The Collaborative International Dictionary of English v.0.48:

Petrology \Pe*trol"o*gy\, n. [Petro + -logy.]
   1. The department of science which is concerned with the
      mineralogical and chemical composition of rocks, and with
      their classification: lithology.
      [1913 Webster]

   2. A treatise on petrology.
      [1913 Webster]

WordNet (r) 3.0 (2006):

petrology
    n 1: the branch of geology that studies rocks: their origin and
         formation and mineral composition and classification [syn:
         petrology, lithology]