Formation of Tabular Masses of Stony Lava on steep slopes. 59 



by M. Hartung and the author. In that island the principal 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 second- 

 ary centre of eruption interfered with the slope of the volcanic for- 

 mations, and where the order of their succession and superposition 

 is uninterrupted, there occur, both in Madeira and Etna, deep cra- 

 teriform 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 irreconcile- 

 able 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 in the rock of Musarra, where 

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

 lavas, are inclined at 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 di/ces 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 erup- 

 tion. But instead of ascribing to this mechanical force, as many 

 have done, nearly all, or about four-fifths of the whole dip, Sir C. 

 Lyell considers that about one-fifth may, with more probability, be 

 assigned as the effect of such movements. 



Tlie alleged ])arallelism and uniformity of thickness in the rolcauic 

 beds of the Val del Bove, when traced over wide areas, is next con- 

 sidered, and the author remarks that neither in the northern nor 

 southern cscarj)iii"nts of the great valley, could he and his compa- 

 nion verify the existence of siicb parallelism. Examples of a marked 

 deviation from it are given, both in cliffs seen from a distance, and 

 in others which were closely inspected, even in cafses where these last, 



