In the vicinity of the western boundary of Renfrew County, eastern Ontario, in an area between Barry's Bay and Rosenthal, supracrustal metamorphic tectonites of the Grenville Supergroup structurally overlie a migmatite complex known as the Radcliffe Hybrid Gneiss. It is shown that the latter can be subdivided by qualitative structural analysis into two units of differing tectonic complexity. A polymigmatitic unit displaying the effects of at least five phases of deformation and containing at least the same number of leucosomal phases lies to the northwest of a less complex migmatitic unit displaying three phases of deformation and only two leucosomal phases. The former is identified as pre-Grenvillian basement and the latter as the basal portion of the Grenville Supergroup cover sequence which was transgressed by a rising migmatite front during the Grenvillian orogeny. Criteria for distinguishing basement and cover migmatites are discussed in the local context as are some problems involved in mapping tectonostratigraphic units in migmatite terrains.

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