88 DOMAIN I. SIDEROUS. 



if the base assume the granitic form, it must be 

 \ regarded as a granitic porphyry. 



The Swedish porphyry, already mentioned, 

 approaches nearer to a porphyrin ; specimens 

 of which are common in most porphyritic re- 

 gions, forming the passage from basaltin to por- 

 phyry. 



MODE VII. PORPHYRON. 



When the crystals of felspar exceed an inch 

 in length, and are distant from each other, cir- 

 cumstances which occur in the large scale of 

 nature, the rock may be termed Porphyron. 

 The utility of these divisions will be more fully 

 understood, as the science becomes more and 

 more studied. 



MODE VIII. PORPHYROID. 



This denomination includes such substances 

 as approach the porphyritic structure. In a 

 strict derivation of the term porphyry, as al- 

 ready explained, the black and green kinds 

 could only be termed porphyroids; but as this 

 severity would too much violate common usage, 



