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 rocks round the chimney, until they escape at the surface, and fumaroles or fume jets of carbon dioxide and so forth are produced. Solfataras are jets of sulphurous vapours formed in the same way. Hydrochloric and boric acids are also sometimes given off, as in Nea Kameni, near Micra, in the Grecian Archepalago, and in the lagoons of Tuscany, respectively.

''5th Stage. Geyser Stage.'' — This is the last expiring manifestation of the volcano, when superficial water wins its way to the still hot centre of the volcano, and returns back to the surface impelled by the steam generated at the contact. The steam accumulates slowly, and every now and again drives out the water in front of it with considerable violence; then the geyser spouts. When all the steam is expended the water falls back to wait for another accumulation of steam pressure to come up again. As hot water under pressure is a strong solvent of silica, the water in geysers holds a large quantity of this substance in solution. When it issues on the surface, temperature and pressure are reduced, and the silica, becoming insoluble, is deposited as siliceous sinter round the surface of the geyser. Beautiful pools and terraces are formed in this way; those of Rotamahana, in the northern part of New Zealand, were one of the seven wonders of the world, but they were blown into the air in the renewed activity of the volcano Tarawera, and the sinter terraces of the Yellowstone Park are the best example we now have of them. A volcano does not necessarily go through all these stages; activity may stop at any one stage, and the chimney may be blocked with debris of non-volcanic material of the first stage, fine non-volcanic dust, or