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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 : 1 st, 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 1354 

 by M. Hartung and the author. In that island the principal chain 

 of volcanic vents, running east and west, and 30 miles long, attains at 

 one point a height of 6000 feet. Parallel to it, at the distance of 

 two miles, a shorter and lower, secondary chain once existed, but 

 was afterwards overflowed and buried to a great depth by lavas 

 issuing from the higher and dominant chain. The space between 

 the two axes, like the space which separated the two cones of Etna, 

 has been filled up with lavas in part horizontal. On the north side 

 of Madeira, as probably on the west side of Etna, where no second- 

 ary centre of eruption interfered with the slope of the volcanic for- 

 mations, and where the order of their succession and superposition 

 is uninterrupted, there occur, both in Madeira and Etna, deep cra- 

 teriform valleys (the Curral and the Val del Bove) intersecting the 

 products of the two axes of eruption. 



