Geysers (Icelandic, geysa, to burst forth violently), intermittent hot springs found in various parts of the world. In Iceland the principal geysers are in the S.W. part of the island, about 35 m. N". W. of Hecla, and 70 m. from Reykiavik, the chief town. In a circuit of about two miles are more than 100 springs which send forth hot water, 50 or more in the space of a few acres. These are on the lower slope of a small hill of trappeanrock, and above them in the steeper part of the hill under the cliffs of this rock are banks formed by the incrustations of ancient and now nearly extinct geysers. The springs are of different dimensions, and exhibit various degrees of activity; some are uniformly full and quiet, others are constantly boiling, and others only at intervals, with explosive discharges of water and steam. The vapors rising from them form clouds that are seen miles away. They are attended with sulphurous odors; and the geysers of other localities on the island deposit sulphur derived from the decomposition of the iron pyrites in the clays through which the hot waters penetrate.

The chief spouting springs of the group are the Great geyser and the Great and Little Strokr. The Great geyser when quiet presents the appearance of a circular mound of silicious incrustations, enclosing a pool, with sides sloping inward at an average angle of 13°, and outward at a mean inclination of 8°. The height of the mound is about 20 ft. on the lower side, but only half as much on the upper side. The diameter of the basin varies from 50 to 60 ft., and its average depth is 4 ft. In its centre is the mouth of the vertical tube which connects it with the subterranean passages. This tube is about 9 ft. in diameter at its mouth, and 70 ft. in depth. When the geyser is inactive, the basin is filled to the edge with clear water, which has a mean temperature of'185° F. and runs gently down the mound, emitting clouds of steam; but for several hours after an eruption the tube is empty to the depth of 4 or 5 ft. At intervals of about an hour and a half a rumbling noise is heard, and the water heaves up in the centre, throwing an increased quantity over the margin. The great eruptions take place at irregular intervals, sometimes exceeding 30 hours.

At these times loud explosions are heard beneath the surface, the water is thrown into violent agitation, it boils furiously, and at last is suddenly sent forth in a succession of jets, which increase in force till they become an immense fountain that is lost to view in the clouds of steam in which it is enveloped. The heights reached by these jets have been variously estimated by different travellers. The lowest estimate is 60 or 70 ft.; that of Yon Troil in 1772 is 92 ft.; of Sir John Stanley in 1789, 96 ft.; of Lieut. Ohlsen, a Danish officer, in 1804, determined by a quadrant, 212 ft.; of Sir George Mackenzie in 1810, 90 ft.; and of Henderson in 1815, 150 ft. Later visitors, Lord Dufferin, Mme. Ida Pfeiffer, J. Ross Browne, and others, estimate the height at from 60 to 70 ft. The eruptions appear to be diminishing in force and frequency, and it is not improbable that they will cease altogether before the lapse of another century. The discharge continues only about five minutes, when the geyser subsides to a state of tranquillity.

The Great Strokr, so named either from the Icelandic word meaning churn, or from stroka, to agitate, is only 300 or 400 ft. from the Great geyser, from which it differs in appearance in being an irregularly formed well, incrusted with silicious deposits, but having no basin at its mouth. Its orifice is about 8 ft. in diameter, diminishing to about 10 in. at the depth of 27 ft.; the whole depth is a little over 44 ft. The water for the greater part of the time is 10 or 12 ft. below the surface, and is continually boiling and seething, but at intervals of about half a day it breaks forth in a great eruption, throwing its water generally from 40 to 60 ft.; but Bunsen, who saw it in 1846, estimates it to be 151 ft. high. By throwing turf or stones into the well of the Strokr, an eruption can be brought on in a few minutes. The Little Strokr exhibits the same phenomena on a smaller scale. In the same vicinity are two large and quiet wells remarkable for their beautifully blue water. These were once active, and one of them is described by an English traveller as the Roaring geyser. It became tranquil immediately after an earthquake in 1789, when the Great Strokr first broke forth.

The deposits of silica which accumulate around the geysers are derived from the small amount of this material which is taken up in solution by the hot water. By the analysis of Dr. Black, made upon 10,000 grains (about 5 1/3 gills), it would appear that the whole amount of solid matter remaining dissolved in the cold water is only a little more than 1/1000 of the whole, the quantity examined yielding as follows: soda, 0.95; alumina, 0.48; silica, 5.40; muriate of soda, 2.46; dry sulphate of soda, 1.46; in all, 10.75. An analysis of the geyserite, or solid deposit, made by Forchhammer, gave the following result: silica, 84'43; water, 7.88; alumina, 3.07; iron, 1.91; lime, 0.70; soda and potassa, 0.92; magnesia, 1.06; total, 99.97. As the water evaporates and is chilled, the excess of silica is added to the surface around, filling the interstices of the mosses and grass, and making of these silicious petrifactions, while the living plants still thrive and shoot above the strong substance that binds together their roots and stems. Where the waters are found at a temperature of 98° 0. (208.4° F.), M. Descloiseaux observed that the confervae still flourished.

The true theory of the cause of geyser eruptions is due to Bunsen. When in Iceland in 1846, he proved by a series of careful experiments that the temperature of the water in the geyser tube varies at different depths, as also at different periods between two eruptions, the changes always taking place in the same manner and with considerable regularity. Immediately before the eruptions there is a maximum temperature at the bottom of the well estimated at 260.6° F., and a minimum immediately after of 253.4°. The temperature of boiling water at the depth reached by the thermometer should be about 276° F. The water therefore in no part of the tube is hot enough to generate steam under the conditions. But the higher you ascend in the tube, the lower is the temperature at which water will boil. If then the column be thrown up by the generation of steam in the underground channels, the water at the bottom, which is near the boiling point, is brought to a height where it is sufficiently relieved from pressure to be converted into steam. The water in the tube is lifted still higher, until the steam condenses by contact with the cooler water, to which it imparts its latent heat. Each condensation makes a detonation, the subterranean explosion which precedes an eruption.