XXXII. On the structure of lavas which have consolidated on steep slopes; with remarks on the mode of origin of Mount Etna, and on the theory of "craters of elevation."

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Abstract
The question whether tabular masses of lava having a compact and stony texture, and a thickness of many feet, can be formed on slopes inclined at angles of from 10° to 40°, has of late years acquired considerable importance, since geologists of high authority have assumed that if the inclination exceed 5° or 6°, a lava-current will be scoriaceous in texture, fragmentary in structure, and insignificant in thickness. Such steeply-inclined currents, it is said, can never give rise to beds of compact rock, comparable to those solid layers which we see, alternating with scoriæ and tuff, in the older parts of volcanic mountains, such as the escarpments of Somma in the case of Vesuvius, or the cliffs surrounding the Val del Bove in the case of Etna. It has even been laid down as a rule by one geologist of eminence, the late M. Dufresnoy of Paris, that lavas, to be compact and crystalline, must have consolidated on a slope not exceeding 1° or 2°. He states, in his memoir on "Vesuvius and its Environs” (1834), “Les laves ne sont compactes et cristallines que lorsqu’elles se sont répandues sur un sol ayant 1 degré à 2 degrés au plus d’inclinaison. . . . . Lorsque la pente du terrain est superieure a 2° la texture compacte commence à s’effacer, les laves deviennent bulleuses et meme scoriacées. Les coulées, qui se présentent sous un angle de 4° ne sont plus que des agglomerations de fragments incohérents." M. Elie de Beaumont, in his "Recherches sur le Mont Etna,” says (in the same volume, p. 184) that the solid beds of ancient lava in the Val del Bove (which dip often at 28° and upwards) resemble those portions only of a modern current which have flowed over ground almost flat, or not sloping at an angle of more than 3°. And again, speaking of the Vesuvian lavas seen in the Fosso Grande, he observes, “they attain a thickness of 4 or 5 metres when horizontal, but are thin on slopes of 5° or 6°” (ib. p. 169).

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