130 Sir Charles Lyell [April 15, 



the quaquaversal dip of the beds as above explained, and by the con- 

 vergence of a certain class of greenstone dikes towards the axis of 

 Trifoglietto. Von AYaltershausen has also shown that the lavas and 

 volcanic formations in the middle of the precipices at the head of the 

 Val del Bove, from the Serra Giannicola to the Rocca del Corvo 

 inclusive, are horizontal and unconformable to the highly inclined beds 

 in the lower half of the same precipice ; or where some of the superior 

 beds are inclined, they dip in such directions as would imply that they 

 slope away from the higher parts of Mongibello. All these facts were 

 fully confirmed by Sir C. Lyell in his explorations of these lofty pre- 

 cipices in 1857 and 1858. 



The double axis of Etna was then compared to the twofold axis of 

 the volcanic island of Madeira, as observed by Mr. G. Hartung, and 

 the lecturer, in 1853-54 ; and it is observed that the admission of this 

 theory is entirely adverse to the hypothesis of craters of elevation, for 

 it implies that the force of upheaval plays no more than a subordinate 

 part in the cone-making process. Although one cone of eruption may 

 envelope and bury an adjoining cone of eruption ; it is obviously impos- 

 sible that one cone of upheaval should mantle round and overwhelm 

 another cone of upheaval. 



It is, however, conceded that in some parts of the central nucleus 

 of Etna, there are lavas which dip at higher angles than can with any 

 probability be ascribed to the original steepness of the sloping flanks of 

 an active cone. Some of these instances are regarded as exceptional, 

 and due to local disturbance ; others may be connected with the 

 abundance of fissures, often of great width, which have been filled with 

 lava in the central nucleus of the mountain, forming dikes which are 

 much less frequent and sometimes entirely wanting at points remote 

 from the centre. The injection of so much liquid matter into count- 

 less rents may imply the gradual tumefaction and distension of the vol- 

 canic 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 dip. 

 Sir C. Lyell considers that about one-fifth may, with more proba- 

 bility, be assigned as the effect of such movements. 



The alleged parallelism and uniformity of thickness in the volcanic 

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

 sidered ; and the lecturer remarked that neither in the northern nor 

 southern escarpments of the great valley, could he verify the existence 

 of such parallelism. Drawings exemplifying a marked deviation 

 from it were exhibited ; these views being taken from the northern 

 and southern cliffs of the Val del Bove. 



The discovery that lava is capable of forming continuous and 

 tabular masses of crystalline rock on steep slopes, often exceeding 30", 

 enables us henceforth to dispense with that paroxysmal and terminal 

 upheaval, which the advocates of " craters of elevation " legitimately in- 

 ferred from their premises, for it was as necessary for them, so long as 

 the volcanic beds were assumed to have been originally horizontal, to 



