392 M. Descloizeaiix's Physical and Geological Observations 



ascertained by a series of observations carried on for several 

 days, and consequently they are far from announcing a great 

 eruption ; but when this is approaching, detonations stronger 

 and more frequent than the first are heard, the uprisings of 

 the central column become more and more considerable, and 

 all at once an immense column of water, three metres in dia- 

 meter at its base, is projected into the air to a height varying 

 from thirty to fifty metres, spreads like a fountain at its summit, 

 and falls again in part into the large reservoir, eighteen metres 

 in diameter, which forms the basin of the spring; the rest of 

 the mass of water flows over the external sides of the cone, 

 and spreads out into the little rills which furrow the plateau 

 on which this cone rests. The column of water does not 

 generally attain its greatest height until several consecutive 

 jets at very short intervals have prepared the observer for the 

 magnificent spectacle which the Geysir reserves for the close 

 of the eruption, like a skilful firework-maker who terminates 

 his exhibition by a splendid bouquet. These eruptions last 

 from five to seven minutes. 



In the Strokkur, the circumstances attending the eruption 

 are a little different, and the form of the channel which dis- 

 charges the emitted waters is doubtless the principal cause 

 which gives the pheenomenon a peculiar character. In this 

 channel, which represents a well even with the ground, the 

 water is at about three metres below the surface, and it boils 

 up with incessant oscillation. When it is attempted to produce 

 an artificial eruption b}' covering the surface of the water 

 with clods of earth or turf, or when a natural eruption is 

 about to take place, this boiling ceases for some instants, 

 and a pointed, lanceolate jet issues suddenly to a certain 

 height; it is soon followed by another, which is in its turn 

 succeeded by a series of slender columns, very pointed at the 

 summit, with serrated outlines, which cannot be better com- 

 pared than to immense yew-trees. These columns, which 

 succeed one another sometimes for twelve to fifteen minutes, 

 attain various heights, which reach, as in the Geysir, to fifty 

 metres. When the eruptions of the Strokkur take place 

 naturally, the jetting water is quite as limpid as that of the 

 Geysir; and, if the weather is calm and the sun bright, no- 

 thing can exceed the magnificence of these immense columns 

 of clear water, perfectly vertical, and reflecting the brilliant 

 colours of the rainbow. If however the Strokkur is excited 

 by clods of earth or turf, the water is coloured black, often 

 of a very deep tinge, and the pha?nonienon loses much of its 

 splendour. 



Before passing to the results of the thermometric experi- 

 ments and the various observations made at the Geysir and 



