1859.] on the Conical F&rm of Volcanoes. 129 



lecturer first visited Etna. These changes are very striking ; the fresh 

 currents having run from the head of the Val del Bove both in a north-east 

 and in a south- east direction for a distance of six miles, with a breadth in 

 each case of two miles, and having been piled up one over the other in 

 some places (as at the Portella of Calanna) to a depth of more than 100 

 feet. The longitudinal and nearly parallel ridges on the surface of 

 this new lava field are from 20 to 70 feet high ; and there is now a 

 black and monotonous wilderness in many places, where, in 1 828, there 

 were verdant forests. 



One branch of this lava of 1852 cascaded over a precipitous 

 declivity 500 feet high, at the head of the Valley of Calanna, and con- 

 solidated at angles of 35°, 45°, and even 49°. The scoriaceous crust 

 having been partially washed off, the surface of a continuous crystalline 

 and stony mass is exposed to view, only moderately vesicular, and 

 having the steep inclinations above alluded to. This same 

 current rests on an older one, that of 1819, which passed down 

 the same steep cliff, and which has at some points a dip of more 

 than 40°. 



The structure of the nucleus of Etna, or of the oldest visible part 

 of the volcano, as shown in sections in the Val del Bove, was next 

 treated of, and the doctrine of a double axis of eruption deduced from 

 the varying dip of the beds. The oldest of these beds, composed of 

 trachyte and trachytic tuff at the base of the lofty precipices at the 

 head of the Val del Bove, are inclined at angles of 20° to 30" to the 

 north->^est, or towards the present great central axis of eruption. 

 Other similar beds, two miles to the south-east, in the hill of Zoccolaro, 

 dip in an opposite direction ; while in the north and south escarpments 

 of the Val del Bove, the dips are north-east and south-east respec- 

 tively. On the whole, there is a quaquaversal dip away from some 

 point situated in the centre of the area called the Piano di Trifoglietto. 

 Here a permanent axis of eruption seems to 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, were illustrated by an ideal sec- 

 tion 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 the upper 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 in 1857) by Baron Sartorius von 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 



» See Phil. Trans., Part 2, for 1858, p. 740. 



