250 



cade of the lava of 1852, descending a precipitous declivity 500 feet 

 high, called the Salto della Giumenta, and the stony character of the 

 layers which encrust the steep slope at angles of more than 35 and 

 even 45, are commented upon. This lava has overflowed that 

 of 1819, which congealed on the same precipice; and it is shown that 

 in such cases the junction-lines separating two successive currents 

 must be obliterated, the bottom scoriae of the newer dovetailing 

 into the upper scoriae of the older current. 



The structure of the nucleus of Etna, as exhibited in sections in 

 the Val del Bove, is next treated of, and the doctrine of a double 

 axis is deduced from the varying dip of the beds. The strata of 

 trachyte and trachytic agglomerate in the Serra Giannicola seen at 

 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 arid 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 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 Waltershausen has also 



