ALLUVIUM RECKNT FOR3IATIONS. 



781 



exceeded that of boiling water ; that it has rendered a mass of lead fluid in four minutes, 

 when the same mass, placed on red-hot iron, would have required double that time to 

 enter into fusion ; and that even stones have been melted when thrown into the lavas 

 of Etna and Vesuvius. 



Obsidian, another volcanic product, is a black glassy lava, containing a large propor 

 tion of silica, named, according to Pliny, after its first observer, Obsidius. Scoriae, 

 cinders, ashes, and stones are identical in mineral composition with the lava currents, and 

 are portions of them, abstracted while yet in the fiery funnel, and shot upwards by the 

 explosive energies in operation. Vesuvius of which a view of the crater is annexed as 



it appeared in 1829, at its 

 first recorded eruption, in 

 A. D. 79, ejected scoriai arid 

 ashes vastly exceeding its 

 own bulk, which buried for 

 ages Herculaneum and Pom 

 peii ; and its eruptions con 

 tinued to be of the same 

 nature for more than a thou 

 sand years, the first observed 

 flow of lava occurring in 

 the year 1066. Ashes from 

 its summit have repeatedly 

 fallen at Constantinople, in 

 Egypt, and in Syria ; and 

 among its peculiarities the 

 emission of boiling water 

 from its flanks has been 

 often remarked. This is 

 not uncommon with refer 

 ence to transatlantic volca 

 noes, together with torrents of mud a compost of water and ashes forming a fetid 

 clay. The power exerted by volcanic agency may be estimated by the amount of matter 

 protruded, by the distance to which masses of rock have been projected, and by calculat 

 ing the force requisite to raise lava to the tops of existing craters from their base. 

 Vesuvius hurled stones eight pounds in weight to Pompeii, a distance of six miles, and 

 has cast similar masses 2000 feet above its own summit ; while Cotopaxi has projected a 

 block of 109 cubic yards in volume to the distance of nine miles. The annexed table 

 gives the height of the respective volcanoes named above the sea, with the force requisite 

 to cause lava to flow over their tops, and the initial velocity which such a force would 

 produce : 



The Crater of Vesuvius in 1829. 



