the amount of space uncovered, more interesting and valuable works of art have been disinterred than in Pompeii. When these showers of ashes fall into the ocean, they gradually sink to the bottom, where they must eventually become consolidated into rock, which may be raised again to the surface in the course of the changes which are continually going on in the relative positions of sea and land. Thus are formed very extensive masses of stratified rock, which are at the same time both eruptive and sedimentary, or Pluto-Neptunean, as they have sometimes been called, as belonging to the two domains of the mythological rulers of the realms of fire and water. It is by the constant addition made to their exterior by the falling masses of lava, ashes, and lapilli, that the cones of volcanoes are built up,- not only the dominating one of each volcano, but the secondary or minor ones, which are sometimes very numerous. These smaller cones form on the fissures which open frequently in the main cone, and which connect with the seat of action in the chimney of the volcano, just as that is connected with a still larger eruptive mass deep in the interior of the earth. Etna has more than seven hundred of these smaller cones around its base, some of which attain respectable dimensions, one reaching seven hundred feet in height, and another four hundred or more. On Vesuvius a fissure opened, in 1794, about nine hundred feet below the summit; this was two thirds of a mile in length, and eight new craters, with cones of scoriæ, were formed upon it. Besides ashes and scoriæ, we expect, in most volcanic eruptions, to see rock rendered fluid by heat issuing from the crater, and it is to this molten rock that the name of lava is properly applied. The volcanic bombs, lapilli, and ashes are of course not fluid when ejected, although some of the larger masses sometimes reach the ground in a semi-plastic condition, so as to flatten themselves out into a sort of cake, as before mentioned. Different volcanoes and volcanic regions differ greatly in respect to the fluidity of their ejections. Those of Java, for instance, do not now throw out any molten lava, but only breccia, cinders, and ashes. The Hawaiian volcanoes, on the other hand, seem never to have ejected anything but lava of a high degree of fluidity. Vesuvius and Etna furnish both fluid and solid materials in abundance. As a general rule, the very lofty cones do not emit currents of molten lava. Thus the great South American volcanoes throw out, almost exclusively, cinders and ashes. It may be stated that, in by far the larger number of instances, the great masses of lava which exist have come from low volcanoes, or still oftener from great fissures without any cones at all, in the form of "massive eruptions," as they are called, in which form, probably, by far the larger portions of the older volcanic rocks have come to the surface. It is easy to see why, in lofty volcanoes, the lava should seek and find in many cases an issue at some point far below the summit. The higher the column, the greater the hydrostatic pressure, and when the resistance offered by this exceeds that which the sides of the mountain can oppose to it, the latter must give way, and the lava find a vent at the lowest available point. The constant battering of the internal walls of the chimney, kept up by the explosive forces within, gradually destroys the cohesive power of the material, breaks it up into fragments, or threads it in every direction with cracks, so that it finally yields to the repeated shocks, just as a piece of artillery fired with very heavy charges becomes at last too weak to resist any longer, and bursts into pieces. It is in this way that the fissures originate and become filled with molten lava, which solidifies in them, forming the dikes which are so common in volcanic masses, and which are so beautifully displayed in Etna, where its internal structure is revealed by the great cut into its heart called the Val del Bove. The flow of lava, in volcanic eruptions, take place in very different ways, according to its consistency and the position of the point from which it issues. In general the crater fills up gradually, until the fiery liquid rises high enough to pour over the edge at the lowest point, when it runs down the slope with a degree of rapidity proportioned to its fluidity. The Vesuvian lava is usually very thick and ropy. One of the greatest currents of that volcano,- that of 1794, which was over a thousand feet broad and from twenty to thirty deep, ran two and a half miles in six hours, or at the rate of 2,160 feet in an hour. The lava of Mauna Loa, on the other hand, is so liquid, that when it issues from the crater it pours down the steep slope of the mountain, sometimes with amazing velocity. Thus Mr. Coan says of the eruption of 1855: "In one place only we saw the river [of lava] uncovered for thirty rods, and rushing down a declivity of from ten to twenty-five degrees. The scene was awful, and the momentum incredible. The fusion was perfect, and the velocity forty miles an hour." This lava, in making its way down the mountain-side, leaps over precipices in literal cascades of fire, presenting a most sublime spectacle. It occasionally forces its way out from a side fissure, under immense pressure of course, when it plays as a fountain, and the jets of liquid fire are reported by trustworthy authorities as rising sometimes to the height of six hundred or eight hundred feet. - Lava streams, however fluid the material may be, soon become covered, as they run down the sides of the volcano, with a consolidated crust. This hardened surface gradually thickens, and the bottom and sides also become more or less congealed, so that the flow continues through a sort of tunnel, as if it were being poured out of a sack made of its