Icebergs, and Ice Islands, floating masses of ice gathered on the coast of polar regions, and set adrift by force of winds and currents. Many icebergs are produced from glaciers, which, thrust down from the elevated snowy lands in the interior, are moved onward into the deep waters, where the fragments broken off from the advance border are floated away. The edges of glaciers extending many miles along a precipitous coast have been seen to fall with terrific violence into the sea beneath, and at once be transformed into floating islands of ice. These carry with them the masses of rock gathered up by the ice in its progress as a glacier, and transport them to new localities in warmer latitudes. (See Diluvium, and Gla-cier.) Ice islands of vast extent are also produced by the breaking up of the great fields of sea-made ice which accumulate along the shores of the frigid waters. In 1817 the ice covering several thousand square miles of the sea N. of Iceland, and chiefly on the E. coast of Greenland, most of which, it is believed, had not been moved for nearly 400 years, was suddenly broken up and dispersed over the waters of the North Atlantic. Portions of it were carried far to the eastward of the usual range of icebergs from the north, and approached within 800 m. of Ireland, or to Ion. 32° W. The breaking up of this ice led to the expedition of Capt. Ross, the second of the present century in search of a northwest passage, the opinion prevailing that the climate had essentially changed, and that the northern seas would continue open.

The drift of the northern icebergs is with the great polar currents, one of which sets in a S. S. W. direction between Iceland and Greenland, and another along'the W. side of Baffin bay, meeting the former near the coast of Labrador. They are brought against the American continent and the W. shores of its bays in consequence of not catching at once the more rapid rotating motion of the earth as they pass upon larger parallels, and so allowing this to slip from under them. The greatest numbers are produced on the W. side of Greenland; and, as observed by Dr. Kane, "perhaps the most remarkable place for the genesis of icebergs on the face of the globe " is at Jacob's bight, an inlet a little N. of Disco island, in about lat. 71° and Ion. 56°. From Labrador the ice is floated with the current past Newfoundland, and meeting near the Great Bank the warming influences of the Gulf stream, it usually disappears about lat. 42°. The extreme limit is in lat. 40°. Sometimes the ice is carried as far to the eastward as the Azores. In the southern hemisphere icebergs drift still nearer to the equator, being occasionally seen off the cape of Good Hope. As they reach their southern limit in the northern hemisphere their influence is felt in sensibly cooling the waters of the Gulf stream for 40 to 50 m. around, and on approaching them the thermometer has been known to fall 17° or 18°. When driven, as they sometimes are, in large numbers into Hudson bay, they diffuse intense cold over the northern portion of the continent.

The floating masses assume a variety of forms. Some spread out into sheets, which cover hundreds of square miles and rise only a few feet above the water. These are called fields, or, when their whole area can be defined from the mast head, floes. A number of sheets succeeding each other in one direction constitute a stream, or lying together in great collections, a pack. The surface of the sheets is often diversified by projections above the general level, which are called hummocks; they are forced up by the floes pressing against each other, and are sometimes in the form of great slabs supported by one edge. Dr. Kane noticed that these become bent by their own weight, even when the thermometer continues far below the freezing point. The most solid clear ice exhibits this yielding property of its particles. The surface of the ice fields is usually covered with snow, and when the ice is no more than 2 ft. thick it gives no trace of salt on the surface. The thicker ice contains open pools of fresh water. The bergs are real floating mountains of ice, rugged and picturesque, with peaks jutting high into the air, and strange forms in the glittering hard blue ice, which one easily converts into imaginary castles and grotesque architectural designs.

They are occasionally seen in great numbers moving on together. Dr. Kane in his first cruise counted 280 in sight at one time, most of which exceeded 250 ft. in height, and some even exceeded 300 ft. The dimensions of the largest are measured by miles. Lieut. Parry in the first expedition of Ross encountered one in Baffin bay, 7 leagues from land, the length of which was 4,169 yards, its breadth 3,869, and its height 51 ft. It was aground in 61 fathoms. Its cliffs recalled those of the chalk on the coast of England W. of Dover. Dr. Kane saw one aground in soundings of 520 ft. which with every change of tide swung round upon its axis; and Capt. Ross describes several he saw aground together in Baffin bay in water 1,500 ft. deep. The officers of the French exploring expedition in the Southern ocean measured several bergs from 2 to 5 m. each in length, and from 100 to 225 ft. high. Capt. Dumont d'Urville reports one in the Southern ocean 13 m. long, with vertical walls 100 ft. high. The portion of these masses of ice seen above the water is only about an eighth part of their entire bulk. Such bodies, weighing hundreds of millions of tons, moved on by a broad current of water, exert a power against obstacles of which we can form little idea.

In their action upon the bottom of the sea, as explained in the article Diluvium, many geologists recognize a repetition of the phenomena accompanying the distribution of the drift formation, and the production of its sands and gravel and rounded bowlders. Dr. Kane remarks of the display of power exhibited by the movements of these huge bodies as follows: "Nothing can be more imposing than the rotation of a berg. I have often watched one, rocking its earth-stained sides in steadily deepening curves, as if to gather energy for some desperate gymnastic feat; and then turning itself slowly over in a monster somerset, and vibrating as its head rose into the new element, like a leviathan shaking the water from its crest. It was impossible not to have suggestions thrust upon me of their agency in modifying the geological disposition of the earth's surface." - Icebergs occur in great numbers in the North Atlantic in the latter part of the summer, and form the chief danger which then besets the navigation between Europe and North America. These mountains and fields of ice, however, have sometimes served as a means of safety to persons who have taken refuge on them, or floated off with them accidentally.

Several members of Hall's exploring expedition were in 1872 rescued from a floe on which they had drifted 196 days and a distance of 2,000 miles. (See Arctic Discovery.)