LYELL — ON CRATERS OF ELEVATION. 



317 



cascaded into tlie Cava Grande. Tlie lava there consoHdated into 

 a central stony mass on an inclination of 85 degrees. 



During the eruption of 1852, the lava cascaded more than once 

 over the steep precipice of the Salto della Giumenta, more than 400 

 feet high, which intervenes between the hills of Calanna and Zocco- 

 laro, and at this spot measurements of inclined stony lava, at angles 

 varying from 85 to 45 degrees, were taken with the clinometer, and 

 in one case even 50 degrees were ascertained. 



The next remarkable instance given by Sir Charles is that of a 

 steeply inclined continuous sheet of lava, 5,000 feet higher than the 

 last mentioned, near the top of the great precipice of the Yal del 

 Bove, not far below the Cistema. Thirty, thirty-five, and even 

 thirty-eight degrees are there attained. 



Other similar instances are given, and quite enough is done even 

 in this first part of the paper to prove the essential point, that lavas 

 can be consolidated on slopes of considerable steepness. 



The second part of the paper enters into the subject of the struc- 

 ture and position of the older volcanic rocks of Mount Etna, as seen 

 in the Yal del Bove, as also on the proofs of a double axis of elevation ; 

 and by these means the " crater of elevation-hypothesis" is again re- 

 futed, and the opinions of M. Elie de Beaumont, both as to theory 

 and many important matters of fact, are controverted. 



From a point in the Yal del Bove called the Piano di Trifoglietto, 

 midway between the Serra Giannicola and the hill of Zoccolaro, the 

 beds of lava radiate in all directions (sho^vn by the arrows in the 

 map of the region of Etna and the Yal del Bove at page 821) ; and 

 from this quaquaversal dip Sir Charles assumes this point (T in the 

 map referred to) to have been an ancient centre of eruption dis- 

 tinct from the present cone of Etna, and that Etna had therefore at 

 one period a double axis, or two points of permanent eruption, 

 "vvith an intermediate valley, or intercoUine space, between the two 

 cones, which became gradually filled up by lavas and fragmentary 

 matter. For the sake of distinguishing these, the extinct axis is 

 termed the axis of Trifoglietto, and the present centre of activity 

 the axis of Mongibello, — the modern Sicilian appellation of Mount 

 Etna. 



The former existence of an old centre of eruption in the Piano del 

 Trifoglietto had been inferred independently by S. Yon Walters- 

 hausen from the convergence towards a middle point in that area of 

 thirteen or more dikes of greenstone, one of them of enormous 

 dimensions, visible in the surrounding escarpments. The same 

 geologist also observed in the gigantic buttresses of the cliffs, 2,000 

 and 8,000 feet high, between the Giannicola and the Rocca del Corvo, 

 that while in the lower part of the precipices the lava-beds dip at 

 high angles inward towards the escarpment, or away from the Yal 

 del Bove, those in the middle portion become horizontal, and those 

 nearer the summit dip towards the Yal del Bove, as if they were 

 sloping away from some other point near the present great centre of 

 Mongibello. 



VOL. 11. B B 



