12 



HARDWICKE'S SCIENCE-GOSSIP. 



ON THE ORIGIN AND STRUCTURE OF 

 VOLCANIC CONES. 



By Dr. H. J. Johxston-Lavis, F.G.S., t^c. 



IT is observable in certain volcanoes that the lava 

 frequently strewn around after an eruption con- 

 tains more or less perfect spheres, consisting of a 

 hard external coat and more scoriaceous contents, 

 and these from their resemblance are known as vol- 

 canic bombs. Their contents may be divided into 

 two classes : 



1. Scoriaceous vesicular lava, identical in composi- 

 tion with the external shell. 



2. Miscellaneous, such as altered masses of lapilli, 

 loose blocks of foreign materials caught up in the 

 current of lava. These balls are generally considered 



PUrjTA tJASCMS 



MONTE SOMMA 

 PREHISTJC A.D.79 



Fig. II. — A, Cone of eruption built up in the crater of 1878 ; B, Vesuvius 

 proper, this cone composed of alternate lava streams (5, and lapilli «,built up since 

 A.D. 79. It occupies the crater of Somma ; C, This is composed like the 

 latter, of alternate beds of lava c' and lapilli c ; B, C, Deposits of pumice and 

 trachytic fragments, &c., capping all exposed parts of Somma derived from 

 the eruption of a.d. 79; D, Beds of late tertiary period containing shells 

 existing at present in the Mediterranean ; E, Basis, consisting of denuded 

 surface of Apennine limestone (cretaceous ?) ; F, Chimney or vent. 



to be formed by the masses being ejected to great 

 heights and cooling as they whirl through the atmos- 

 phere. 



This seems improbable, as on falling they would 

 inevitably smash into a thousand fragments. It 

 would appear more likely that they are simply con- 

 cretionary in structure around a nucleus of low 

 temperature, solidifying on the surface a layer form- 

 ing a crust of lava. Let us now direct our attention 

 to the minor particulars, such as the changes of the 

 crater, and metamorphism, or alteration of the already- 

 ejected materials. If the volcano has already reached 

 some considerable dimensions, effected by one or 

 many eruptions, we shall find that certain definite 

 changes have taken place in the chimney. The 

 eruption is reduced in force, there are spasmodic puff- 

 like ejections of lapilli, and occasionally small streams 

 of lava emitted. Before entering farther into our 



subject we must return a step 'or two. It has been 

 mentioned that the inclination of the outer slope of 

 the cone is that of the "angle of repose" of the 

 rock fragments. We should therefore conclude that 

 the inner or chimney side would be much the same. 

 This, however, is not generally the case, the inner 

 retaining a greater slope than the outer. It is due 

 chiefly to the fusion, or cementing together of the 

 fragments by the intense heat and the presence of 

 lava, which, so to speak, solders each mass to its 

 neighbour. Each is retained in position by the fluid 

 column occupying the internal cavity, and when 

 this has disappeared the temperature is necessarily 

 lowered, and thus there is formed a lining to the tube 

 by the semifusion of its superficial components. Never- 

 theless, the upper edges crumble away, falling into 

 the vents, thence to be again ejected. This process 

 continually repeated will result in the 

 majority of the lapilli falling on the 

 outer slope, leaving the chimney of 

 the form of a true funnel, that is to 

 say, a cavity whose sides descend for 

 a certain distance at a moderate angle, 

 say roughly 45°, and then suddenly in- 

 creasing to nearly a perpendicular. The 

 consequence of this is a basin-like cavity 

 of sloping walls with the volcanic vent 

 situated at its centre. The materials 

 now ejected by the volcano, supposing 

 it to be in a comparatively quiescent 

 state, will tend to build up a fresh cone 

 occupying this basin. It is not a thing 

 unknown for such a concentric arrange- 

 ment of cones and craters to be ex- 

 tended to many repetitions. Let us take 

 for example the crater of Somma (fig. 11) 

 occupied by the cone of Vesuvius, 

 and this again enclosing within its own 

 walls, the little cone of eruption, A, 

 We may perhaps represent it thus : — 

 A : B : : B : C. Such a repetition is 

 recorded as being quadruple, thus giving to the 

 mountain near its apex, a step-like appearance. 



From various irregularities and accidents, the vent 

 may shift its position and become excentric, and 

 thus produce an overlapping of the newer cone upon 

 the older. This is well illustrated by the island of 

 Vulcano at this moment. In fact, the little hill of 

 scoria surrounding the active bocca, or mouth, of 

 Vesuvius is situated right away to the E.N.E. of the 

 crater, and consequently, the lava streams are more 

 abundant on that side of the mountain (fig. 12). 



The escape of the lava and vapour is the next thing 

 to require our attention. Little more, however, has 

 to be said. The lava rarely mounts the edge of the 

 cone of eruption, generally escaping near its foot, by 

 forcing itself a passage through the loose materials or 

 some pre-existing fissure according to hydrostatical 

 laws. The vapour is the real agent in keeping a 



