The lake superior region.  In eastern Canada an area of pre-Cambrian rocks, Archean and Algonkian, estimated at two million square miles, stretches from the Great Lakes and the St. Lawrence River northward to the confines of the continent, inclosing Hudson Bay in the arms of a gigantic U. This immense area, which we have already studied as the Laurentian peneplain, extends southward across the Canadian border into northern Minnesota, Wisconsin, and Michigan.  The rocks of this area are known to be pre-Cambrian; for the Cambrian strata, wherever found, lie unconformably upon them.

The general relations of the formations of that portion of the area which lies about Lake Superior are shown in Figure 262.  Great unconformities, UU’ separate the Algonkian both from the Archean and from the Cambrian, and divide it into three distinct systems, —­the lower Huronian, the upper Huronian, and the Keweenawan.  The Lower and the Upper Huronian consist in the main of old sea muds and sands and limy oozes now changed to gneisses, schists, marbles, quartzites, slates, and other metamorphic rocks.  The Keweenawan is composed of immense piles of lava, such as those of Iceland, overlain by bedded sandstones.  What remains of these rock systems after the denudation of all later geologic ages is enormous.  The Lower Huronian is more than a mile thick, the Upper Huronian more than two miles thick, while the Keweenawan exceeds nine miles in thickness.  The vast length of Algonkian time is shown by the thickness of its marine deposits and by the cycles of erosion which it includes.  In Figure 262 the student may read an outline of the history of the Lake Superior region, the deformations which it suffered, their relative severity, the times when they occurred, and the erosion cycles marked by the successive unconformities.

Other pre-Cambrian areas in north America.  Pre-Cambrian rocks are exposed in various parts of the continent, usually by the erosion of mountain ranges in which their strata were infolded.  Large areas occur in the maritime provinces of Canada.  The core of the Green Mountains of Vermont is pre-Cambrian, and rocks of these systems occur in scattered patches in western Massachusetts.  Here belong also the oldest rocks of the Highlands of the Hudson and of New Jersey.  The Adirondack region, an outlier of the Laurentian region, exposes pre-Cambrian rocks, which have been metamorphosed and tilted by the intrusion of a great boss of igneous rock out of which the central peaks are carved.  The core of the Blue Ridge and probably much of the Piedmont Belt are of this age.  In the Black Hills the irruption of an immense mass of granite has caused or accompanied the upheaval of pre-Cambrian strata and metamorphosed them by heat and pressure into gneisses, schists, quartzites, and slates.  In most of these mountainous regions the lowest strata are profoundly changed by metamorphism, and they can be assigned to the pre-Cambrian only where they are clearly overlain unconformably by formations proved to be Cambrian by their fossils.  In the Belt Mountains of Montana, however, the Cambrian is underlain by Algonkian sediments twelve thousand feet thick, and but little altered.