The Perth Road pluton is a lenticular, grossly conformable body emplaced in metasedimentary rocks of the Grenville series during a period in which the enclosing gneisses deformed by plastic flow. The body is zoned and from the center outward comprises gabbro, diorite, monzonite (the dominant rock-type), quartz monzonite, and granite. The change from one rock-type to the next is gradual and the progressive acidification toward all the contacts, without regard to the type of enclosing gneiss, is taken as an indication of cogenesis of the plutonic assemblage; modal mineral variations of quartz, alkali-feldspar, plagioclase, and color index demonstrate this mineral progression.The enclosing crystalline metasedimentary rocks have been metamorphosed to the hornblende granulite facies, and although recrystallization has kept pace with deformation some cataclasis pervades all rocks. The pluton occupies a flattened crestal position of a doubly plunging fold, and all folds show a combination of fold-styles depending on the relative competence of the various rock-types. Flow models demonstrate that the early magmatic precipitates collected in regions of maximum velocity of the magma during folding of the enclosing envelope of gneisses and that syntectonic differentiation in this manner obtains for the Perth Road and other plutons in the area.