§ 9. 



LAVA CURRENTS, DIKES, AND VEINS. 



819 



the great mass of a volcanic mountain, are rent and torn 

 by the earthquakes, which generally precede every fresh 

 eruption; and these fissures and chasms become filled by 

 subsequent injections of molten rock. In this manner 

 dikes and veins are formed in the trachytic and scoriaceous 

 masses of Vesuvius, Etna, and other active volcanoes 

 {Lign. 184), resembling, on a small scale, the intrusive 

 trap -dikes in the ancient strata, of which we have already 

 noticed many examples. 



Lign. 184. — Dikes and veins in Lava. 



Fig. 1. Veins and dikes of slaggy lava in volcanic tuff; Stromboli. 

 2. Lava dikes in scoriae and sand; Etna. 



The loose sand, scoriae, and ashes, which have been either 

 wafted by the winds and fallen into the sea, or washed 

 down by torrents on the plains, become agglutinated toge- 

 ther, and form an earthy conglomerate, which is termed 

 volcanic tuff. This substance is frequently traversed 

 by veins and dikes of lava, thrown up by subsequent 

 eruptions. It often happens that the beds thus permeated, 

 being formed of materials that readily decompose, are 

 partially or wholly worn away, while the durable in- 

 truded dikes remain, and stand out in relief, sometimes 

 forming vertical walls or buttresses, of great thickness 

 and extent ; of which the celebrated Yal del Bove, of 

 Etna, to be noticed in tbe sequel, affords the most re- 

 markable examples. 



VOL. II. 3 H 



