Page:Popular Science Monthly Volume 18.djvu/800

780 Etnean lava, the height of the crater, what is the unit of pressure at the sea-level?

The outward pressure of the lava will increase in proportion to the depth. Also the cone wall necessarily increases in thickness from above downward. This, therefore, tends to counteract the augmentation of pressure from within. Nevertheless, when this is so great inside that the inner layer of the chimney must necessarily be compressed outward, and therefore the circumference made larger, the consequence is that at one point it begins to yield, forming the commencement of a perpendicular fissure, radiating from the central axis, and, by the same course of circumstances, this will gradually spread outward. Mr. Mallet, in his paper describing these mechanical effects, aptly compares them to the bursting of a gun where the greatest strain is on the inner lining, and consequently the fissure commences in this and radiates outward. In a volcano, as the fissure is formed, it is immediately occupied by the fluid lava. If the fracture extends far enough it may reach the surface, where it may form one or more parasitic cones. By the explosion of vapor from the lava, these cones are generally formed in a row, radiating from the mountain axis, and in a step-like arrangement. This is attributed to the fact that, as the lava and vapor escape, the former reaches a lower level, and here forms the second, third, fourth, and so on in succession. This was well illustrated in 1861 at Vesuvius, where seven such hollow mounds were formed, the first being the largest, and gradually diminishing downward, as the igneous forces became exhausted. The pressure of the contained fluids may be so great that the entire side of the mountain may be rent asunder with the rapid escape of the contained lava, thus forming a breached cone. In the above-mentioned paper, in fact, it is supposed by the author that all such have originated in this manner. A third condition of things may be brought about: this fissure may only extend a certain distance from the chimney, never showing itself superficially, and the lava occupying the fissure will gradually become cooled and consolidated, forming a perpendicular sheet of rock or dike, as it is called, radiating from the mountain axis. These are well illustrated in the Val de Bove of Etna and the escarpment of Monte Somma. In the former, Sir Charles Lyell adopted the plan of endeavoring to find-the orientation or point of convergence of these dikes, to localize the site of the old crater supposed to have produced this curious cavity. This was followed by the untiring work of Mr. Mallet in the latter locality, to determine where the axis of Somma should be placed. In the latter case twenty-seven of the largest were chosen, but, when their directions were taken by a careful survey, they were found not to converge at one point, but in some there were discrepancies of upward of two