Laurentian Rocks

Laurentian Rocks, the lowest series of Archaean rocks in North America, named by Sir WIBiam Logan from their development along the shores of the St. Lawrence, where they cover an enormous area. They are at least 30,000 feet thick, but their base is unknown. Logan divided them into two series - the Bower, over 20,000 feet thick, of granitoid gneiss, quartzite, and crystalline schists, with iron-ore and limestone; and the Upper, said to rest unconformably on the lower, 10,000 feet thick, of gneiss with much Labradorite (q.v.), as well as schist, iron-ore and limestone. The succeeding Huronian (q.v.) is said to rest unconformably on the Upper Laurentian. In the Lower Laurentian occurs the structure known as Eozoon (q.v.), once thought to be of animal origin. Archaean rocks of a similar lithological character to those of Canada, such as the Hebridean gneiss of the Highlands, were formerly termed Laurentian.