The borehole is located on the west flank of the Precambrian core of the Wind River Mts. Rock types cored range from quartz-dioritic to quartz-monzonitic gneiss; the proportions of K-feldspar and plagioclase vary greatly, as do textures. Neither composition nor texture show any systematic relation to depth and retrograde features are found sporadically. Final mineral assemblages attained seem to be largely conditioned by local variation in H 2 O concentration. Modal composition diagrams are included.