ORIGIN AND STRUCTURE OF VOLCANIC CONES. 781 



kilometres between the points of melting. This -we can well under- 

 stand when we know how irregulai'ly 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 dikes, 

 threading their way along planes of least resistance. There is another 

 source of error that is, that so little of the j^rojecting edge of the 

 dike 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, at the surface C C" of the subjacent 

 rock, we observe it forms a wave-like line in section. It is again to 

 Mr. Mallet * that credit is due for the explanation of this somewhat 

 anomalous appearance. It is known that the ground under high tow- 

 ers and other heavy structures is gradually compressed by the immense 

 superincumbent weight. At the same time a corresponding elevation 

 takes place around the base of the structure. This is just what occurs 

 in a volcanic mountain. The immense pressure of superposed mate- 

 rial compresses, to a variable degree, the subjacent rock, according to 

 its yielding power. This will be greatest where the column of materi- 

 als is highest, that is to say, exactly under the crater edge as at C\ in 

 the diagram. This causes a corresponding rim-like elevation around 

 the base, or at the toe of the cone as at C", in the diagram. 



The materials which go to form the cone are the subjects of our 

 next consideration. 



Taking as our standpoint the old but useful division of lavas into 

 basaltic or basic, and trachytic or acidic, let us look at the characters 

 presented by these two great classes of rocks. Basalt and its con- 

 geners are generally heavy, compact, dark-colored, more or less crys- 

 talline. Very rarely vitreous in structure, and only in email patches. 

 Excessively fluid in the molten state, losing heat and fluidity slowly, 

 and then passing rapidly from the liquid to the solid state, the liquid 

 fragments of which, when ejected from the crater, generally fall still 

 plastic, and, when cold, form an excessively ragged, hard, angular mass. 

 The surface or scoria of the lava-stream also is hard, and not easily 

 broken, the main mass itself being very apt to form the well-known 

 columnar structure. On the other hand, the trachytic or acidic lavas, 

 when molten, are very viscid, which condition increases rapidly as it 

 loses its heat, so that it flows very short distances, often stopping mid- 

 way down the steep side of the cone, as in the island of Vulcano, or 

 forming a large, boss-shaped mass around the vent.f When cooled 

 slowly it crystallizes, but it is much more liable to form a vitreous mass 

 or obsidian than the basaltic rocks, resulting probably from its high 

 percentage of silica. In fact, it behaves very much like glass or slag 

 in its physical transformations. As on the surface of the glass pot is 



* R. Mallet, F. R. S. : " Hitherto Unnoticeci Circumstances affecting the Piling up of 

 Volcanic Cones " (" Proc. Geol. Soc," London, p. 740). 

 t P. Scrope, F. R. S., "Volcanoes," 1862. 



