145 



are so imbedded, or where crystals of one, or 

 more, are imbedded in a granular, or a compound 

 base. 



The crystals are commonly minute, not ex- 

 ceeding the tenth of an inch in dimensions ; oc- 

 casionally, they attain to the size of an inch or 

 more. 



The porphyritic,structure varies in the num- 

 ber of the imbedded crystals ; and becomes 

 evanescent by the loss of their forms, or in con- 

 sequence of being obscured by the granular na- 

 ture of the base, or, lastly, by their entire disap- 

 pearance. 



Like the granular, it may be united to any of 

 the larger modifications of structure, or to the 

 amygdaloidal. 



