Chap. 47-1 Itnmejifs Depth of Volcanoes. 32 r 



lied, in 1767, by the projectile force of the fteam, a 

 quarter of a mile from the crater. In an eruption of 

 jEtna, a ftone, fifteen feet long, was ejected from the 

 crater to the diftance of a mile, and buried itfelf eight 

 feet deep in the ground. 



A volcano broke forth in Peru, in 1600, accom- 

 panied with an earthquake, and the fand and afhes 

 which were ejected covered the fields ninety miles 

 one way and one hundred and twenty another. Dread- 

 ful thunders and lightning were heard and feen for 

 upwards of ninety miles round Araquapa during this 

 eruption, which feemed to denote fome connection 

 between the electric matter and thefe volcanic fires** 

 and this fact is ftrongly confirmed by the very accu- 

 rate obfervations of Sir William Hamilton, which 

 I {hall afterwards have occafion to notice more at 

 large. 



Both the infide of the crater and the bafe of many 

 volcanoes confift of lava, either entire or decompofed, 

 nearly as low as the level of the fea, but they finally 

 reft either on granite, as in Peru, cr fchiflus, as the 

 extinguifhed volcanoes of Hefle and Bohemia, or on 

 lime-ftone, as thofe of Silefia, Mount Vefuvius, &c. 

 No ore is found in thefe mountains, except that of 

 iron, of which lava contains from twenty to twenty- 

 five parts in the hundred, and fome detached frag- 

 ments of the ores of copper, antimony, and arfenic. 

 Vefuvius ejected, from the year 1779 to 1783, 

 309,658,161 cubic feet of matter of different kinds; 

 we muft therefore, conclude the feat of thefe fires to 

 be feveral miles, perhaps hundreds of miles, below 

 the level of the fea ; and as iron makes from one- 

 fourth to one-fifth of thefe ejections, we may infer that 



* -Dr. Hooke's podhumous Works, p. 304. 



VOL. II. Y the 



