Page:Popular Science Monthly Volume 17.djvu/472

456 along the lines of unequal density where the continents join the ocean-beds.

The quantity of lava that a volcano emits during an eruption surpasses anything we can imagine. The volume of the lava-stream of Kilauea, in the great eruption of 1840, was estimated at five and a half milliards of cubic metres, and a still larger mass was thrown out in 1855 from the crater at the summit of Mauna Loa, of which Kilauea is the smaller outlet. But these are trifling compared with the mass of matter emitted by the Iceland volcano of Skapta-Jökull, in 1783, which was estimated to equal the volume of Mont Blanc, or not less than five hundred milliards of cubic metres! According to the probably exaggerated estimate of Zollinger, the total volume of scoriae and ashes thrown out in 1815 from a volcano in the island of Sumbawa (Tomboro), to the distance of five hundred kilometres, equaled twice that of Mont Blanc. We have more exact data concerning the eruption of Coseguina, a small volcano of Central America, which, in 1835, rained pumice-stone on the land and sea over a radius of fifteen hundred kilometres, and discharged daily not less than fifty milliards of cubic metres. When we consider the stupendous force required to raise and throw to a distance such volumes of matter, it is difficult to believe that the underground forces that feed the volcanoes, and which we know have been active from a very remote period, are mere accumulations of matter in fusion. Still more difficult is it to suppose that the heat of these fires is due to chemical action developed in the bosom of the earth. We can not but seek, in the wide-spread, incandescent mass under the thin crust that varies, possibly, from twenty to one hundred kilometres in thickness, the proximate cause of volcanic phenomena. The objection based on the non-coincidence of eruptions of volcanoes situated in the same region disappears when the mechanism of the eruption is explained by the more or less fortuitous deposition of infiltrated water.

The question appears to be reduced to deciding whether the central nucleus on which the mass of lava rests is itself liquid or whether it is solid. This is a much disputed point, and great ingenuity has been shown on both sides of the argument. The hypothesis of a liquid nucleus has long been favored, and it has many adherents. It has been objected that a liquid nucleus would be subject to tides that would break in an instant the thin envelope and produce terrific cataclysms. Ampère, in particular, felt it impossible to reconcile this consequence of the hypothesis with the calm that reigns on the surface. "Those who maintain the idea of a liquid nucleus," said he, "do not appear to have considered the effect of the moon's attraction on this enormous liquid mass, which would cause tides analogous to those of our seas, but far more terrible, by reason of their extent and the density of the liquid. It is difficult to conceive how the earth's crust could withstand the action of a kind of hydraulic lever 1,400 leagues long."