Precambrian schists and gneisses are exposed in much of the Ruby Mountains of southwestern Montana. The major rock types include abundant quartz-feldspar gneiss, hornblende gneiss, amphibolite, quartzitic gneiss, and marble. Minor rock types important in the study area include anthophyllite gneiss, hornblende-pyroxene-plagioclase gneiss, quartz-hypersthene-garnet granulite, iron-formation, and hornblende-hypersthene granulite. These rocks form a concordant series folded into a broad, open antiform-synform pair in the study area. This volume is a revised edition containing the complete original text plus the addition of a number of appendices. These include all maps, data tables, field notes and petrographic descriptions. I have included a new structural analysis treating the area as a single domain with all 342 strike and dip measurements, and a structure cross-section cartoon and a suggested pre-metamorphic sedimentary section. These appendices allow the reader to assess the evaluation from original descriptions.