Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/261

Rh VOLCANIC ACTIO.'.] miles or more, its sides should at last give way, and large divergent ﬁssures should be opened down its cone. Again, the hydrostatic pressure of the column of lava must have a potent inﬂuence. At a depth of 1000 feet below the top of the column the pressure exerted on each square foot of the surrounding walls must amount to more than 80 tons. We may well believe that such a force, acting upon the walls of a funnel already shattered by a succession of terriﬁc explosions, will be apt to prove too great for their resistance. lVhen this happens, the lava pours forth from the outside of the cone. So ﬁssured is the cone some- times that the lava issues freely from many points. A volcano so affected has been graphically described as “sweating ﬁre.” More usually the lava issues only from one or two points. Should these lie well down on the cone, far below the summit of the lava—column, the lava, on its li1'st escape, driven by hydrostatic pressure, will sometimes I spout up high into the air—a fountain of molten rock. This was observed in 1794 on Vesuvius, and in 1832 on Etna.‘ In the eruption of 1852 at Manna Loa, an unbroken fountain of lava, from 200 to 700 feet in height and 1000 feet broad, burst out at the base of the cone. Similar “ geysers ” of molten rock have subsequently been noticed in the same region. Thus, in March and April 1868, four ﬁery fountains, throwing the lava to heights varying from 50!) to 1000 feet, continued to play for several weeks. In a lofty volcano, therefore, the chances are always rather against the lava rising to the lip of the crater and llowlllg‘ out there. It does so now and then; but more fre- quently it escapes from some fissure or oriﬁce in a weak part of the cone. In minor volcanoes, on the other hand, where the explosions are less violent, and where the thickness of the cone in proportion to the diameter of the funnel is often greater, the lava very commonly rises in the crater. Should the crater walls be too weak to resist the pressure of the molten mass, they will give way, and the lava will rush out frail] the breach. This is seen to have happened in several of the puys of Auvergne, so well ﬁgured and described by Mr Scrope. But if the crater be massive enough to with- stand the pressure, the lava, if still impelled upward by the struggling vapour, will at last ﬂow out from the lowest part of the rim. It was at one time supposed that lava beds could not consolidate on such steep slopes as those of most volcanoes, and that their present inclined position was to be attributed to a central upheaval of each mountain. This idea formed the subject of the famous theory of elevation—craters (Brice- Innzf/slcratere) of L. von Buch, E. de Beaumont, and other geologists. It was a matter of prime importance in the interpretation of volcanic action to have this question settled. To Constant Prevost belongs the merit of having completely exposed the fallacy of this theory. He pointed out that there was no more reason why lavas should not consolidate I on steep slopes than that tears or drops of "wax should not do so. M r Poulett Scrope also showed conclusively that the steep slope of the lava-beds of a volcanic cone was original. Sir Charles Lyell and Mr Hartung subsequently obtained abundant additional evidence from the Canary Islands, Etna, and other volcanic districts, to disprove the elevation theory. Geologists are now agreed that thick sheets of lava, with all their characteristic features, can consolidate on slopes of even 35° and 40°. The lava in the Hawaii Islands has cooled rapidly on slopes of 25°, that from Vesuvius, in 1855, is here and there as steep as 30'. On the east side of Etna, a cascade of lava, which poured, in 1689, into the vast hollow of the Cava Grande, has an in- clination varying from 18° to 48°, with an average thickness of 16 feet. On Manna Loa some lava-ﬂows are said to have congealed on slopes of 49°, 60°, and even 80°, though in these cases it could only be a layer of rock stiffening and GEOLOGY 247 adhering to the surface of the steep slope. Even when it consolidates on a steep slope, a stream of lava forms asheet with parallel, upper, and under surfaces, a general uniformity ofhthiclipess, pndfqlften greater evenness of surface than w ere t e ang e o escent is low. At its ﬁrst appearance, where it issues from the mountain, the lava glows with a white heat, and flows with a motion which has been compared to that of honey or of melted iron. It soon becomes red, and, like a coal fallen from a hot ﬁreplace, rapidly grows dull as it moves along, until it assumes a black, cindery aspect. At the same time the surface conceals and soon becomes solid enouoh to su J ort I a heavy bldck of stone. Its aspect depends, dot merelly) on
 * the composition and ﬂuidity of the lava, but on the point

I of egress, whether from the crater or from a ﬁssure, on the form of the ground, the angle of slope, and the rapidity of flow. Lavas which have been kept in ebullition within the central chimney are apt to acquire a rough cellular tex- ture. rough brown or black cinder-like slags, and irregular rugged cakes, which, with the onward motion, grind and grate against each other with a harsh metallic sound, sometimes rising into rugged mounds or getting seamed with rents and gashes, at the bottom of which the red—hot glowing lava may be seen. When lava escapes from a lateral ﬁssure it may have no scoriae, but its surface will present froth-like, curving lines, as in the scum of a slowly ﬂowing river, or successively ﬂowed over each other and congealed. These and many other fantastic coiled shapes were exhibited by the lava which ﬂowed from the side of Vesuvius in 1858. A large area which has been ﬂooded with lava is perhaps the most hideous and appalling scene of desolation anywhere to be found on the surface of the globe. A lava stream at its point of escape from the side of a volcanic cone occupies a comparatively narrow breadth; slowly. The sides of the moving mass look like huge em- bankments, or like some of the long mounds of “ clinkers” one sees in a great manufacturing district. The advancing end of the mass is often much steeper, creeping onward like a great wall or rampart, down the face of which the rough blocks of hardened lava are ever rattling. The rate of movement is regulated by the ﬂuidity of the lava, by its volume, and by the form and inclination of the ground. Hence, as a rule, a lava—stream moves faster at ﬁrst than afterwards, because it has not had time to stiffen, and its slope of descent is considerably steeper than further down the mountain. One of the most ﬂuid and swiftly ﬂowing lava-streams ever observed on Vesuvius was that erupted on 12th August 1805. It is said to have rushed down a space of 3 Italian (3% English) miles in the ﬁrst four minutes, but to have widened out and moved more slowly as it descended, yet ﬁnally to have reached Torre del Greco in three hours. A lava erupted by Manna Loa in 1852 went as fast as an ordinary stage-coach, or 15 miles in two hours. Long after a current has been deeply crusted over with slags and rough slabs of lava it continues to creep slowly forward for weeks or even months. It happens sometimes that, as the lava moves along, the pressure of the still molten mass inside bursts through the outer hardened and deeply seamed crust, and rushes out with, at ﬁrst, a motion much more rapid than that of the I main stream; but such an offshoot rapidly congeals and I comes to rest, though sometimes not before doing much damage to vineyards, gardens, houses, or other property in its course. Any sudden change in the form or slope of the ground, too, will affect the ﬂow of the lava. Thus, should the stream reach the edge of a steep deﬁle or cliﬂ‘, it will 1 pour over it in a cataract of glowing molten rock, with The surface of the moving stream breaks up into- but it usually spreads out as it descends, and moves more a
 * will be arranged in curious ropy folds, as the layers have