The Iceland geysers, mentioned by Saxo Grammaticus, are situated about 30 m. N.W. of Hecla, in a broad valley at the foot of a range of hills from 300 to 400 ft. in height. Within a circuit of about 2 m., 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 about 60 ft. in diameter and 4 ft. 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 10 ft. in diameter and 70 ft. 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° C., or from 168° to 188° F. 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 A. L. O. Descloizeaux in 1847 were as follows (cf. _Pogg. Ann._, vol. 72 and _Comptes rendus_, vol. 19): About three hours after a great eruption on July 6, the temperature 6 metres from the bottom of the shaft was 121.6° C; at 9.50 metres, 121.1°; at 16.50 metres, 109° (?); and at 19.70 metres, 95° (?). About nine hours after a great eruption on July 6, 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. 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, according to Sir W. J. Hooker and Sir George S. Mackenzie, its columns were 100 or 90 ft. high, and rose at intervals of 30 hours, while, according to Henderson, in