The Collaborative International Dictionary of English v.0.48:

Lithology \Li*thol"o*gy\, n. [Litho- + -logy: cf. F.
   lithologie.]
   1. The science which treats of rocks, as regards their
      mineral constitution and classification, and their mode of
      occurrence in nature.
      [1913 Webster]

   2. (Med.) A treatise on stones found in the body.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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