By successive efforts enough of the superincumbent column is thrown off to raise nearly all the water in the tube to the boiling point, until at last the relief from pressure is sufficient to permit the ejection of the contents of the tube. This ejection continues until all the reservoirs around the geyser are emptied, when it subsides until the proper conditions are established again. A boiling spring becomes in time a geyser if, in building up around itself a mound of precipitated mineral, it forms a vertical tube of sufficient height and regularity to give a certain pressure of confined water; and when the tube reaches such an altitude that the water below cannot, in consequence of the increased pressure, reach the boiling point, the eruptions cease and the geyser becomes a mere cistern. It is a singular fact in the history of Iceland that no mention is made of the geysers until they are spoken of by Svenson, bishop of Skalholt, in the 17th century; and this is the more remarkable, as Ari Frode, who wrote of the geography and history of the island in the 11th century, spent his youth in their immediate vicinity.

They bear unmistakable evidences of having been in operation in this district, if not in the exact places where they are now found, from remote periods.-The geysers of New Zealand are in the island of New Ulster, the most northerly of the group. About the centre of the island, near the ever active volcano of Tongariro, thermal springs, mud fountains, and geysers rise in more than 1,000 places, exhibiting phenomena more remarkable than those in Iceland. A portion of Lake Taupo boils and smokes as if heated by subterranean fires, and the average temperature of its water is about 100° F. North of it, a valley through which the Waikato river flows contains a great number of geysers, 76 having been counted in one group. These jets of water are of various height, and play alternately. About half way between the lake of Taupo and Plenty bay, on the coast, is the little lake of Rotomahana, covering 120 acres, whose temperature, raised by the hot springs which feed it, is about 78° F. This lake is surrounded by springs and fissures, from which steam, sulphurous gases, water, and mud are continually escaping. The most remarkable of these, the Tetarata (tattooed rock), is at the N. E. end of the lake, about 80 ft. above its level.

It is described by Von Hochstetter as a crater-like excavation, with steep reddish sides, 30 to 40 ft. high, which are open toward the lake only. The basin of the spring is about 80 ft. long and CO wide, and is filled to the brim with clear transparent water, which against the white incrusted sides appears of a beautiful blue color. Immense clouds of steam continually rise from it, obstructing the view of the surface, and the noise of boiling is always audible. At the margin the temperature is 183° F., but in the centre, where the water is continually in a state of ebullition to the height of several feet, it probably reaches the boiling point. The deposit, like that of the Iceland springs, is silicious, and the incrustations made by the overflow have formed on the slope a system of terraces, from 2 to 6 ft. in height, as white and almost as regular as if cut from marble, on each of which are circular basins, resplendent with blue water. These terraces, which cover an area of about three acres, have the appearance of a cataract plunging over natural shelves, which as it falls is suddenly turned into stone. Each stage has a small raised margin, from which slender stalactites hang down on the next below.

At ordinary times but very little water ripples over these terraces, and only the principal discharge on the side forms a hot steaming fall; but sometimes, say the natives, the whole body of water is thrown up in an enormous column, emptying the pool. On the highest stage is an extensive platform, with a number of basins, from 5 to 6 ft. deep, the water showing a temperature of from 90° to 110° F. In the middle of this platform rises, close to the brink of the main basin, a rock island, about 12 ft. high, covered with mosses and ferns. From it a full view may be had of the interior of the boiling caldron, without danger. The rocks from which these springs derive their silica are rhyolites and rhyolithic tufas, which contain over 70 per cent. of it. An analysis of the solidified incrustation of the Tetarata, made by Mayer, gave the following result: silica, 84-78; water and organic substances, 12.86; sesqui-oxide of iron and alumina, 1.27; lime, magnesia, and alkalies, 1.09; total, 100.-In the United States, volcanic boiling springs exist in numerous localities west of the Rocky mountains. In the Colorado desert, between lat. 33° and 34°, and Ion. 115° and 116°, are remarkable mud volcanoes and boiling springs.

The desert at this point is below the level of the sea. The springs cover a space not more than a quarter of a mile square. This area is covered with soft mud, through which water and steam are constantly escaping, with a noise audible at a distance of ten miles. In some places the vapor rises steadily, with a sharp hissing sound; in others it bursts forth with a loud explosion, throwing water and mud to the height of 100 ft. Some of the boiling springs throw up a column of water 20 or 30 ft.; some have cones formed around them, and some have basins 100 ft. in diameter, in which the blue paste-like mud is ever bubbling and hissing. Many are incrusted with carbonate of lime, others with deposits of sulphur. The steam which rises from them is strongly impregnated with sulphur. Similar springs exist in New Mexico and in some of the other territories.-The so-called geysers of California are in Sonoma county, in a lateral gorge of the valley of Napa, called the "Devil's Canon," near the Pluton river. The narrow ravine, which is always filled with vapor, is shut in by steep hills, the sides of which, marked with evidences of volcanic action, are smoking with heat and bare of vegetation. A multitude of springs gush out at the base of the rocks.