Glacier (Fr. glacier, from glace, ice), a vast body of ice, filling some alpine valley, down which it slowly moves, the outlet of the snows which accumulate in the elevated portions of the mountain group. Glaciers may be found in all countries where extensive tracts lie above the snow line. In such localities the snows are ever accumulating, and the temperature not rising sufiiciently for any considerable pro-portion to be melted and flow down, they fill the spaces between the summits. By the pressure exerted by these vast collections the yield-ing material is forced through whatever opening is presented for its passage, and the great valleys leading to the base of the mountains are packed full of ice, which results from the snow being solidified by pressure, or by its own melting and freezing again. This, solid as it appears, is steadily though imperceptibly urged onward, conforming to all the irregularities of its channel, split sometimes by immovable ledges of rock, which stand like islands in its course, yet closing again below them with no trace of the fissure. These bodies of ice extend down the valleys till they reach a region where the temperature is sufficiently elevated to melt away the supplies as they arrive.

Though these have gradually diminished toward the lower extremity of the glacier, so that this has flattened away somewhat like a wedge, and has also become narrower, the termination is frequently abrupt and even inaccessible. It presents an apparently stationary wall of ice, which, though seen to be constantly wasting, may yet by observations continued several days be found steadily advancing from the mountain. During the summer currents of water formed from superficial thaws flow over its surface, at least in the daytime, and fall in cascades into the numerous chasms, which extend across the glacier. They continue their course, hollowing out through the lower layers of the ice arched channels, which at the lower end appear like dark caverns extending far up into the icy mass. In high polar latitudes, where the line of perpetual snow comes down to the sea level, the phenomena of glaciers are displayed upon the grandest scale. Thus they were seen in lat. 79°-80° by Dr. Kane in 1855, spreading over the western coast of Greenland, and sloping so gently toward the water that the effect of an inclined plane was perceived only by looking far into the interior toward the east.

In this long range the angle of the slope was from 7° to 15°. Yet the whole icy crust of this portion of the continent was always advancing and stretching itself out into the western bay, where masses of it were constantly detached and floated off as icebergs. From this glacier to the southern extremity of Greenland, more than 1,200 m., Dr. Kane imagined a deep unbroken sea of ice might extend along the central portions nearly the whole length of the continent.-The study of the geology of California had enabled Prof. Whitney to point out the traces of immense glaciers which at a time geologically recent had existed in the mountains of the Sierra Nevada. The alteration of the climate and the diminution of the rainfall consequent upon comparatively recent geological changes, have however caused the disappearance of the greater part of these, and it was not till 1870 that Mr. Clarence King discovered actual glaciers on the N. side of the extinct volcano of Mt. Shasta in northern California. From the summit, 14,440 ft. above the sea, according to him, we look down on three considerable glaciers. One of these has a breadth of three or four miles, and sends branches four or five miles down the canons.

Its thickness is estimated in places to be 1,800 ft. or more, and its surface presents great crevasses, some of them 2,000 ft. long, and 30 or 40 ft. wide. Mr. S. F. Emmons has also found glaciers on Mt. Rainier or Tachoma in Washington territory, and Mr. Arnold Hague on Mt. Hood in Oregon; while more recently Mr. John Muir has succeeded in finding small glaciers much further southward in the sierras near the Yosemite valley on Mts. Lyell, McClure, and Hoffmann. They have the -structure and movement of true glaciers, but the largest is not more than a mile in length, and they vary in breadth from half a mile to a few feet.-The phenomenon of glaciers reaching the sea and becoming icebergs was noticed by Darwin in the gulf of Penas, Patagonia. In northern Europe, it has been observed in Norway, in lat. 67° N., and in America on the W. coast of Greenland. Upon the Himalaya mountains the glaciers appear from the accounts of modern travellers to be exhibited in masses of stupendous height, as well as of vast extent.

In the "Himalayan Journals of Dr. Joseph Hooker, those of the eastern portion of the range, in the territories of Sikkim and Nepaul, are described in detail, and mention is made of one which presents a vertical height of 14,000 ft., the source of which is the great Kinchinjunga, whose summit reaches the elevation of 28,000 ft. above the sea. Other gigantic glaciers in the central Himalaya are described by Dr. Thomas Thomson ("Western Himalaya and Tibet"), and by Col. Madden and Capt. Richard Strachey, in the Asiatic Researches," vol. xiv. Iceland, Spitzbergen, the Caucasus, and the Altai have their glaciers, which have been described by travellers; but no regions have afforded such convenient opportunities for studying them in detail as the Alps of Switzerland, Savoy, Piedmont, and Tyrol. Here, in the heart of Europe, they are found covering in detached portions an aggregate area computed at 1,484 sq. m. Between Mont Blanc and the borders of Tyrol 400 are reckoned, of which the greater number are between 10 and 20 m. long, and from 1 to 2 1/4; m. broad. Their vertical thickness in many places is rated at 600 ft.; their range is from above the snow line, which is from 7,500 to 8,000 ft. above the sea, down to the level of 3,500 to 3,000 ft.

Lateral ravines have their glaciers, which join as branches the ice currents of the great valleys. This interesting region was studied by De Saussure in the latter portion of the last century, and his views were published in his Voyages dans les Alpes (1796). Charpentier is distinguished among later explorers as the able advocate of the theory explaining the motion of the glaciers, afterward sustained by Agassiz in his Etudes sur les glaciers (1840); and Prof. James D. Forbes of Edinburgh published in 1843 his Travels in the Alps," etc, with observations on the phenomena of glaciers made in visits to them repeated in ten different summers, in which he crossed the principal chain 27 times by 23 different passes. Many other distinguished naturalists have aided not only to develop the true nature of glaciers, but to apply their phenomena to the explanation of past changes upon the earth's surface.-Spread over the broad valleys, glaciers appear immovable. The snow disappears from their face in summer, and thousands of streams are.then produced, which waste their material; but with the return of winter the covering of snow is renewed, and no change may be perceived in the great mass except such as can be referred to these superficial causes.