Ch. XXVI.] 



PROOFS OF DOUBLE AXIS OF EEUPTION. 



9 



tlie Val del Bove is on a scale as far exceeding tliat of Somnia 



esuvins in magnitude. 



-)f the mountain and proofs of 



i 



•When 



am 



played sncli an arrangement of the beds as showed that the 



structure 



mountain 



from that which the escarpment of Somma exhibits. I 

 imagined that the sloping away of the strata from a central 

 axis to all points of the compass^ or what has been called the 

 quaquaversal dip^ Avas wanting in the Yal del Bove. But 

 when I revisited the same district in 1857-8^"^ I discovered 

 that the lower portion of the volcanic beds exposed to view in 

 the great precipices A*^ ^, at the head of the valley li^ i^ Tc^ of the 

 section fig. 72^ page 12^ dipped steeply to the west^ and this 

 arrangement of the strata, together with that observed in the 

 other cliffs bounding the valley, can only be explained by 

 assuming that there was once a great centre of eruption at 



or near the point in the annexed map (fig. 71) which I have 

 marked with a cross as indicating the axis of Trifoglietto. 

 The direction of the arrows a^ h^ c, d^ e^ f g^ h, % indicate the 

 various points of the compass towards which the strata have 

 been observed to be inclined. I was accompanied in 1857 by 

 Signer G. G. Gemmellaro, when we made out this quaqua- 

 versal dip, and came to the opinion that the point marked 

 with a cross or the axis of Trifoglietto had been an ancient 



centre of eruption, f 



In confirmation of this theory, Baron S. von Waltershausen 



has observed that there is an ancient set of greenstone dikes. 



more in number, which radiate from 



i^ 



w , 



axis alluded to and are seen traversing the surrounding 

 precipices. These greenstone dikes are distinguishable by 



their 



miner 



composition from those of mor 



modern 



from 



summit 



may 



from the modern name of the mountain, the axis of Mongi- 



^^ See Paper on Mount Etna by the come to the same conclusion ; for that 



Author, Phil. Trans. Part II. for 1858. part of his Atlas in which he announced 



t I was not then aware that Baron this opinion was not published till after 



S. Von "Waltershausen had previously my return to England. 



d 



