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Show 464 GEYSERS OF ICELAND. ne hundred cubic yards in volume,. to a mas.s of rockf , aicbrhotu to or m. ne m1" 1 es, and we may well concei.v e the distance o e o f t the escape of such an expansive that the slightest obstruc wn'd~. bl tract in South America. nvulse a consi era e force may co ,, s Michell, "when they find a vent are " If these vapo~rs, say ntr to the distance of ten or twenty capable of shakmg a cou { we not expect from them miles round .the volca~~~' w ~~ ~l;;e is no do~lbt that aqueous when they ate confine . b d nt of the aeriform products vapour co~lstitute~ the ~nost a b:nw:ll to consider attentively a of volcamc eruptions, ~t mayl . l the movincr power-the . 1 . h team IS exc us I ve y o . f case m w uc s '"rhese intermittent hot springs rise rom Geysers of Iceland·.e d to a cons1" d e1. a ble depth by a stream of a large tract, covet l t and apertures evolving steam, lava; and where ther,m; wa e~sGeyser rises out of a spacious are very common. r le f?real und composed of siliceous . 1 · t of a c1rcu ar mo ' , baslll at ~ le summi . ' f· the spray of its waters. r~e incrustations deposi~ed rom . ne direction is fifty-six diameter of the basm or crater, m o ' feet, and forty-six in another. Vie~v qf t/&c Crater qf the great Gty <er ;,. I celalld •. • · · e seventy-e1. g ht feet in perpendiCu1l1a r In the centre IS a ptp .r t I·n di'ameter but gradua y · 1 t to ten 1ee ' · depth, and from eig ~ 1 b . 'I'he inside of the basm "deni'ncr as it opens mto t le asm. WI o · I . b w J. Iloo k er, M ·D ., in his " Tour m ce- '41 Reduced from a sketch gLvcu y . land," vol. i., P· 149. CAUSE OF THEIR INTERMITTENT ACTION. 465 js whitish, consisting of a siliceous incrustation, and perfectly smooth, as are two small channels on the sides of the mound, down which the water makes its escape when filled to the margin. The circular basin is sometimes empty, as represented in the above sketch, but is usually filled with beautifully transparent water in a state of ebullition. During the rise of the boiling water up the pipe, especially when the ebullition is most violent, and when the water flows over or is thrown up in jets, subterranean noises are heard, like the distant firing of cannon, and the earth is slightly shaken. The sound then increases and the motion becomes more violent, until at length a column of water is thrown up perpendicularly with loud explosions, to the height of one or two hundred feet. After playing for a time like an artificial fountain, and giving off g1·eat clouds of vapour, the pipe is evacuated, and a column of steam then rushes up with amazing force and a thundering noise, after which the eruption terminates. If stones are thrown into the crater they are instantly ejected, and such is the explosive force, that very hard rocks are sometimes shivered into small pieces. I-Ienderson found that by throwing a great quantity of large stones into the pipe of Strockr, one of the Geysers, he could bring on an eruption in a few minutes*. The fragments of stone as well as the boiling water were thrown in that case to a much greater height than usual. After the water had been ejected, a column of steam continued to rush up with n deafening roar for nearly an 1wur; but the Geyser, as if exhausted by this effort, did not give symptoms of a fresh eruption when its usual interval of rest had elapsed. In the different explanations offered of this singular phenomenon, all writers agree in supposing a subterranean cavity where water and steam collect, and where the free escape of the steam is intercepted at intervals, until it acquires sufficient force to discharge the water. Suppose water percolating from the surface of the earth to penetrate into the subterranean cavity A D by the fissures F F, while at the same time, steam, at an extremely; high temperature, such as is commonly given out from th~ rents of lava-currents during congelation, emanates from th~ fissures CC. A portion of the steam is at fit·st condensed into VoL, r. "' Journal of a Residence iu Iceland, p. 74. |