It is the purpose of this paper to present briefly the results of a study of the genetic relations of the migmatite structures- of the area named Praia do Tombo, ilha Santo Amaro, State of São Paulo, Brazil. The migmatites in the area in question present the following petrographic features: the old rock, a biotite-hornblende gneiss, was intruded by granitic material whose main constituent is microclne. The illustrations give an idea of forming process. Pegmatite dikes cutting through the migmatite rock has provoked the formation of a characteristic greisening zone by a process of metasomatic alteration