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THE WONDERS OF GEOLOGY. Lect. VIII. 



of the hard lavas. In some of the ancient Vesuvian lavas 

 there are decided indications of a concretional and pris- 

 matic structure, and a tendency to divide into columns, 

 like the basaltic rocks. 



Aqueous vapours are emitted in abundance from vol- 

 canoes, and often from their condensation give rise to 

 copious springs. The gaseous emanations from the fuma- 

 roles, or lesser vents, frequently contain chlorides of lead, 

 iron, copper, ammonia, soda, &c. 



The lofty volcanic peaks which reach far above the limits 

 of perpetual snow, as those of the Andes, (Cotopaxi, which 

 is 19,070 feet high,) are frequently the cause of frightful 

 inundations from the sudden melting of the snow, occa- 

 sioned by the evolution of heat during an eruption. Torrents 

 of water, bearing along heated masses of scoriae and blocks 

 of ice, rush down the sides of the mountains, and over- 

 whelm the plains below. Water from the melted snow is 

 also continually finding its way into the hollows and fissures 

 of the trachytic rocks, and vast subterranean lakes are thus 

 formed in the interior of the volcanic mass: and when 

 these reservoirs are burst open by the earthquakes that 

 precede eruptions, water, and tufaceous mud, not unfre- 

 quently accompanied with swarms of fishes that inhabited 

 the internal pools, are ejected with great violence. 



The chief constituents of lavas are the substances 

 termed felspar and augite, and titaniferous iron, and the 

 lavas are classed according as either of these ingredients 

 predominates. When the felspar prevails, the mass is 

 called Trachyte, which is generally of a coarse grain, with 

 a harshness of texture, and a degree of porosity ; when the 

 grain is fine and compact, but irregular, it constitutes 

 Trachytic Porphyry ; when the particles are so fused as to 

 have a resinous or glassy texture, it forms Pitchstone and 

 Obsidian. If Augite or titaniferous iron constitute a large 

 proportion of a rock, it is termed Basalt ; and when the 



