MODE V. PORPHYRY. 77 



rectangular, but sometimes oval or otherwise 

 irregular. When they are scarcely visible to 

 the naked eye, the substance is here called por- 

 phyrin ; and when they exceed an inch in size 

 the term porphyron may be applied. 



Genuine porphyry abounds in many parts of 

 the world, and often forms entire mountains. 

 Like siderite it has been found to alternate with 

 gneiss, and it occurs in a columnar form. Among 

 the defects of orology, and even of the Wer- Werner's 

 nerian theory of formations, maybe chiefly par- 

 ticularised the classification of the porphyries, 

 vaguely so called, which are arranged under one 

 head, whether the base be keralite, felsite, pitch- 

 stone, or even serpentine, or indurated clay ; 

 while felspar, like mica, may be occasionally 

 found in most rocks, and these pretended por- 

 phyries ought all to be referred to their several 

 bases. The name has even been extended to 

 rocks with calcareous or other crystals : and as 

 strict definitions form the first foundation of every 

 science, no argument can more clearly evince 

 the necessity of new and abundant denomina- 

 tions of rocks, than this confusion of substances 

 of a nature wholly remote ; and so frequent and 

 important, that no geological work can be pro- 

 perly understood, except the author use much 

 circumlocution. For to extend the term por- 



