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Major rock units of the central and eastern Beartooth Mountains are granitic gneiss, amphibolite, and biotite schist. Migmatitic interlayering and gradational sequences are common. In the western Beartooth Mountains, biotite schist and quartzite are dominant units and granitic gneiss is relatively minor.

A summary of available data shows that amphibolite has a composition similar to tholeiitic basalt, and biotite schist is probably a metamorphosed pelitic rock. Granitic gneiss is similar in composition to magmatic granitic rocks.

Summary data are evaluated by use of four basically different models for the origin of granitic gneiss: sedimentary-volcanic depositional, magmatic, anatectic, and metasomatic models. Combinations of processes are likely in nature; therefore, combined models such as the magmatic-metasomatic are also considered. Depositional and anatectic models are inadequate to explain field relationships and composition of major rock units. The magmatic model is plausible only if magma injection can take place over hundreds of square miles and vertical distance of thousands of feet without significantly disturbing the homogeneity of fabric and conformable nature of contacts. Rounded zircons in granitic gneiss are evidence against the magmatic model. If gradational sequences are explained by reaction between magma and country rock, there is a problem of disposal of elements, mainly aluminum, that are shown not to be needed in the reaction. The metasomatic model explains many of the observed relationships, but little evidence is available on source of material and transport mechanism. Some experimental evidence suggests that metasomatism under conditions of upper amphibolite facies may produce granitic rocks similar in composition to magmatic or anatectic granites.

No single-process model can explain the observed relationships in the Beartooth Mountains. The author is most convinced by a theory of metasomatism as the dominant process, but the evidence can also be interpreted to favor other processes, particularly a combination of synkinematic magmatism and metasomatism.

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