812 



THE WONDERS OF GEOLOGY. Lect. VIII. 



ance the variegated strata of the Trias {ante, p. 543). Veins 

 of chalcedony and opal occur, and pumice-stone and obsidian 

 are abundant. Dikes and veins of trachyte intersect the 

 tuff in every direction {IAgn. 184, jig. 1), like the intrusions 

 of trap in the ancient sedimentary formations. 



6. Ischia and Vesuvius. — The celebrated mountain of 

 Vesuvius, or Somma, is about four thousand feet high, and 

 its crest is now broken and irregular ; but when northern 

 Italy was first colonized by the Greeks, " its cone was of a 

 regular form, with a flattish summit, where the remains of 

 an ancient crater, nearly filled up, had left a slight depression, 

 covered in its interior by wild vines, and with a sterile plain 

 at the bottom." From the earliest period to which tradition 

 refers, to the first century of the christian era, this mountain 

 was in a dormant state, and the neighbouring isles of Ischia 

 and Procida were the theatres of constant explosions and 

 earthquakes. The early Greek colonists who attempted a 

 settlement, were obliged to abandon the territory, in conse- 

 quence of the frequency and violence of the subterranean 

 movements. 



Ischia. — But subsequently to the great outburst of 

 Vesuvius, Ischia has been almost entirely dormant ; and it 

 is therefore inferred that the latter volcano was the vent 

 through which the elastic fluids and incandescent materials 

 of the subterranean fires of Italy escaped, before Vesuvius 

 resumed its activity. Ischia has numerous cones ; the central 

 one, Epomeo, is 2,600 feet high, and has traces of two large 

 craters on its summit. This mountain appears to have been 

 submarine at its origin, but, since its elevation above the sea, 

 other eruptions have burst out at various points ; and a lava- 

 stream that issued from its base is still arid, and covered in 

 parts with cinders and scoriae. The materials erupted by 

 the cones of Ischia are, for the most part, trachytes, or 

 felspathic lavas. 



Vesuvius. — In the year 63 of the christian era, Vesuvius 



