Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/257

Rh VOLCANIC Ac'r1o1'.] Lower Carboniferous date at Dunbar, in Scotland. When the ejected fragment of lava has a rough irregular form, and a porous structure like the clinker of an iron—furnace, it is known as a slug. The fragmentary materials erupted by a volcano and deposited around it acquire by degrees more or less con- solidation, partly from the mere pressure of the higher upon the lower strata, pa.rtly from the inﬂuence of inﬁltrating water. It has been already stated (part ii., p. 239) that different names are applied to the rocks thus formed. The coarse, tumultuous, unstratiﬁed accumulation of vol- canic debris within a crater or funnel is called agglomerate. When the debris, though still coarse, is more rounded, and is arranged in a stratiﬁed forn1, it is a volcanic con- glomerate. The ﬁner-grained varieties, formed of dust and lapilli, are included in the general designation of tuffs. These are usually pale—yellowish, greyish, or brownish, some- times black rocks, granular, porous, and often incoherent in texture. Organic remains sometimes occur in tuff. Where the volcanic debris has accumulated over the ﬂoor of a lake, or of the sea, the entombing and preserving of shells and other organic objects must continually take place. But even in the tuffs of a volcanic cone traces of animals and plants may be preserved. Professor Guiscardi of Naples has found about 100 species of marine shells of living species in the old tuffs of Vesuvius. Marine shells l1ave been picked up within the crater of Monte N uovo, and have been frequently observed in the old or marine tuft‘ of that district. The showers of ash which fall on the outer slopes of a volcano, or the sheets of mud which sometimes are spread out there, cover over and often preserve the land-shells, insects, and vegetation living on the area at the time. The older tuffs of Vesuvius have yielded many remains of the shrubs and trees which at successive periods have clothed the ﬂanks of the mountain. § 2. l'olcam'c Action. We have now to consider the circumstances under which the various solid, liquid, and gaseous products of a volcano are emitted. Volcanic action may be either constant or periodic. Stromboli, in the Mediterranean, so far as we know, has been uninterruptedly emitting l1ot stones, steam, and lava, from the earliest period of history. Among the Moluccas the volcano Sioa, and in the Friendly Islands that of Tofua, have never ceased to be in eruption since their first discovery. The lofty cone of Sangay, among the Andes of Quito, is always giving off hot vapours; Cotopaxi, too, is ever constantly active. But, though examples of unceasing action may thus be cited from widely different quarters of the globe, they are nevertheless exceptional. The general rule is that a volcano breaks out from time to time with greater or less fury, and after longer or shorter intervals of quiescence. To what particular cause or series of causes any special eruption may be due is a question to which at present no deﬁnite answer ca11 be given. An attempt has been made to show that the explosions of a volcano are to some extent regulated by the conditions of atmospheric pressure over the area at the time. I11 the case of a volcanic funnel like Stromboli, where, as Mr Scrope poi11ted out, the expansive subterranean force within, and the repressive effect of atmospheric pressure without, just balance each other, any serious disturbance of that pressure might be expected to make itself evident by a change in the condition of the volcano. Accordingly, it has long been remarked by the ﬁshermen of the Lipari Islands that in stormy weather there is at Stromboli a more copious discharge of steam and stones than in ﬁne weather. They make use of the cone as it weather-glass, the increase of its activity indicating a falling, and the diminution a rising barometer. In like GEOLOGY 243 manner Etna, according to S. von Waltershausen, is most active in the winter months. When we remember the con- nexion now indubitably established between a more copious discharge of ﬁre-damp in mines and a lowering of atmo- spheric pressure, we may be prepared to ﬁnd a similar inﬂu- ence affecting the escape of vapours from the upper surface of the lava column of a volcano ; for it must not be forgotten that it is not so much to the lava itself as to the expansive vapours accompanying it that the manifestations of volcanic activity are due. Among the Vesuvian eruptions since the middle of the l7th century, the number which took place in winter and spring was to that of those which broke out in summer and autumn as 7 to 4. But there may be other causes besides atmospheric pressure concerned in these differences ; the preponderance of rain during the winter and spring may be one of these. At present we must wait for further data. Kluge has sought to trace a connexion between the years of maximum and lninimum sun-spots and those of greatest and feeblest volcanic activity, and has constructed lists to show that years which have been specially characterized by terrestrial eruptions have coincided with those marked by few sun-spots and diminished magnetic disturbance.‘ Such a connexion cannot be regarded as having yet been satis. factorily established. Again, the same author has called attention to the frequency and vigour of volcanic explosions about the middle of August, at or near the time of the yearly meteoric shower. But in this case, likewise, the cited examples can hardly yet be looked upon as more than coincidences. The case of Kilauea, in Hawaii, seems to show a regular system of eruptive periods. Dana has pointed out that outbreaks of lava have taken place from that volcano at intervals of from eight to nine years, this being the time required to ﬁll the crater up to the point of outbreak, or to a depth of 400 or 500 feet. But the great eruption of 1868 did not occur until after an interval of 18 years. The same author suggests that the missing eruption may have been submarine. The approach of an eruption is not always indicated by any premonitory symptoms, for many tremendous explosions are recorded to have taken place in different parts of the world without any perceptible warning. Much in this respect would appear to depend upon the condition of liquidity of the lava, and the amount of resistance offered by it to the passage of the escaping vapours through its mass. In Hawaii, where the lavas are reniarkably liquid, vast out- pourings of lava have taken place quietly without earth- quakes during the present century. But even there the great eruption of 1868 was accompanied by tremendous earthquakes. The eruptions of Vesuvius are often preceded by a failure or diminution of the wells and springs in the district. But more frequent indications of an approaching outburst are conveyed by sympathetic movements of the ground beneath. Rumblings and groanings from a subterranean source are heard ; slight tremors succeed, increasing in frequency and violence till they become distinct earthquake shocks. The vapours from the crater rise more abundantly into the air. All this time the lava column in the pipe or funnel of the volcano has been slowly ascending, forced upward and kept in perpetual agitation by the passage of the elastic vapours through its mass. If a long previous interval of quiescence has elapsed, there may be much solidiﬁed lava towards the top of the vent which will restrain the ascent of the still molten portion underneath. A vast pressure is thus exercised on the sides of the cone. Should these be too weak to resist, they will open in one or more rents, and the 1 Ucber ;S'ynchron2's7nus mad .1-2ztagom's7nus, p. 72.