Formation of Tabular Masses of Stony Lava on steep slopes. 57 



upward and outward pressure on beds originally horizontal or nearly- 

 horizontal. For in all such mountains there are found to exist some 

 stony layers dipping at 10°, 15°, 25°, or even higher angles; and 

 according to the assumed law, such an inclined position of the beds 

 must have been acquired subsequently to their origin. 



After giving a brief sketch of the controversy respecting " Craters 

 of Elevation," the author describes the results of his recent visit 

 (October, 1857) to Mount Etna, in company with Signor Gaetano 

 G. Gemmellaro, and his discovery there of modern lavas, some of 

 known date, which have formed continuoiis beds of compact stone 

 on slopes of 15°, 36°, 38°, and, in the case of the lava of 1852, more 

 than 40°. The thickness of these tabular layers varies from 1^ foot 

 to 26 feet ; and their planes of stratiticatiou are parallel to those of 

 the overlying and underlying scoriae which form part of the same 

 currents. The most striking examples of this phenomenon were 

 met with — 1st, at Aci Reale ; 2ndly, in the ravine called the Cava 

 Grande near ISIilo, where a section of the lava of 1689 is obtained; 

 Srdly, in the precipice at the head of the Val di Calaima, in the lava 

 of 1852-53; and 4thly, at a great height above the sea near the 

 base of the ^lontagnuola. 



Sir C. Lyell then alludes to the extraordinary changes which had 

 taken place in the scenery of the Valley of Calanna and the Val del 

 Bove since his former visit to Mount Etna in 1828 — changes effected 

 by the eruption of 1852-53, one of the greatest recorded in history. 

 A brief account is given, extracted from contemporary narratives and 

 illustrated by a map, compiled with the assistance of Dr. Giuseppe 

 Gemmellarp, of the course taken in 1852-53 by various streams of 

 lava, some of them six miles in length, flowing during nine succes- 

 sive months from the head of the Val del Bove to the suburbs of 

 Zafarana and ^lilo. The present aspect of this lava-field, parts of 

 it still hot and emitting vapour, and the numerous longitudinal ridges 

 and furrows on its surface are described. As to the origin of these 

 superficial inequalities, the author inquires whether they may be due 

 to the flowing of lava in subterranean tunnels, or whether they be 

 anticlinal and synclinal folds caused by fresh streams pouring over 

 preceding and half-consolidated ones, so that these last may be bent 

 and crumpled by the newly superimposed weight, like soft yielding 

 ground on which a railway embankment has been made. The cas- 

 cade of the lava of 1852, descending a precipitous declivity 500 feet 

 high, called the Salto della Giumenta, and the stony character of the 

 layers which encrust the steep slope at angles of more than 35° and 

 even 45°, are commented upon. This lava has overflowed that 

 of 1819, which congealed on the same precipice; audit is shown that 

 in such cases the junction-lines separating two successive currents 

 must be obliterated, the bottom scoriic of the newer dovetailing 

 into the upper scoriae of the older current. 



The structure of the nucleus of Etna, as exhibited in sections ia 

 the Val del Bove, is next treated of, and the doctrine of a double 

 axis is deduced from the varying dip of the beds. Tlic strata of 

 trachyte and trachytic agglomerate in the Scrra Giannicola seen at 



