222 



HARDWICKE'S SCIENCE-GOSSIP. 



things may happen. If the cone which now forms, 

 as it were, an embankment around the lava, is of 

 sufficient strength to withstand the pressure of the 

 fluid mass contained within it and the continual ex- 

 plosive vibrations, the liquid rock pours out over the 

 edge of the crater down the side of the cone, and may 

 continue its course for variable distances from its 

 starting-point. Or if, on the other hand, the cone is 

 too weak to support the strain, it may break away 

 and give free passage to the lava through the breach. 

 This condition is well illustrated in many of the Puys 

 of central France. There is another series of events, 

 that is to say, the formation of dykes, about which 

 we shall have more to say anon. 



The lava may form a series of little streams over 

 the cone sides, changing their situation according to 

 the point at which the crater is lowest. Here it will 

 cool, forming a buttress of rock on the slopes of the 

 cone. These masses will be covered again by lapilli, 

 other buttresses formed in same manner, and thus the 

 cone built up higher and made stronger. If we see it in 

 section, as in fig. 136, it will present a stratification of 

 alternate beds of rock and cinders. This, however, 

 is misleading. The lava streams do not form a con- 

 tinuous sheet surrounding the cone — see diagram, 

 fig. 136, where they are seen cut through in transverse 

 section. When a mountain of some height has been 

 formed, it then becomes liable to fracturing, and the 

 formation of so-called volcanic dykes. Mr. Mallet, 

 in a communication to the Geological Society,* 

 thoroughly explained this condition of things. As we 

 have seen, the cone may form an embankment around 

 the column of lava occupying the chimney and crater, 

 consequently there is an enormous pressure put upon 

 the supporting wall of loose material. Let us begin by 

 taking the pressure of a column of water thirty-two feet 

 high, then let us say another 4000 feet, roughly the 

 altitude of Vesuvius, and compare that with a column 

 of molten lava, whose specific gravity is two or three 

 times that of water. This would be an interesting 

 calculation : given the specific gravity of 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 neces- 

 sarily increases in thickness from above downwards. 

 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 there- 

 fore 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 outwards. 

 Mr. Mallet,t in his paper describing these mechanical 



* Proc. Geol. Soc. Lond. vol. xxxii. part iv. page 478. 

 t Proc. Geol. Soc. Lond. vol. xxxii. p. 478. 



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 outwards. 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 vapour from the lava, 

 these cones are generally formed in a row, radiating 

 from the mountain axis, and in a step-like arrange- 

 ment. This is attributed to the fact that as the lava 

 and vapour 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 downwards, 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 perpen- 

 dicular sheet of rock or dyke, 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, f Sir Charles Lyell 

 adopted the plan of endeavouring to find the orien- 

 tation or point of convergence of these dykes, to 

 localise 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 

 upwards of two kilometres between the points of 

 melting. This we can well understand when we 

 know how irregularly the cone is constructed, and how 

 buried coulees of lava may derange the direction of the 

 fracture, such as we exaggerately see illustrated in 

 some old denuded trap dykes, threading their way 

 along plains of least resistance. There is another 

 source of error, that .is, that so little of the projecting 

 edge of the dyke is exposed to accurately take its 

 strike, thus rendering us unable to determine by this 

 means the locality of an old volcanic axis. 



If we look at the figure 136, at the surface C C" of 

 the subjacent rock, we observe it forms a wave-like 



* Proc. Geol. Soc. Lond. vol. xxxiii. p. 740. 

 t Sir C. Lyell. "Lavas of Mount Etna." Phil. Trans. 

 1858. 



