76 -DOMAIN I. SIDEROUS. 



classical term porphyry to many substances, 

 which have only a very faint and distant resem- 

 blance. 



The term porphyry is therefore here restrict- 

 ed to its proper and peculiar sense of a base 

 sprinkled with crystals of felspar. The word in 

 the Greek implies a purple, or rather red stone ; 

 and in severe classical precision ought to be 

 confined to that colour, common among the 

 monuments of antiquity : but as denominations 

 derived from colour, the worst of all distinctions, 

 have been forced to be extended, the black, the 

 grey, the bluish, and even the green, having 

 the same base of trap or basaltin, must be in- 

 cluded. But the base being the sole ground of 

 the present classification, all the other kinds are 

 considered as Intrites, and reserved for separate 

 descriptions. 



Base. It was long imagined that the base or ground 



of porphyry consisted of jasper ; but this suppo- 

 sition has been finally rejected, and it has been 

 found to be trap, from its fusibility and other 

 chemical properties, and likewise from its exter- 

 nal attributes. Like basaltin, it presents crys- 

 tals of siderite, grains of quartz, and sometimes 

 glandules of chalcedony and of steatite, which 

 last perhaps forms the green matter in Swedish 

 porphyry. The crystals of felspar are generally 



