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Ch. XXV.] 



RECENT DIKES. 



021) 



-x- 



since received no accession of height, but, on the contrary, 

 has ever since been diminishing in elevation. 



I shall endeavour to show that this hypothesis of Von 

 Buch, whether applied to the modern cone of Vesuvius or to 

 the more ancient cone called Somma, is wholly untenable. 

 But before enlarging on this topic, I may mention some facts 

 recorded by M. Abich in his account of the Vesuvian erup- 

 tions of 1833 and 1834, because they might seem at first 

 ioht to favour the possibility of such a mode of origin. f In 



si 



At length this flat area 



the year 1834, the great crater of Vesuvius had been filled 

 up nearly to the top with lava, which had consolidated and 

 formed a level and unbroken plain, except that one small 



i 



cone of scorige had been thrown up, which rose in the middle 

 of the plain like an island in a lake, 

 of lava was broken by a fissure which passed from N.E. to 

 S.W., and along this line a great number of minute cones 

 emitting vapour were formed. The first act of formation of 

 these minor cones consisted, according to Abich, of a partial 

 upheaval of beds of lava previously horizontal, and which 

 had been rendered flexible by the heat and tension of elastic 

 fluids, which, rising from below, escaped from the centre of 

 each new monticule. There would be considerable analogy 

 between this mode of origin and that ascribed by Von Buch 

 to Vesuvius and Somma, if the dimensions of the upraised 

 masses were not on so different a scale, and if it was safe to 

 reason from the inflation of bladders of half-fused lava, from 

 15 to 25 feet in height, to mountains attaining an altitude 

 of several thousand feet, and having their component strata 

 strengthened by intersecting dikes of solid lava. 



At the same time M. Abich mentions, that when, in August 

 1834, a great subsidence took place in the platform of lava 

 within the great crater, so that the structure of the central 

 cone was laid open, it was seen to have been evidently formed, 

 not by upheaval, but by the fall of cinders and scorise which 

 had been thrown out during successive eruptions. 



Mr. Scrope, writing in 1827, attributed the formation of a 



+ 



* Von Buch, Descrip. Phys. des lies 

 Canaries, p. 342. Paris, 1836. 

 t Abich, Vues Jllust. de Pheriom. 



Geol. sur le Vesuye et l'Etna. Berlin, 



1837. 



J Ibid. p. 2. 





