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Cir. XXVI.] 



PEOOFS OF DOUBLE AXIS OF ERUPTION. 



11 



Description of Fig. 71. — (Map of Etna, p. 10). 



Groiiud plan of the Val del Bove showing the dip of the beds on opposite sides of 



the axis of Trifofrlietto. 



Arrows a, h, c, d, e,f, g, li, i, showing the dip 

 of the beds in opposite directions from the 

 centre of ernption or axis of Trifoglietto. 



k. Arrow showing the direction of the dip of 

 the beds in the Cisterna (see also k in the 

 section fig. 72), where they are inclined 6^ 

 south-east, whereas the beds in the lower parts 

 of the same precipice as indicated by the arrow 



6, dip in qnite a different direction, or at an 

 angle of more than 20° to the west, 



L Horizontal beds in the great precipice 

 above the Serra Giannicola, resting on bods of 

 trachyte and trachitic tuff and conglomerate, 

 which last dip at angles of from 20° to 28° 

 N.W. as indicated by the arrow a. 



M. N. Line of section of fig. 72. 



bello. In I8585 wlieii I paid a third visit to tlie Yal del 

 Bove, I found that there was a great thickness of beds [l^ 

 of Map fig. 71)5 in that part of the precipice between the 

 Piano del Lago and Giannicola, which are horizontal — a fact 

 perfectly reconcilable with the theory that the structure 

 of Mount Etna is due to its having been formed by the 

 pouring out of lava and scorise from two great distinct centres 



of eruption before alluded to, viz., that of Trifoglietto and 

 that of Mongibello ; the latter having eveiitually obtained 

 such an ascendancy as to overwhelm the products of the 

 former, and reduce the whole mountain to one symmetrical 



chasm 



compa 



ratively modern date. The accompanying section will explain 

 the theory of the structure of Etna above alluded to, or the 

 hypothesis of a double axis by which all the dips in opposite 

 directions in the Val del Bove. annarentlv so 



com 



together with the horizontality of the beds immediately below 

 the edge of the Piano del Lago, are found to be resolv- 

 able into a very simple arrangement, such as is exemplified 

 in not a few of the great Javanese volcanos described by 

 Junghuhn. That author has particularly called attention to 

 the fact, that when there are two centres of eruption in the 

 same volcanic mountain, there is a certain area between them, 

 which he calls a saddle, where the beds of lava, or the 

 showers of ashes, are level or horizontal, 

 instances he alludes to a saddle connecting the twin cones 

 of Gede and Panggerango which is 7,870 feet high. The 

 largest of the two cones, although truncated like Etna, i 

 9,226 feet high, and the lesser cone has a deep valley on one 



Among other 



s 



