662 



VOLCANIC EOCKS OF 



[Oh. XXX. 



Dikes of Somma. — The dikes seen in the great escarpment which 

 Somma presents towards the modern cone of Vesuvius are very 

 numerous. They are for the most part vertical, and traverse at right 

 angles the beds of lava, scoriae, volcanic breccia, and sand, of which 

 the ancient cone is composed. They project in relief several inches 

 or sometimes feet, from the face of the cliff, being extremely compact, 

 and less destructible than the intersected tuffs and porous lavas. In 

 vertical extent they differ from a few yards to 500 feet, and in breadth 

 from 1 to 12 feet. Many of them cut all the inclined beds in the 

 escarpment of Somma from top to bottom, others stop short before 

 they ascend above half way, and a few terminate at both ends, either 

 in a point or abruptly. In mineral composition they scarcely differ 

 from the lavas of Somma, the rock consisting of a base of leucite and 

 augite, through which large crystals of augite and some of leucite 

 are scattered.* Examples are not rare of one dike cutting through 

 another, and in one instance a shift or fault is seen at the point of 

 intersection. 



In some cases, however, the rents seem to have been filled laterally, 

 when the walls of the crater had been broken by star-shaped cracks, 

 as seen in the accompanying wood-cut (fig. 715). But the shape of 



Fie. T15. 



Dikes or veins at the Punto del Nasone on Somma. (decker. t) 



these rents is an exception to the general rule ; for nothing is more 

 remarkable than the usual parallelism of the opposite sides of the 

 dikes, which correspond almost as regularly as the two opposite faces 

 of a wall of masonry. This character appears at first the more inex- 

 plicable, when we consider how jagged and uneven are the rents caused 

 be earthquakes in masses of heterogeneous composition, like those, 

 composing the cone of Somma. In explanation of this phenomenon, 



* L. A. Necker, Mem. de la Soc. de Phys. et d'Hist. Nat. de Geneve, torn, it, 

 parti., Nov. 1822. 



f From a drawing of M. Necker, in Mem. above cited. 



