The principal formations of the Lac McGillivray region are biotite and amphibole gneiss. They contain levels of leptynites, amphibolites, meta-gabbro, and pyroxene gneiss. This series can be considered of sedimentary origin and the agreement with the diagrams ACF and A′KF of the chemical analyses permits a comparison between the different formations of the gray-wackes and those of the marl. Some pyroxene amphibolites and the metagabbros show evidence of weak magnetic activity during sedimentation.The following parageneses characterize an amphibolite facies type of metamorphism: Quartz — Plagioclase — (Microcline) — Biotite — (Almandine) — (Muscovite), Quartz — Plagioclase — (Microcline) — Biotite — Hornblende — (Almandine) — (Muscovite), Quartz — Plagioclase — (Almandine) — Hornblende — (Diopside).Three principal phases of flexible deformations are in evidence: (a) an axial folding N 90° of which the traces are strongly obliterated, (b) an axial folding N 40°–N 60°, and (c) an axial folding N 130°–N 150° with very distinct traces, because it is the last one.The intersection of the last two fold-sets brought about the formation of domes or of structures of short elongation. Two kinds of fractures, N–S and E–W, are the last tectonic events of this region.