ARCHAIAN SYSTEM

This geolog- — ical division includes rocks of greater antiquity than the oldest fo il-bearing strata, and has a world-wide distribu- — tion, goresdins E, all the others. Its other name is Pre-Cam brian, since it in der lies the Cam brian or Eozoic system, in which the first faint traces f once living organisms appear. In certain regions the rocks of this system resemble some of the meta rocks of the British Isles, but no precise sorrel a tions have yet been made. The inest development of Pre-Cam brian ‘ocks yet described occurs in the basin yf the St. Lawrence and in the adjacent arts of the U.S., and also in the Allefhany and Rocky Mts. Their thickne s estimated at 100,000 ft., and their ~omplexity is very great. 'They contain sroups of coarse and finely crystalline sediments and unaltered sediments, and volcanic rocks. Logan, their di overer, described two divisions: (1) Lauren tian, or Lower Division, comprismg the coarse gnei es and schists, sranitoid rocks, and metamorphosed i me stones; (2) Huronian, or Upper Division, of finer crystalline rocks, and uitered shales, grits, and limestones. Sertain unaltered volcanic and sedimentary rocks are now also regarded as Pre-Cam brian. Recent Amer. geologists 1ave modified this grouping. Enormous areas in China are Pre Cam brian, and similar rocks have been found in India, Australia, and Africa. In Britain they are largely hidden peneath later formations, but in the N.W. Highlands, the Malvern Hills, Sharn wood Forest, and other isolated wreas, they have become exposed (after prolonged denud ation) along the axes wr cores of ancient hill ranges. They also project as prominent crags above areas of younger deposits which once covered them. They have been found also in deep borings. From the fundamental position of these ‘ancient’ rocks and their properties arose the idea. now generally abandoned, that they represented the primitive crust of the earth. In some Pre-Cam brian rocks distinct traces of recognizable ge have been found, chiefly in America.