Ch. X.] DIKES OF SOMMA. 121 



from an ordinary limestone altered by heat and volcanic va- 

 pours. 



Carbonate of lime enters into the composition of so many of 

 the simple minerals found in Somma, that M. Mitscherlich, 

 with much probability, ascribes their great variety to the action 

 of the volcanic heat on subjacent masses of limestone. 



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, like the dikes of Etna already described (see wood- 

 cut No. 19), being, like them, extremely compact, and less 

 destructible than the intersected tuffs and porous lavas. In 

 height they vary from a few yards to 500 feet, and in breadth 

 from one to twelve 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 com- 

 position 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 *. Exam- 

 ples are not rare of one dike cutting through another, and in 

 one instance a shift or fault is seen at the point of intersection. 

 We observed before f, when speaking of the dikes of the 

 modern cone of Vesuvius, that they must have been produced 

 by the filling up of open fissures by liquid lava. In some ex- 

 amples, however, the rents seem to have been filled laterally. 



* Consult the valuable memoir of M. L. A. Necker, Mem. de la Soc. (le Phys. 

 et d'Hist. Nat. de Geneve, tome ii. part i. } Nov. 1822. 

 f Vol. i. chap. xx. 



