177 



These vapors rise in abundance ; and, what is 

 remarkable enough, through crevices which 

 seem to have no communication with the aper- 

 tures that emit aqueous vapors. We may be 

 convinced of the presence of the sulphurous acid, 

 by examining the fine crystals of sulphur, which 

 are every where found in the crevices of the lava. 

 This acid, combined with the water with which 

 the soil is impregnated, is transformed into sul- 

 phuric acid by contact with the oxygen of the 

 atmosphere. In general, the humidity in the 

 crater of the Peak is more to be feared than the 

 heat ; and they who seat themselves for a while 

 on the ground find their clothes corroded. The 

 porphyritic lavas are affected by the action of 

 the sulphuric acid : the alumin, magnesia, soda, 

 and metallic oxids, gradually disappear ; and 

 often nothing remains but the silex, which unites 

 in mammillary plates, like opal. These siliceous 

 concretions *, which Mr. Cordier first made 

 known, are similar to those found in the Isle of 

 Ischia, in the extinguished volcanoes of Santa 

 Fiora, and in the Solfatara of Puzzuoli -f% It is 

 not easy to form an idea of the origin of these 



* Opalartiger kieselsinster. The siliceous gurh of the vol- 

 canoes of the Isle of France contains, according to Klaproth, 

 0*72 silex, and 0 21 water $ and thus comes near to opal, 

 which Karsten considers as a hydrated silex. Miner Tabelkn, 

 1800, p. 70. 



+ Breislak, Introduzzione alia Geologia, t. ii, p. 236. 

 VOL. I. N 



