,1 





62 8 



RECENT DIKES. 



[Ch. XXV. 



mentioned 



have been produced by the filling up of open fissures with 

 liquid lava ; but of the date of their formation we know 

 nothing farther than that they are all subsequent to the year 



more moder 



all the lavas and scoriee which they intersect. A consider- 

 able number of the upper strata are not traversed by them. 

 That the earthquakes, which almost invariably precede erup- 

 tions, occasion rents in the mass, is well known ; and, in 



months 



evolving hot vapours, were numerous 



It is clear that such 



matter 



of lava rises, so that the origin of the dikes is easily ex- 

 plained, as also the great solidity and crystalline nature of 

 the rock composing them, which has been formed by lava 

 cooling slowly under great pressure. 



Scacchi, in his detailed narrative of what happened from 

 day to day in the eruption of 1850, gives an account of a long 



N.N 



Vesuvius, from one part of which lava issued. This chasm 



become 



p- the mountain. When 



chasm 



on the slope of the cone, though even then it had been 

 partly filled by the lava of 1857, which descending from the 

 lip of the crater had flowed into it. 



It has been suggested that the frequent rending of volcanic 



—m m i *m 1 



may 



mass 



as to increase the inclination of the beds composing the 

 cone ; and in accordance with the hypothesis before proposed 

 for the origin of Monte Nuovo, Von Buch supposes that the 

 present cone of Vesuvius was formed in the year 79, not by 

 eruption, but by upheaval. It was not produced by the 

 repeated superposition of scoriae and lava cast out or flowing 

 from a central source, but by the uplifting of strata pre- 

 viously horizontal. The entire cone rose at once, such as we 

 now see it. from the interior and middle of Somma, and has 







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