Rocks representing at least four periods of basaltic intrusion are present in the Beartooth Mountains: (1) pregranitization and early granitization Archean ortho-amphibolites (and meta-serpentinites), derived from pre-existing mafic intrusive masses, (2) late granitization Archean metadolerites, (3) late Precambrian quartz dolerites, and (4) Tertiary(?) olivine dolerites. The Archean metadolerites show a differentiation sequence similar to that of layered mafic plutons. Ultramafic dikes, which include metaharzburgites and metabronzitites, resemble crystal cumulates and are considered crystal-mush intrusions with 60 per cent or more solid phases. Leopard-rock dikes are also crystal-mush intrusions with 50–60 per cent plagioclase phenocrysts. They are similar in composition to anorthositic gabbro or norite. Nonporphyritic borders of such dikes probably resulted from flowage differentiation rather than composite intrusion. Metanorite dikes are most common, but metadolerites, iron-rich metadolerites, and metamorphosed mafic pegmatite are also present. The Desolation Lake dike contains mafic and ultramafic rock types in complex relationships; the dike was largely crystalline when intruded. Intrusion-breccia dikes were emplaced in zones of shattered country rocks; noritic magma intruded these zones and engulfed and assimilated the fragmented country rocks. The metadolerites were metamorphosed under water-deficient conditions, probably at temperatures of 500°–600°C in the highest subfacies of the amphibolite facies. Late Precambrian dolerites are undifferentiated and commonly contain three coexisting pyroxenes: augite, magnesian pigeonite, and bronzite. This indicates incomplete equilibrium. Tertiary(?) dolerites are alkali basaltic.