238 DOMAIN XI. DECOMPOSED. 'l 



NOME II. D. PORPHYRY. 



In the same work, Karsten has given the fol- 

 lowing examples : 



" 208. A piece of porphyry in which the fel- 

 spar is indeed entirely, but the basis only slight- 

 ly, decomposed ; from Norway. 



" 209. Porphyry in which the felspar is partly 

 actually decomposed, but partly appears barely 

 without lustre, the basis is become perfectly fri- 

 able ; from the vicinity of Regensburg. 



" Rem. It is very frequently passed for 



tarras." 



The remarkable stone which composes the 

 Puy de Dome, where Pascal made his celebrated 

 observations on the barometer, is a porphyry, 

 which seems to be decomposed by volcanic heat. 

 According to the experiments of Saussure, the 

 base is an earthy felspar, or felsite. 



But the most celebrated decomposed porphyry 

 is the saxum metalliferum of Baron de Born, 

 which serves as a gangart to many rich mines of 

 gold and silver in Hungary ; and even to the 

 noble opal, only found in that country. It is 

 surprising that so many mistakes should have 



