268 Geological Gleanings. 



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 sug- 

 gested 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 Sartorius v. Waltershausen, and has been since supported 

 in the fifth 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 Walters- 

 hausen has also shown that the superior lavas and volcanic for- 

 mations 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 inclined 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 phy- 

 sical geography has been altered by three causes : 1st, the inter- 

 ference of the two foci of eruption (Trifoglietto and Mongibello) ; 

 2ndly, the truncation 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 by M. Hartung and the author. In that island the principal 



