Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/255

Rh VOLCANOES.] it is believed, about 17 centuries before that date. From the craters of the Eifel, Auvergne, the Vivarais, and central Italy, though many of them look as if they had only re- cently been formed, no eruption has been known to come during the times of human history or tradition. From these examples it is clear that no real distinction can be drawn between dormant and extinct volcanoes. Also, from the fact above mentioned, that volcanoes have often appeared on areas occupied by those of previous geolo- gical periods, it is equally evident that volcanic action is apt to show itself again and again even at vast intervals within the same regions and over the same sites. § 1. Volcanic Products. The materials erupted from volcanic vents may be classed as (1) gases and vapours, (2) lavas, (3) fragmentary sub- stances. A brief summary under each of these heads may be given here; the mode and order of appearance of the several products will be stated in § 2. 1. Cases aml l'apom's.——These play an important part l'lV0lC3.|1lC activity‘; they show themselves in the earliest stages of a volca.uo’s history, and continue to appear for centuries after all the other evidences of subterranean action have ceased to be manifested. By much the most abundant of them all is steam. It has been estimated to form {=‘U‘g’Utl1s of the whole cloud which hangs over an active volcano. In great eruptions it rises in prodigious quantities, and is rapidly condensed into a heavy rainfall. M. Fouqué calculated that during 100 days Etna had ejected vapour enough to form, if condensed, 2,100,000 cubic metres of water. But even from volca11oes which, like the Solfatara near Naples, have been dormant for many cen- turies it sometimes still rises without intermission and in considerable vol111ne. Jets of vapour rush out from clefts in the sides and bottom of a crater with a noise like that made by the steam blown off by a locomotive. The number of these funnels is often so large, and the amount of vapour so abundant, that only now and then, when the wind blows the dense cloud aside, can a momen- tary glimpse be had of a part of the bottom of the crater; while at the same time the rush and rear of the escaping steam remind one of the din of some vast factory. Aqueous vapour rises likewise from rents on the outside of the volcanic cone. It issues so copiously from the molten lava which pours down the slopes of the cone that the stream of rock is almost concealed from view by the cloud ; and it continues to escape from fissures of the lava, far below the point from which the molten matter proceeded, for a long time after the rock has solidiﬁed and con1e to rest. So saturated, as it were, are many crystalline lavas with the vapour of water that Mr Scrope even suggested that they derive their mobility from this cause. Probably in no case is the steam mere pure vapour of water. It is associated with other vapours and gases dis- engaged from the potent chemical laboratory underneath. Of these probably the most abundant is sulpkm'etted- 7zy(l7'0_r/€12, readily traceable by its characteristic odour. Its liability to decomposition leads to the deposition of a yellow crust of sulphur about the oriﬁces from which it rises,- perhaps, also, in great part to the alteration of this gas must we attribute the sulplmric acid so frequently observed at volcanic vents. Another gas especially abundant at Vesuvius, but not detected in all volcanoes. is Ia-_2/droc/cleric acid, which comes away with the steam, and gives it its pungent suffocating fumes. C'a7'bomIc acid appears in most volcanoes, partly as one of their usual products, but chieﬂy given off during the feebler conditions of activity and in the last stages of vulcanism. After an erup- tion of Vesuvius this gas has sometimes been discharged so copiously that hundreds of hares, pheasants, and part- GEOLOGY ‘.241 ridges have been suffocated by it. In the ancient volcanic regions of the Eifel and Auvergne, it still rises in pro- digious quantities. Bischof estimates that the quantity of carbonic acid evolved in the Brohl Thal amounts to five millions of cubic feet, or 300 tons of gas in one day. lV'itro_qe7z., derived doubtless from the decomposition of atmospheric air dissolved in the water which penetrates into the volcanic foci, has been frequently detected among the gaseous emanations. With these gases and vapours are associated many sub- stances which, sublimed by the volcanic heat, appear as deposits along crevices and surfaces wherein they reach the air and are cooled. Besides sulphur, which has been already mentioned, there are several chlorides (particularly that of sodium, and less abundantly those of iron, copper, and lead), sal-ammoniac, specular iron, oxide of copper (tenorite), boracic acid, and other substances. Sodium chloride sometimes appears so abundantly that wide spaces of a volcanic cone, as well as of the newly erupted lava, are crusted with salt, which can even be proﬁtably removed by the inhabitants of the district. Considerable quantities of these chlorides may thus be buried between successive sheets of lava, and in long subsequent times may give rise to mineral springs, as has been suggested with reference to the saline waters which issue from volcanic rocks of Old lied- Sandstonc and Carboniferous age in Scotland. The iron- chloride forms a bright yellow and reddish crust on the crater walls, as well as on loose stones on the slopes of the- coue. Specular iron forms abundantly as thin lamellze in the ﬁssures of Vesuvian lavas. Tenorite may be seen at the edge of the crater of Vesuvius, condensing into ﬁ11e ﬁlaments which are blown away by the wind. An order of succession has been observed to take place in the ap- pearance of the different volcanic gaseous discharges In connexion with the aqueous vapour of volcanoes, reference may be made here to the abundant discharges of water which accompany volcanic explosions Three sources of this water may be assigned :———(I) from the melting of snow by a rapid accession of temperature previous to or during an eruption,——this takes place from time to time on Etna, and still more markedly in Iceland and among the snowy ranges of the Andes; from the condensation of the vast clouds of steam which are discharged during an eruption,—this undoubtedly is the chief source of the destructive torrents so frequently observed to form part of the phenomena of a great volcanic explosion ; and (3) from the disruption of reservoirs of water ﬁlling subterranean cavities, or of lakes occupying crater—basins,——this has several times been observed among the South American volcanoes, where immense quantities of dead fish, which inhabited the water, have been swept down with the escap- ing torrents. never been known to discharge anything but water. In the large crater of Idjen, one of the volcanoes of Java, lay’ a hot steaming lake of acid water. In the beginning of the- year 1817 an eruption took place, by which the water was discharged with frightful destruction down the slopes of the mountain. After the explosion the basin ﬁlled again with water, but its temperature was no longer high. In many cases the water rapidly collects volcanic dust as it rushes down, and soon becomes a pasty mud, or it issues at first in this condition from the volcanic reservoirs, from which, after violent detonations, it is discharged. Hence arise what are termed mud-lavas or aqueous lavas, which in many respects behave like true igneous lavas. This mud eventually consolidates into one of the numerous forms of tuff,—a rock which, as has been already stated (p. 239), Varies greatly in the amount of its coherence, in its composition, and in its internal arrangement. Obviously, unless where subsequently altered, it can possess none of the crystalline X. — 31 The volcano of Agua, in Guatemala, has.