Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/258

Rh 244 liquid lava will issue from the outer slope of the mountain ; or the energies of the volcano will be directed towards clearing the obstruction in the chief throat, until, with tremendous explosions, and the rise of a vast cloud of dust and fragments, the bottom and sides of the crater are finally blown out, and the top of the cone disappears. The lava. may now escape from the lowest part of the lip of the crater, while, at the same time, immense numbers of red-hot bombs, scorize, and stones are shot up into the air, most of them falling back into the crater, but many descending upon the outer slopes of the cone, and some even upon the country beyond the base of the mountain. The lava rushes down at ﬁrst like one or more rivers of melted iron, but, as it cools, its rate of motion lessens. Clouds of steam rise from its surface, as well as from the central crater. Indeed, every successive paroxysmal convulsion of the mountain is marked, even at a distance, by the rise of huge ball-like Wreaths or clouds of steam, mixed with dust and stones, forming a vast column which towers sometimes a couple of miles above the summit of the cone. By degrees these diminish in frequency and intensity. The lava ceases to flow, the showers of stones and dust dwindle down, and after a time, which may vary from hours to days or months, even in the 7'égz'me of the same mountain, the volcano be- comes once more tranquil. Let us now consider some of the aspects of this action which have more particular geological interest from the permanent changes with which they are connected, or from the way in which they enable us to detect and realize con- ditions of volcanic energy in former periods. Fissures.—During the convulsions which culminate in the formation of a volcano, as well as in the subsequent progress of the mountain so formed, the ground at and around the focus of action is liable to be rent open by fissures. These tend to diverge from the focus; but around the vent where the rocks have been most exposed to con- cussion the ﬁssures sometimes intersect each other in all directions. In the great eruption of Etna, in the year 1669, a series of six parallel ﬁssures opened on the side of the mountain. One of these, with a breadth of two yards, ran for a distance of 12 miles, in a somewhat winding course, to within a mile of the top of the cone. Similar ﬁssures, but on a smaller scale, have often been observed on Vesuvius; and they are recorded from many other volcanoes. Two obvious causes may be assigned for the production of ﬁssures :-—(1) the enormous expansive force of the im- prisoned vapours acting upon the walls of the funnel and convulsing the cone by successive explosions; and (2) the hydrostatic pressure of the lava-column in the funnel, amounting to about 125 lb per square inch, or 8 tons on the square foot, for each 100 feet of depth. Both of these causes may act simultaneously. Into the rents thus formed the molten lava naturally ﬁnds its way. It is indeed forced into them, and solidiﬁes there like iron in a mould. The cliffs of many an old crater show how marvellously they have been injected by such veins or (lg//ces of lava. Those of Somma, and the Val del Bue on Etna, have long been known. The dykes project now from the softer tuﬂ's like great walls of masonry. Such wedges of solid rock driven into the cone nmst widen its dimensions, and at the same time from their length and ramiﬁcations must bind the substance of the cone together, and thus strengthen it against the effects of future convulsions. VVe can understand, therefore, how there should be for a time an alternation in the character of the eruptions of a volcano, depending in great measure upon the relation between the height of the cone on the one hand and the strength of its sides on the other. When the sides have been well braced together by interlacing dykes, and further thickened by the spread of volcanic materials GEOLOGY [1II. DYI‘.‘lMICAL. all over their slopes, they may resist the effects of explosion and of the pressure of the ascending lava-column. In this case the volcano may ﬁnd relief only from its summit, and if the lava ﬂows forth it will do so from the top of the cone. As the cone increases in elevation, however, the pressure from within upon its sides augments. Eventually egress. is once more established on the ﬂanks by means of fissures, and a new series of lava-streams is poured out over the lower slopes. Though lava very commonly issues from the lateral ﬁssures on a volcanic cone, it may sometimes approach the surface without actually ﬂowing out-. The great ﬁssure on Etna in 1669, for example, was visible even from a distance by the long line of vivid light which rose from the incan- descent lava within. Again, it frequently happens that minor volcanic cones are thrown up on the line of a ﬁssure. This may arise either from the congelation of the lava round. the point of emission, or from the accumulation of ejected scoriae round the ﬁssure-vent. Of the former structure examples occur in Hawaii, where the lava is remarkably liquid, and rapidly hardens into tears or drops, like wax down the sides of a candle. Where in viscid lavas the steam tears oil‘ and throws up many scoriae and bombs, a cone of such loose materials will naturally form round the oriﬁce by which the lava escapes from the ﬂank of the mountain. Explosions.—Although volcanic materials may be erupted to a large extent without the appearance of visible ﬁssu1‘es, they cannot in such cases reach the surface without some explosive action, and the consequent displacement and removal of previously existing rock. Vents are thus blown out of the solid crust, the volcanic energy being, as it were, concentrated on a given point, which we may suppose must usually be the weakest in the structure of that part of the crust. The operation has often been observed in volcanoes already formed, and has even been witnessed on ground previously unoccupied by a volcanic vent. The history of the cone of Vesuvius brings before us a long series of such explosions, beginning with that of 79—~the greatest which has occurred within the times of human history—and coming down to the present day. Even now, in spite of all the lava and ashes poured out during the last eighteen centuries, it is easy to see how stupendous must have been that earliest explosion, by which the southern half of the ancient crater was blown out. At every successive important eruption, a similar but minor operation takes place within the present cone. The hardened cake of lava forming the floor is burst open, and with it there usually disappears much of the upper part of the cone, and some- times, as in 187:2, a large segment of the crater-wall. In the year 1538 a new volcano, Monte Nuovo, was formed in 24 hours on the margin of the Bay of Naples. A cavity was drilled by successive explosions, and such quantities of stones, scoriae, and ashes were thrown out from it as to form a hill 440 English feet above the sea-level, and more than a mile and a half in circumference. Most of the fragments now to be seen on the slopes of the cone and inside its beautifully perfect crater are of various volcanic rocks, many of them being black scoriae _: but pieces of llom-an pottery, together with fragments of the older underlying tuﬂ', and some marine shells, have been obtained; and these doubtless formed part of the soil and subsoil dislocated and ejected during the explosions It is not necessary, and it does not always happen, that any actual solid or liquid volcanic rock is ejected even when explosions of considerable violence take place, whereby the rocks through which the funnel rises are much shattered. Thus among the cones of the extinct volcanic tract of the Eifel, some occur consisting entirely, or nearly so, of com- minuted debris of the Devonian greywacke and slate which