own substance. The surface gets broken up into great angular masses, which, by the motion beneath, are thrown into disorder and piled up on each other, as cakes of ice are on the sudden breaking up of one of our great rivers, the St. Lawrence for instance. In the great eruption of Mauna Loa, already mentioned, the lava made its way seventy miles reckoned by the course of its flow, and forty in a direct line, to Hilo; and after its surface had become quite hard all the way, and there was no evidence of activity visible except the columns of vapor ascending from its head and foot, Mr. Coan believed that the interior was still moving downwards. This stream of lava was three miles wide on an average, and in some places three hundred feet deep. The masses of broken crust were piled up on it to the height of a hundred feet at various points. The lava of Vesuvius seems more variable in its consistency than that of almost any other volcano. In the eruption of 1805, the velocity with which it issued from the crater was almost equal to that of the Mauna Loa current; on the other hand, the stream of 1822, when it reached Resina, moved at the rate of only five or six feet an hour. That of 1819 was in motion, at the rate of three feet an hour, nine months after its issue. The rate of motion, measured by Dolomien, of one stream was a mile a year. The low conducting power of lava is the reason why the interior of the mass can remain fluid so long and run beneath a erust of its own substance. The exterior hardens, can be walked over, or perhaps even cultivated, while the interior is still red-hot. This internal heat lasts for a long time. The lava of Jorullo was hot enough to light a cigar twenty-one years after its issue; and sixty-six years later it was still perceptibly heated, sufficiently so to give rise to fumaroles. One of the lava flows of Etna - that of 1787. spread over a mass of snow, which, in 1830, still remained under it unmelted, while the overlying mass of rock was quite hot. The snow was preserved from melting by a cover of ashes, through which the heat was conducted with extreme slowness. The manner in which volcanoes are built up by successive ejections of ashes, scoriæ, and lava, and the question whether the vast size of some cones is due in part to any other cause than this simple one of the piling up of erupted materials around a central orifice, now remain to be discussed. The simplest possible form of a volcanic accumulation is that of the ordinary cinder cone, built up by a single eruption. Such cones are among the most common, as well as the most characteristic, features of almost every volcanic district. The coarse fragments thrown out heap themselves around the orifice as they fall, in the form of a circular bank, which, as the eruptive action continues, increases in size until it becomes a hill, having the form of a truncated cone, with a funnel-shaped hollow at the summit. A section of one will show that they are rudely stratified, and that the inclination of the strata decreases with the distance from the centre. These cones are of all sizes, from that of a hay-cock to that of a mountain. The "Puys," as they are called, of Central France, - Auvergne, Velay, and the Vivarais,—are hills of scoriæ thrown up in this way. Near Clermont-Ferrand there are above sixty cones strung together on a line more than sixty miles in length, and the fissure on which these were built up is continued in Velay and the Vivarais, with two hundred or more such cones arranged in a belt twenty miles long. The shape of these accumulations of ejected materials varies with the conditions under which they are formed. When the wind blows steadily from one quarter, the materials will be more heaped up on one side, and this effect is very marked in the region of the trade-winds. A great many causes may be effective in modifying the cones thus formed. One is the issuing from them of a current of lava, by which the mass is broken down on one side; such breached cones are among the most common features of many volcanic regions. An ordinary cone resulting from a single eruption consists, then, of a pile of scoriæ, lapilli, and other loose materials, with a single current of lava, which may have flowed from the summit, the side, or the base of the elevation, and which will be found spreading itself out over the adjacent region in a sheet or stream, proportioned in size to the extent of the eruption and the nature of the surface over which it has found room to extend itself. The result of repeated eruptions occurring from the same vent will be the gradual building up of a mass, which grows in size constantly but has the same kind of structure from top to bottom. Beds of solid lava alternate in it with others of fragmentary materials, and the whole system dips in all directions from the centre. It is not to be supposed, however, that any one of the beds of lava entirely surrounds the cone; on the contrary, if a horizontal section were made through such an accumulation, it would be seen that each outflow of molten. rock has only added to the mass a portion of a concentric belt, so that the cone is built up by gradual additions of ejected materials, first on one side and then on another. Besides, there would be found, in many cases, a net-work of dikes of lava ramifying through the, lower interior portion of the cone, and produced in a way which has already been indicated. Almost all the older authors and many modern ones suppose that all volcanic cones have been built up in this simple manner. The theory originated by Humboldt and elaborated by Buch, and called the "crater-of-elevation theory," has found many warm supporters even among those who have worked long in volcanic regions, while it has been persistently opposed by most of the English geologists, especially by Lyell |