xenolith: (geology) a piece of rock of different origin from the igneous rock in which it is embedded
rock: a lump or mass of hard consolidated mineral matter
erythromycin: an antibiotic (trade name Erythrocin or E-Mycin or Ethril or Ilosone or Pediamycin) obtained from the actinomycete Streptomyces erythreus; effective against many Gram-positive bacteria and some Gram-negative