5 



THE POPULAR SCIENCE MONTHLY, 



pilli, and occasionally small streams of lava emitted. Before entering 

 further 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. 



N 



PONTA NflSONE 



fVOMTE SONOMA 

 PREHIST.TOA.D. 79 



Fig. 1. a. Cone of Eruption built up in the Crater of 1878: B. Vesuvius proper, this cone 

 composed of alterrDate lava-streams 6, and lapilli a, built up since a. d. 79. It occupies the 

 crater of Somma ; C. This is composed like the latter, of alternate heds of lava c' and lapilli c ; 

 B, C. Deposits of pumice and trachytic fragments, etc., capping all exposed parts of Somma 

 derived from the eruption of a. d. 79 ; D. Beds of late Tertiary period containing shells exist- 

 ing at present in the Mediterranean; E. Basis, consisting of denuded surface of Apennine 

 limestone (cretaceous ?) ; F. Chimney or vent. 



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 to- 

 gether, of the fragments by the intense heat and the presence of lava, 

 which, so to speak, solders each mass to its neighbor. Each is re- 

 tained 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. Nevertheless, the upper edges crumble away, 

 falling into the vents, thence to be again ejected. This process con- 

 tinually 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 mod- 

 erate angle, say roughly 45, and then suddenly increasing to nearly 

 a perpendicular. The consequence of this is, a basin-like cavity of 

 sloping walls, with the volcanic vent situated at its center. The ma- 

 terials now ejected by the volcano, supposing it to be in a compara- 

 tively 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 extended to many repetitions. Let us 

 take for example the crater of Somma (Fig. 1) occupied by the cone 

 of Vesuvius, and this again inclosing w^ithin its own walls the little 

 cone of eruption, A. We may perhaps represent it thus : A : B : : B: C. 



