NEWER PLIOCENE PERIOD. 



Dikes or veins at the Punto del Nasone on Somma. 



The reader will remember our description of the manner in 

 which the plain of Jerocarne, in Calabria, was fissured by the 

 earthquake of 1783 *, so that the Academicians compared it to 

 the cracks in a broken pane of glass. If we suppose the side 

 walls of the ancient crater of Vesuvius to have been cracked in 

 like manner, and the lava to have entered the rents and become 

 consolidated, we can explain the singular form of the veins 

 figured in the accompanying wood-cut -f-. 



Parallelism of their opposite sides. Nothing is more re- 

 markable than the parallelism of the opposite sides of the dikes, 

 which usually correspond with as much regularity as the two 

 opposite faces of a wal^ of masonry. This character appears at 

 first the more inexplicable, when we consider how jagged and 

 uneven are the rents caused by earthquakes in masses of hete- 

 rogeneous composition like those composing the cone of 

 Somma ; but M. Necker has offered an ingenious and, we 

 think, satisfactory explanation of the phenomenon. He refers 

 us to Sir W. Hamilton's account of an eruption of Vesuvius in 

 the year 1779, who records the following facts. f The lavas, 

 when they either boiled over the crater, or broke out from the 

 conical parts of the volcano, constantly formed channels as 

 regular as if they had been cut by art, down the steep part of 



* See vol. i. chap, xxiv., wood-cut No. 22. 

 f From a drawing of M, Necker, ibid. 



