Scattered outcrops of crystalline rock were examined within an area of approximately 3600 square miles in central Minnesota. From field data and petrographic evidence, the variety of igneous rock types can be referred to the following historical sequence: (1) intrusion of the McGrath gneiss and regional dynamic metamorphism (Early Algoman), (2) intrusion of a series of intermediate igneous rocks (Late Algoman), (3) intrusion of the Stearns magma series (Middle Keweenawan), and (4) intrusion of felsite and basalt dikes (Middle Keweenawan).

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