Garnetiferous amphibolites are abundant throughout the Cambro-Ordovician Lardeau Series on the eastern side of Kootenay Lake, but are absent in the younger, Carboniferous-Triassic Milford and Slocan Series. Garnet-bearing schists are most plentiful in the Cambrian Plaid Lake Formation, and in formations younger than the Ordovician. The syntectonic Kootenay Intrusives are notably peraluminous, and carry garnet.

The amphibolites examined, both garnetiferous and non-garnetiferous, are remarkably similar in chemical composition. They compare well with the C.I.P.W. norm of normal tholeiitic basalts and dolerites, and could conceivably be related to the Kaslo Volcanics. The schists examined are more variable in composition, and are best regarded as metamorphosed semipelites.

New data are presented for twenty-five garnets: fifteen from amphibolites, eight from schists, and two from acid igneous material. The average molecular composition of garnets from amphibolites is found to be Alm_52.3Sp_4.5Pyr_11.9Gro_23.2And_8.0 while those from schists and acid igneous material average Alm_74.8Sp_3.5Pyr_11.0Gro_1.4And_9.3 and Alm_66.8 Sp_22.0 Pyr_2.5Gro_0.9And_4.8 respectively. All the garnets from the central Kootenay Lake amphibolites plot outside the "solubility field" limits for pyralspites proposed by Winchell (1951).

A linear relationship between the physical parameter ration a₀/N and the CaO content is demonstrated for the Kootenay Lake garnets irrespective of their host rock type. No systematic trends in garnet composition related to metamorphic grade are found for garnets either from schists or amphibolites. The CaO and MnO content of garnets from schists, and the MnO content of garnets from amphibolites, vary despite almost uniform whole rock CaO and MnO content. However, the CaO content of garnets from amphibolites varies sympathetically with whole rock CaO content.

The coexistence of garnetiferous and non-garnetiferous amphibolites within the central Kootenay Lake area is considered to be neither a function of T nor P differences, nor of substantial chemical compositional differences. The development of felsic-rich selvages or rims about garnets from amphibolites is interpreted as the result of retrograde metamorphic change.

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