510 OVERLYING ROCKS. 



together. Occasionally also, the felspar is opake 

 and dull, as if in an incipient state of decompo- 

 sition ; sometimes, it is even in a powdery state ; 

 and, in a few instances, it would not be supposed 

 that it had ever existed ; the rock having a carious 

 appearance, and the original seats of the crystals 

 being imperfectly filled with an ochry powder. 



The colours of the bases are, in all these cases, 

 subject to the same variations as the simple rocks. 

 Hence arise numerous varieties of aspect, still 

 further multiplied by the colours of the imbedded 

 crystals, by their proportion to the base, by their 

 size, and by different modes of disposition which 

 it is unnecessary to specify. Of these latter, the 

 twinning or the crossing of the crystals is the 

 most remarkable. 



By the gradual exclusion of the crystals, these 

 porphyries pass into the simple rocks ; and as the 

 two frequently exist in one mass, a specimen is 

 not always a criterion of the nature of the i;ock. 

 Hence the term porphyry, when used in geolo- 

 gical description, must not always be taken too 

 strictly in its mineralogical sense. 



These rocks are abundant in nature, and occur 



