144 



ter of a single example being determined by ex- 

 perience respecting the predominant examples in 

 that rock. 



As the crystals in the granular structure may 

 sometimes possess a greater dimension in one 

 direction than in another, a fibrous structure 

 more or less perfect may result ; but this modifi- 

 cation being rare, and less decided than the ordi- 

 nary kind, is, apparently, with more propriety 

 ranked under the head of texture. 



In the solitary case of graphic granite, the 

 granular structure becomes imperfectly prismatic; 

 but where nature has made no decided limits, it 

 is impossible to preserve an artificial classification, 

 like this, pure. 



The granular structure is also known by the 

 name of the crystalline ; a convenient term, had 

 it always been strictly limited. 



PORPHYRITIC. 



In the rigid porphyritic structure, crystals of 

 one mineral are imbedded in a continuous mass 

 of another. But the term is also extended to 

 cases in which crystals of two or more mineral* 



