The Collaborative International Dictionary of English v.0.48:

Porphyritic \Por`phy*rit"ic\, a. [Cf. F. porphyritique.] (Min.)
   Relating to, or resembling, porphyry, that is, characterized
   by the presence of distinct crystals, as of feldspar, quartz,
   or augite, in a relatively fine-grained base, often aphanitic
   or cryptocrystalline.
   [1913 Webster]

WordNet (r) 3.0 (2006):

porphyritic
    adj 1: (of rocks) consisting of porphyry or containing large
           crystals in a fine groundmass of minerals