Page:The American Cyclopædia (1879) Volume VII.djvu/800

 784: GEYSERS lity. The Great Strokr, so named either from the Icelandic word meaning churn, or from strolca, to agitate, is only 300 or 400 ft. from the Great geyser, from which it differs in ap- pearance 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 inter- vals 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 throw- ing 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 ac- tive, and one of them is described by an Eng- lish traveller as the Roaring geyser. It be- came tranquil immediately after an earthquake in 1789, when the Great Strokr first broke forth. The deposits of silica which accumu- late 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^ gills), it would appear that the whole amount of solid matter remaining dissolved in the cold water is only a little more than -^7 of the whole, the quantity examined yielding as fol- lows: 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 ex- cess 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 confervas 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 care- ful 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 mini- mum immediately after of 253'4. The tem- perature 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 condi- tions. 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 con- denses by contact with the cooler water, to which it imparts its latent heat. Each con- densation makes a detonation, the subterranean explosion which precedes an eruption. By successive efforts enough of the superincum- bent 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 sub- sides 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 wa- ter below cannot, in consequence of the in- creased 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 cf 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 llth century, spent his youth in their im- mediate vicinity. They bear unmistakable evi- dences of hating 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 re- markable than those in Iceland. A portion of Lake Taupo boils and smokes as if heated by subterranean fires, and the average temper- ature 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 alter- nately. 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