Fig. 76.—Vesuvius, as seen from Sorrento, half encircled by the crater-ring of Somma.
We shall point out, hereafter, that at most volcanic centres the ejection of trachytic lavas precedes that of the basaltic lavas. Now it is these trachytic lavas which principally give rise to the formation of the light lapilli of which tuff-cones are formed. Hence it is that we so frequently find, as in the case of Vesuvius, Rocca-Monfina, and many other volcanoes, that a great crater-ring, largely composed of tuffs, encloses a cone built up of more basic lavas.
In [fig. 77] we have shown by a series of outline sections the various forms assumed by volcanoes in consequence of the different kinds of eruptive action going on in them:—
1. Is an outline of Fusiyama, an almost perfect cone, with a small crater at its summit. The sides of this volcano admirably illustrate the beautiful double curves characteristic of volcanic cones.
2. Hverfjall in Iceland, a volcanic cone with a large crater, reaching almost to its base.
3. The crater-lake of Bracciano, in which the area of the crater is out of all proportion to the height of the crater-walls.
4. Rocca-Monfina, in Southern Italy, a tuff-cone of large dimensions, in the midst of which an andesitic lava-cone has been built up.
5. Teneriffe, in the Canary Islands, in which a perfect volcanic cone has been built up in the centre of an encircling crater-ring.
6. Vulcano, in the Lipari Islands, in which, by the shifting of the centre of volcanic activity along a line of fissure, a series of overlapping volcanic cones has been produced.
