Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. A treatise on petrology.

Source : WordNet®

petrology
     n : the branch of geology that studies rocks: their origin and
         formation and composition