OLDEST GNEISS COMPLEX--Chiefly layered granitic gneiss, locally migmatitic.  Local masses
     of quartzite, metagraywacke, iron-formation, and other metasedimentary rocks and am-
     phibolite and felsic gneiss thought to be volcanic; metasedimentary rocks in Beartooth
     Mountains contain detrital zircon dated at more than 3,400 Ma.  Inclusions show evidence
     of granulite-facies metamorphism prior to 2,800 Ma.  Mueller and others (1982) suggest
     that large areas in Beartooth Mountains were invaded by Late Archean granite (age about
     2,800 Ma).
  Bighorn Mountains--Dates of metamorphism 3,000+ Ma.  Ugn
  Wind River Range--Includes large bodies of metagabbro.  Overprint pattern indicates area of
     migmatite related to emplacement of 2,600-Ma granite.  Ugn