78o THE POPULAR SCIENCE MONTHLY. 



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 augmenta- 

 tion of pressure from within. Nevertheless, Avhen this is so great 

 inside that the inner layer of the chimney must necessarily be com- 

 pressed outward, and therefore the circumference made larger, the 

 consequence is that at one point it begins to yield, forming the com- 

 mencement 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 frac- 

 ture 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 moun- 

 tain 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 hol- 

 low 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-men- 

 tioned 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 occupy- 

 ing 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,J 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 fol- 

 lowed 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 direc- 

 tions 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 



* " Proc. Geol. Soc," London, vol. xxxii, p. 478. f Ibid., vol. xxxiii. p. 740. 



X Sir C. Lyell, " Lavas of Mount Etna," " Phil. Trans.," 1858. 



