In the case of Monte Nuovo, we have seen that the fissure, when produced, emitted water--at first in a cold, then in a boiling condition--and, eventually, steam and scoriæ. It is probable that through the first cracks which reached the surface, during the heaving of the subterranean forces, water, charged with carbonic acid, flowed abundantly, and that these cold springs, charged with carbonic acid and carbonate of lime, would be succeeded by others which were hot and contained silica in solution. In Hungary, the Western Isles of Scotland, and many other volcanic districts, we find abundant evidence that, before the eruption of lavas in the area, great masses of travertine and siliceous sinter were formed by the action of cold and hot springs. As the volcanic action became more intense by the more perfect opening of the fissures, the evolution of carbonic add gas would be succeeded by the appearance of sulphurous acid, sulphuretted hydrogen, boracic acid, and hydrochloric acid, which recent studies have shown to be successively emitted from volcanic vents as the temperature within them rises. At last lava or molten rock becomes visible within the fissures, and the ejection of the frothy masses--scoriæ, pumice, lapilli and dust--commences, and this is sometimes succeeded by the outflow of currents of lava. That volcanoes originate upon lines of fissure in the earth's crust we have the most convincing proofs. Not only have such fissures been seen in actual course of formation at Vesuvius, Etna, and other active volcanoes, but a study of the volcanoes dissected by denudation affords the most convincing evidence of the same fact. The remarkable linear arrangement seen in groups of volcanoes, which is conspicuous to the most superficial observer, confirms this conclusion. [Illustration: Fig. 81.--Map of the volcanic group of the Lipari Islands, illustrating the position of the lines of fissure on which the volcanoes have been built up.] [Sidenote: SHIFTING OF VENTS ALONG FISSURES.] We have described the action going on at Stromboli as typical of that which occurs at all volcanic vents. Stromboli is, however, one among a group of islands all of which are entirely of volcanic origin. The volcanoes of this group of islands, the Æolian or Lipari Islands, are arranged along a series of lines which doubtless mark fissures in the earth's crust. These fissures, as will be seen by the accompanying map (fig. 81), radiate from a centre at which we have proofs of the former existence of a volcano of enormous dimensions. It is a very interesting fact, which the studies of Prof. Suess have established, that the earthquakes which have so often desolated Calabria appear to have originated immediately beneath this great centre of volcanic activity. [Illustration: Fig. 82.--The Puy de Pariou in the Auvergne, illustrating the shifting of the centre of eruption along a line of fissures.]