Anorthosites, gabbros, and associated rocks in southeastern Ontario are described in detail. Field and laboratory evidence indicates that the anorthosites formed from a magma with a composition about that of gabbroic anorthosite. It is suggested that the more mafic masses are higher in the earth's crust than are the anorthosites. Rising volatile-rich fluids carrying iron, titanium, phosphorus, etc., and assimilation of limey sediments by the magma probably concentrated the dark minerals. The source of the magma may have been a layer in the earth's crust with a composition approaching that of bytownite anorthosite. Two granites are separated by the intrusion of the basic rocks.

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