Any hot spring capable of depositing siliceous material by the evaporation of its water may in course of time transform itself into a geyser, a tube being gradually built up as the level of the basin is raised. And every geyser continuing to deposit siliceous material is preparing its own destruction; for as soon as the tube becomes deep enough to contain a column of water sufficiently heavy to prevent the lower strata attaining their boiling points, the whole mechanism is deranged. In geyser districts it is easy to find thermal springs busy with the construction of the tube; warm pools, or laugs, as the Icelanders call them, on the top of siliceous mounds, with the mouth of the shaft still open in the middle; and dry basins from which the water has receded with their shafts now choked with rubbish.
Geysers exist at the present time in many volcanic regions, as in the Eastern Archipelago, Japan, and South America; but the three localities where they attain their highest development are Iceland, New Zealand, and Wyoming in the United States. The very name by which we call them indicates the historical priority of the Iceland group. It is an old Icelandic word—geysir, equivalent to gusher or rager—from the verb geysa, itself a derivative of gjosa, to gush. In native usage it is the proper name of the Great Geyser, and not an appellative—the general term hver, a hot spring, making the nearest approach to the European sense of the word (see Cleasby and Vigfusson, Icelandic English Dictionary, s.v.).
The Iceland geysers are situated about 50 miles N.W. of Hecla, in a broad valley of alluvial formation, at the foot of a range of hills from 300 to 400 feet in height. Within a circuit of about two miles, upwards of one hundred hot springs may be counted, varying greatly both in character and dimensions. The Great Geyser in its calm periods appears as a circular pool 72 feet in diameter and 4 feet in depth, occupying a basin on the summit of a mound of siliceous concretion; and in the centre of the basin is a shaft, about 9 feet in diameter and 70 feet in depth, lined with the same siliceous material. The clear sea-green water flows over the eastern rim of the basin in little runnels. On the surface it has a temperature of from 76° to 89° Cent., or from 168° to 188° Fahr. Within the shaft there is of course a continual shifting both of the average temperature of the column and of the relative temperatures of the several strata. The results of the observations of Bunsen and Descloizeaux in 1874 were as follows (cf. Poggendorff’s Annalen, loc. cit., and Comptes Rendus, vol. xxiii.):—About three hours after a great eruption on July 6th, the temperature 6 metres from the bottom of the shaft was 121·6° C.; at 9·50 metres, 121·1°; at 16·30 metres, 109° (?); and at 19·70 metres, 95° (?). About nine hours after a great eruption on July 6th, at about 0·3 metres from the bottom, it was 123°; at 4·8 metres it was 122·7°; at 9·6 metres, 113°; at 14·4 metres, 85·8°; at 19·2 metres, 82·6°. On the 7th, there having been no eruption since the previous forenoon, the temperature at the bottom was 127·5°; at 5 metres from the bottom, 123°; at 9 metres, 120.4°; at 14·75 metres, 106·4°; and at 19 metres, 55°. About three hours after a small eruption, which took place at forty minutes past three o’clock in the afternoon of the 7th, the temperature at the bottom was 126·5°; at 6·85 metres up it was 121·8°; at 14·75 metres, 110°; and at 19 metres, 55°. Thus, continues Bunsen, it is evident that the temperature of the column diminishes from the bottom upwards; that, leaving out of view small irregularities, the temperature in all parts of the column is found to be steadily on the increase in proportion to the time that has elapsed since the previous eruption; that even a few minutes before the great eruption the temperature at no point of the water column reached the boiling point corresponding to the atmospheric pressure at that part; and finally, that the temperature about half-way up the shaft made the nearest approach to the appropriate boiling point, and that this approach was closer in proportion as an eruption was at hand. Observations made by Mr Robert Walker in August 1874 remarkably confirm those of Professor Bunsen (see Proceedings of Roy. Soc. of Edinburgh, vol. viii. p. 514). The Great Geyser has varied very much in the nature and frequency of its eruptions since it began to be observed. In 1809 and 1810, e.g., according to Hooker and Mackenzie, its columns were 100 or 90 feet high, and rose at intervals of 30 hours, while, according to Henderson, in 1815 the intervals were of 6 hours, and the altitude from 80 to 150 feet.
