A study has been made of the petrology and geochemistry of a 30-mile (48.3 km) segment of a belt of ultramafic-gabbroic igneous bodies extending past the south side of Lake Abitibi, Ontario. In general, the bodies are sill-like and are differentiated into major layers of peridotite, clinopyroxenite, and gabbro, the layers generally being in that stratigraphic order. In detail, the intrusions fall into four groups: (1) complex sills in which there is a cyclic repetition of layers; (2) simple differentiated sills showing only one sequence of the above rock layers; (3) bodies composed only of peridotite and dunite; and (4) bodies composed wholly of gabbroic rocks.One of the group (2) bodies has a chilled margin equivalent in composition to a tholeiitic basalt. The general structure of the intrusions and their petrographic and chemical features indicate that they are differentiated from basaltic magma by gravity-controlled fractionation. However, it appears that while solidifying, some of the intrusions were open to periodic addition or subtraction of magma. Thus, in the intrusions showing cyclic repetition of layers, it is apparent that the magma was altered prior to the formation of each cyclic unit such that the original order of mineral crystallization was repeated. For other intrusions, it can be inferred that large amounts of partly crystallized liquid were expelled such that each of these intrusions is now largely or wholly represented by ultramafic rocks. The bodies composed wholly of gabbro may be derived from the expelled magma.