What kinds of rocks are included in the Precambrian?
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The rocks of this region, and of the early Precambrian as a whole, are generally granite, schist, or gneiss. The most notable formations are the Keewatin and Coutchiching of Minnesota and the adjoining part of Canada; the Grenville of Ontario, which, however, may be late Precambrian; and the widely distributed Laurentian. The Keewatin series of rocks is composed chiefly of metamorphosed lava, with some sediments; the Coutchiching series is chiefly of sedimentary gneisses and schists. The Grenville limestone, marble, gneiss, and quartzite are predominantly metamorphosed sediments; the Laurentian gneiss and granite are probably younger than the other series, having been forced up through the Grenville as igneous rock. After the appearance of the Laurentian, the Temiskaming, or Sudburian, sediments were deposited, and a second series of gneisses and granites, the Algoman, was formed.
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