The Saganaga Granite of Minnesota-Ontario
The oldest batholith along the Minnesota-Ontario boundary is that at Saganaga Lake. It is the only one older than the Ogishke sediments and is therefore assigned to the Laurentian. Since a still older granite gneiss has been found farther east, the Saganaga granite arose late in Laurentian time. It is a roughly oval mass about 15 X 25 miles across and has nearly vertical walls. The main rock, 85-90 per cent of the mass, has long been known and is very uniform sodic hornblende granite. The local phases, described in some detail, include syenite, shonkinite, and hornblendite. For such an old formation the rocks are very little altered. Measurements and chemical analyses give data for tables and diagrams of the differentiated series. The ratio of potash to soda increases with increasing silica toward the central part of the mass. This series resembles that in the Giants Range which is of Algomian age; also some others of later eras.