Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/264

Rh 250 lava that it hung in pendent stalactites from the branches, which nevertheless, though clasped round by the molten rock, had barely their bark scorched. Again, for nearly 100 years there has lain on the ﬂank of Etna a large sheet of ice which, originally in the form of a thick mass of snow, was overﬂowed by the molten ﬂood, and has thereby been protected from the evaporation and thaw which would certainly have dissipated it long ago, had it been exposed to the air. The heat of the lava has not sufficed to melt it. There seems reason to suspect, however, that in other cases snow and ice have been melted in large quantities by over- ﬂowing lava. The great ﬂoods of water which rushed down the ﬂank of Etna, after an eruption of the mountain in the spring of 1755, have been thus explained. One further aspect of a lava-stream may be noticed here—the effect of time upon its surface. While all kinds of lava must, in the end, crumble down under the inﬂuence of atmospheric waste and, where other conditions permit, become coated with soil and support some kind of vegeta- tion, yet extraordinary differences may be observed in the facility with which different lava-streams yield to this change, even on the ﬂank of the same mountain. Every one who ascends the slopes of Vesuvius remarks this fact. After a little practice it is not difficult there to trace the limits of certain lavas even from a distance, in some cases by their verdure, in others by their barrenness. Five hundred years have not sufficed to clothe with green the still naked surface of the Catanian lava of 1381 ; while some of the lavas of the present century have long given footing to bushes of furze. Some of the younger lavas of Auvergne, which certainly ﬂowed in times anterior to those of history, are still singularly bare and rugged. Yet, on the whole, where lava is directly exposed to the atm0sphel‘e, without receiving protection from occasional showers of volcanic ash, or being liable to be washed bare by heavy torrents of rain, its surface decays in a few years sufficiently to afford soil for a few plants in the crevices. Then these have taken root they help to increase the disintegra- tion. At last, as a more or less continuous covering of vegetation spreads over the rock, the traces of its volcanic origin one by one fade away from its surface. Some of the Vesuvian lavas of the present century already support vineyards. Torrents of Water and JI-ucl.——Ve have seen that large quantities of water accompany many volcanic eruptions. In some cases, where ancient crater-lakes or internal reser- voirs have been shaken by repeated detonations, and ﬁnally disrupted, the mud which has thus been produced issues at once from the mountain. Such “mud-lavas,” on ac- count of their liquidity and swiftness of motion, are more dreaded for their destructiveness than even the true melted lavas. On the other hand, rain or melted snow, rushing down the cone and taking up loose volcanic dust, is con- verted into a kind of mud that grows more and more pasty as it descends. The mere sudden rush of such large bodies of water down the steep declivity of a volcanic cone cannot fail to effect much geological change. Deep trenches are cut out of the loose volcanic slopes, and some- times large areas of woodland are swept away, the debris being strewn over the plains below. During the great Vesuvian eruption of 1622 a torrent of this kind poured down upon the villages of Ottajano and Massa, overthrowing walls, ﬁlling up streets, and even burying houses with their inhabitants. It was by similar streams from the same volcano that some of the Roman cities on its ﬂanks were overwhelmed in the first century. Many of the volcanoes of Central and South America discharge large quantities of mud directly from their craters. Thus in the year 1691 Imbaburu, one of the Andes of Quito, emitted ﬂoods of mud, so largely charged with dead ﬁsh that pesti- GEOLOGY [IIL DY1‘.MICAI.. lential fevers arose from the subsequent effluvia. Seven years later (1698), during an explosion of another of the same range of lofty mountains, Carguairazo (14,706 feet), the summit of the cone is said to have fallen in, while torrents of mud, containing immense numbers of the fish I’_2/mclodus Cyclopum, poured forth and covered the ground over a space of four square leagues. The carbon- aceous mud (locally called 7nn_1/(1) emitted by the Quito volcanoes sometimes escapes from lateral ﬁssures, sometimes from the craters. Its organic contents, and notably its siluroid ﬁsh, which are the same as those found living in the streams above ground, prove that the water is derived from the surface, and accumulates in craters or underground cavities until discharged by voleanic action. Similar but even more stupendous and destructive outpourings have taken place from the volcanoes of Java, where wide tracts of luxuriant vegetation have at different times been buried under masses of dark grey mud, sometimes 100 feet thick, with a rough hillocky surface from which the top of a submerged pahn—tree might have been seen pro- truding. Between the destructive effects of mere water—torrents and that of these mud-floods there is, of course, the notable difference that, whereas in the former case a portion of the surface is swept away, in the latter, while sometimes con- siderable demolition of the surface takes place at first, the main result is the burying of the ground under a new tumultuous deposit by which the surface is greatly changed, not only as regards its temporary aspect, but in its more permanent features, such as the position and form of its water-courses. - ]|[ud—volcanoes.——Though probably seldom if ever strictly volcanic in the proper sense of that term, certain remark- able oriﬁces of eruption may be noticed here to which the names of mud-volcanoes, salses, air-volcanoes, and 1naca— lubas have been applied. These are conical hills formed by the accumulation of ﬁne and usually saline mud, which, with various gases, is continuously or intermittently given out from the oriﬁce or crater in the centre. They occur in groups, each hillock being sometimes less than a yard in height, but ranging up to elevations of 100 feet, or cven sometimes, as in the plains of the lower Indus, to 400 feet. Like true volcanoes, they have their periods of repose, when either no discharge takes place at all, or mud oozes out trauquilly from the crater, and their epochs of activity, when large volumes of gas, and sometimes columns of ﬂame, rush out with considerable violence and CXpl0:~l0l], and throw up mud and stones to a height of several hundred feet. The gases play much the same part therefore in these phenomena that steam does in those of true volcanoes. They consist of carbonic acid gas, carburetted hydrogen, sulphuretted hydrogen, and nitrogen. The mud is usually cold. In the water occur various saline ingredients, among which common salt generally appears. Naphtha is likewise frequently present. Large pieces of stone, differing from those in the neighbourhood, have been observed among the ejections, indicative doubtless of a somewhat deeper source than in ordinary cases. Heavy rains may wash down the minor mud cones and spread out the material over the ground, but gas-bubbles again appear through the sheet of mud, and by degrees a new series of mounds is once more thrown u ). There (Iran be little doubt that these phenomena are to be traced to chemical changes in progress underneath. Dr Daubeny explained them in Sicily by the slow combustion of beds of sulphur. The frequent occurrence of naphtha and of inﬂammable gas points, in other cases, to the dis- engagement of hydrocarbons from subterranean strata. Mud volcanoes occur in Iceland, in Sicily (Macaluba), in