p50 , ON THE FORMATION OF ROCKS. [Augusts 



nion of the inliabitants^ but really for the alum rock, 

 similar to the alum rock of Tolfa, at Sulfaterra, near 

 Naples, and in all the volcanic islands of the West 

 Indies, formed by the lava, bleached by the sulphuric 

 acid. 



The seat of Volcanic lire is not known ; how deep 

 it may originate below the primitive is exceedingly un- 

 certain, or whether its beginning, and progress, is li^ 

 mited to the primitive rocks, and those above them. 

 Experience teaches us tliat volcanoes are often in the 

 primitive, or at no gi'eat distance from it ; and that the 

 greatest part of the substances ejected in a roasted 

 state, and without marks of fusion, are similar to pri- 

 mitive rocks. It is not probable that any new substan- 

 ces have been ejected by volcanoes which Avere not 

 previously found in some of the otlier classes of rocks, 

 particularly in the primitive. Sulphur is the combus- 

 tible substance generally found in and near volcanoes. 

 It is probable that neither sulphur, nor any otlier com- 

 bustible substance, has been jct found in the lowest 

 .;;ranite ; from which it would appear, that the lire of 

 Volcanoes commences either above or below the mass 

 «f granite. 



It is probable that t\^ o thirds of the volcanoes that 

 i?veknow of are upon islands, many of which have been 

 ilirown up from the bottom of the ocean, and consist 

 entirely of volcanic rocks : from wliicli it is probable, 

 ihat the vicinity of the sea is favourable to the coni- 

 nieiicement of volcanic combustion. 



