Geological Obfervatwns. 389 



tries. Sub-marine volcanos have often been 

 obfervecl even in our own times. 



Thefe mountains are of all heights, fome 

 fo low as 450 feet, as that in Tanna *^ but 

 they generally form lofty fpires, internally 

 fhaped like an inverted cone placed on abroader 

 baiis. This cone is called the crater of the 

 volcano, as through it the lava generally 

 paries, thc-ugh fometimes it burfts from the 

 fides, and even from the bottom of the moun- 

 tain ; fometimes the crater falls in and is ef- 

 faced; fometimes in extinguifhed volcanos it 

 is filled with water, and forms thofe lakes 

 that are obferved on the fummit of fome 

 mountains. 



Both the crater and bafis of many volcanic 

 mountains confift of lava either intire or de- 

 compofed, nearly as low as the level of the 

 fea, but they finally reft either upon granite, 

 as the volcanos of Peru, or on fhiftus, as the 

 extinguilhed volcanos of Heffe and Bohemia, 

 or on lime-ftone, as thofe of Sllefia, the Vi- 

 centine Alps^ and Vefauius. The decompofed 

 and undecompofed lavas form irregular firata 

 that are never parallel to each other. No ore 

 is found in thefe mountains, except iron, of 

 which lava contains from 20 to 25 per cent. 



* Fojler, 143. 



C c 3 and 



