58 Royal Society ; — 



the base of the lofty precipice at the head of the Val del Bove are in- 

 clined at angles of 20° to 30° N.W., i. e. towards the present central 

 axis of eruption. Other strata to the eastwards (as in the hill of Zoc- 

 colaro) dip in an opposite direction, or S.E., while, in a great part of 

 the north and south escarpments of the Val del Bove, the beds dip 

 N.E. or N., and S.E. or S. respectively. There is, therefore, aqua- 

 quaversal dip away from some point situated in the centre of the area 

 called the Piano di Trifoglietto. Here a permanent axis of eruption 

 may have existed for ages in the earlier history of Etna, for which 

 the name of the axis of Trifoglietto is proposed, while the modern 

 centre of eruption, that now in activity, may be called the axis of 

 Mongibello. The two axes, which are three miles distant the one 

 from the other, are illustrated by an ideal section through the whole 

 of Etna, passing from west to east through the Val del Bove, or from 

 Bronte to Zafarana. Touching the relative age of the two cones, it 

 is suggested that a portion only of that of Mongibello may be newer 

 than the cone of Trifoglietto. The latter, when it became dormant, 

 was entirely overwhelmed and buried under the upper and more 

 modern lavas of the greater cone. This doctrine of two centres, 

 originally hinted at by the late Mario Gemmellaro, had been worked 

 out (unknown to Sir C. Lyell at the time of his visit) by Baron Sar- 

 torius V. Waltershausen, and has been since supported in the jSfth 

 and sixth parts of his great work called "The Atlas of Etna" both 

 by arguments founded on the quaquaversal dip of the beds as above 

 explained, and by the convergence of a certain class of greenstone 

 dikes towards the axis of Trifoglietto. Von Waltershausen has also 

 shown that the superior lavas and volcanic formations crowning the 

 precipices at the head of the Val del Bove, from the Serra Giannicola 

 to the Rocca del Corvo, inclusive, are unconformable to the highly in- 

 clined beds in the lower half of the same precipice, the superior beds 

 being horizontal, or, when inclined, dipping in such directions as would 

 imply that they slope away from the higher parts of Mongibello. 



According to Sir C. Lyell, the alleged discontinuity between the 

 older and modern products of Etna is, in truth, only partial, and 

 almost confined to that flank of the mountain, where its physical geo- 

 graphy has been altered by three causes: 1st, the interference of the 

 two foci of eruption (Trifoglietto and Mongibello) ; 2ndly, the trun- 

 cation of the cone of Mongibello ; and 3rdly, the formation of the 

 Val del Bove. The truncation of the mountain here alluded to is 

 proved by the remains of the upper portion of a cone, traceable at 

 intervals around the borders of an elevated platform between 9000 

 and 10,000 feet high. These remains bear the same relation to the 

 highest and active cone, nearly in the centre of the platform, which 

 Somma bears to Vesuvius. The manner in which the north and 

 south escarpments of the Val del Bove diminish in altitude as they 

 trend eastward from the high platform, is appealed to as showing 

 that the great lateral valley had no existence till after the time when 

 Mongibello had attained its fullest development and height. 



The double axis of Etna is then compared to the twofold axis of 

 the island of Madeira, as inferred from observations made in 1854 



