistible progress. As the liquid stream erodes and deepens its channels, so the ice-stream rasps and chisels-the function of both being to wear and degrade the old rocks, and to transport the material for the formation of the new. As ice on water, its greater bulk, as compared with that of the water from which it is formed, enables it to float as a protecting surface, preventing the water below from being entirely frozen, and thus preserving a habitable medium, no matter how intense the cold, or however long it may be continued. As ice on water also (the iceberg) it becomes a geological carrier, transporting to the outer depths of the ocean the gravel and shingle and boulders of the rocky shores, and piling them up in long submarine reaches according to the set of the tides and currents by which they are mainly directed. As ice in the rocks and soils, it is ever

splitting and disintegrating; unless within the limits of perpetually frozen ground, as in the tundras of Siberia and the swamps of Arctic America, and there it exercises a conservative effect*-binding the softest soils as hard as rocks, and preserving their embedded organisms fresh and unchanged for ages. In all its aspects, ice is invested with a curious interest; in all its functions it is charged with important results. To us, the inhabitants of an insular and unstable climate, it may appear of little importance; but to those of the higher latitudes and altitudes it assumes the boldest character, and achieves the most gigantic results. And these results, when accumulated for years and ages, present to the geologist, as we shall see in the following Sketch, phenomena as marvellous in magnitude, and as complicated in character, as those produced by any other agency to which the crust of our earth is subjected.

* Even within these icy flats the power of frost is sometimes curiously destructive. "The influence of the cold," says Von Wrangell, speaking of the December temperature of Siberia, which was 58° below freezing, "extends even to inanimate nature. The thickest trunks of trees are rent asunder with a loud sound, which in those deserts falls on the ear like a signal-shot at sea; large masses of rock are torn from their ancient sites; the ground in the tundras and in the rocky valleys cracks and forms wide yawning fissures, from which the waters that were beneath the surface rise, giving off a cloud of vapour, and become immediately changed into ice."

It is chiefly in the frozen sands and gravels of the Siberian lowlands that the remains of the mammoth and woolly rhinoceros are preserved in greatest perfection. Every geological reader is acquainted with the history of the St Petersburg specimen; how hair, wool, and muscle were so fresh, when first discovered, that even the dogs of the Tungusian hunters were tempted to feed upon them, and this after the entombment of ages! The manner of their occurrence is thus described by the authority above quoted :-"The banks of the rivers consist of sand-hills 150 or 200 feet high, and held together by the perpetual frosts which the sunmer is too short to dissolve. Most of these hills are frozen as hard as rock; nothing thaws but a thin outside layer, which, being gradually undermined by the water, often causes large masses of frozen sand to break off and fall into the stream. When this happens, mammoth remains, in more or less good preservation, are usually discovered."