[ PART II.]
HISTORICAL PALÆONTOLOGY.
CHAPTER VII.
THE LAURENTIAN AND HURONIAN PERIODS.
The Laurentian Rocks constitute the base of the entire stratified series, and are, therefore, the oldest sediments of which we have as yet any knowledge. They are more largely and more typically developed in North America, and especially in Canada, than in any known part of the world, and they derive their title from the range of hills which the old French geographers named the "Laurentides." These hills are composed of Laurentian Rocks, and form the watershed between the valley of the St Lawrence river on the one hand, and the great plains which stretch northwards to Hudson Bay on the other hand. The main area of these ancient deposits forms a great belt of rugged and undulating country, which extends from Labrador westwards to Lake Superior, and then bends northwards towards the Arctic Sea. Throughout this extensive area the Laurentian Rocks for the most part present themselves in the form of low, rounded, ice-worn hills, which, if generally wanting in actual sublimity, have a certain geological grandeur from the fact that they "have endured the battles and the storms of time longer than any other mountains" (Dawson). In some places, however, the Laurentian Rocks produce scenery of the most magnificent character, as in the great gorge cut through them by the river Saguenay, where they rise at times into vertical precipices 1500 feet in height. In the famous group of the Adirondack mountains, also, in the state of New York, they form elevations no less than 6000 feet above the level of the sea. As a general rule, the character of the Laurentian region is that of a rugged, rocky, rolling country, often densely timbered, but rarely well fitted for agriculture, and chiefly attractive to the hunter and the miner.
As regards its mineral characters, the Laurentian series is composed throughout of metamorphic and highly crystalline rocks, which are in a high degree crumpled, folded, and faulted. By the late Sir William Logan the entire series was divided into two great groups, the Lower Laurentian and the Upper Laurentian, of which the latter rests unconformably upon the truncated edges of the former, and is in turn unconformably overlaid by strata of Huronian and Cambrian age (fig. 20).
The Lower Laurentian series attains the enormous thickness
Fig. 20.—Diagrammatic section of the Laurentian Rocks in Lower Canada. a Lower Laurentian; b Upper Laurentian, resting unconformably upon the lower series; c Cambrian strata (Potsdam Sandstone), resting unconformably on the Upper Laurentian. of over 20,000 feet, and is composed mainly of great beds of gneiss, altered sandstones (quartzites), mica-schist, hornblende-schist, magnetic iron-ore, and hæmatite, together with masses of limestone. The limestones are especially interesting, and have an extraordinary development—three principal beds being known, of which one is not less than 1500 feet thick; the collective thickness of the whole being about 3500 feet.