Geological Gleanings. 269 



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 secondary centre of eruption interfered with the slope of the 

 volcanic formations, and where the order of their succession and 

 superposition is uninterrupted, there occur, both in Madeira and 

 Etna, deep crateriform valleys (the'Curral and the Val del Bove) 

 intersecting the products of the two axes of eruption. 



" In concluding this part of his memoir, Sir C. Lyell observes, 

 that the admission of a double axis, as explained by him, is irre- 

 concileable with the hypothesis of " craters of elevation ;" for it 

 implies that, in the cone-making process, the force of upheaval 

 merely plays a subordinate part. One cone of eruption, he says, 

 may envelope and bury an adjoining cone of eruption ; but it is 

 obviously impossible that one cone of upheaval should mantle 

 round and overwhelm another cone of upheaval. 



" An attempt is then made to estimate the proportional amount 

 of inclination which may be due to upheaval in those parts of the 

 central nucleus of Etna where the dip is too great to be ascribed 

 exclusively to the original steepness of the flanks of the cone. The 

 highest dip seen by the author was on the rock of Musarra, where 

 some of the strata, consisting of scoriae with a few intercalated 

 lavas, are inclined as 47°. Some masses of agglomerate and beds 

 of lava in the Serra del Solfizio were also seen inclined at angles 

 exceeding 40°. Some of these instances are believed to be excep- 

 tional and due to local disturbance ; others may have an intimate 

 connexion with the abundance of fissures, often of great width, 

 filled with lava, for such dikes are much more frequent near the 

 original centres of eruption than at points remote from them. 

 The injection of so much liquid matter into countless rents may 

 imply the gradual tumefaction and distension of the volcanic mass, 

 and may have been attended by the tilting of the beds, causing 

 them to slope away at steeper angles than before, from the axis of 

 eruption. But instead of ascribing to this mechanical force, as 

 many have done, nearly all, or about four-fifths of the whole class, 

 one-fifth may, with more probability, be assigned as the effects of 

 such movements. 



