§8. VOLCANIC PRODUCTS. 815 



having burst from the sides of the mountain. The vast number of 

 vertical basaltic dikes which intersect the horizontal beds observable 

 in the broken cliffs of the old crater (Atrio del Cavallo) bear witness, 

 however, that the lava was not so frequently elevated to the summit of 

 the mountain, without occasioning numberless cracks and rents in its 

 internal structure. There is great reason to conclude that the old 

 crater of Somma, whose steep walls now half encircle the cone of 

 Vesuvius, was formed by the celebrated eruption of the year 79, which 

 occasioned the death of the elder Pliny, and buried Herculaneum, 

 Pompeii, and Stabiae, beneath a bed of ashes and fragmentary scoriae, 

 &c. from thirty to one hundred feet in thickness." * 



8. Volcanic Products. — Before we pass to the con- 

 sideration of the phenomena attendant on a volcanic erup- 

 tion, we will examine some of the principal minerals which 

 enter into the composition of the lavas and other substances 

 ejected from volcanoes. 



Lava is a term applied to any mineral matter liquefied 

 by heat, that has issued in a stream or current from a 

 volcanic aperture : when consolidated by cooling, it may 

 consist either of scorias, pumice, basalt, trachyte, obsidian, 

 &c. according to its mineral composition, and its slow or 

 rapid refrigeration. The greater or less degree of pressure 

 under which the solidification either of liquid or merely 

 softened mineral substances takes place — as, for example, 

 in the open air, or at the bottom of the sea, or in deep- 

 seated subterranean cavities — appears to be the principal 

 cause of the difference between the ancient plutonic, and 

 the volcanic rocks, f 



Among the products of modern volcanoes, five of the 

 metals occur ; namely, iron, copper, lead, arsenic, and sele- 

 nium. The number of simple minerals found in the rocks 

 of Vesuvius amounts to 400 species ; and many of them are 

 of great beauty. Specular iron is common in the cavities 



* From the Memoir on the Volcanic District of Xaples, by Poulett 

 Scrope, Esq., Geol. Trans, vol. ii. p. 337. 

 t Humboldt. 



