LAURENTIAN SYSTEM (ante). It may be more precisely stated that the Lauren tian system of rocks is the lower period of the eozoic age, or as it is termed by prof. J. D. Dana, the archccan (q.v.), that is, the age of the first appearance of organic life. This arclman age is divided into two periods, the Laurentian beneath and the Huronian above, sometimes called upper and lower Laurentian. The Huronian lies immediately beneath the Cambrian system or age of Sedgwick. The Laurentian rocks proper, or lower Laurentian, are composed chiefly of primitive gneiss, and have been subjected to more change (metamorphism) than any other rocks, unless it be a fact that granite, now called primary, is really derived from stratified rock by igneous action. The Huronian or upper Laurentian rocks are, to a considerable extent, crystalline schists, much less compact than the (lower) Laurentian gneiss, whose strata are also much more convoluted than those of the Huronian. Both together form a series of rocks which in Canada have
a thickness of 40,000 ft., passing into gneiss and granite downwards, and upwards into hornblende gneiss, syenites, diorites, and limestones, the latter being metamorphosed into marbles.
The immense beds of iron ore in the Laurentian rocks are considered as evidence of abundant organic life during that age (see METAMORPHIC ROCKS, ante), as also are the great beds of graphite (q.v.)? In addition to these evidences, there have been found in the Laurentiau limestones of Canada, Bohemia, and other countries, large, irregular, cellular masses, which are regarded as the remains of gigantic rhizopods, and the sup posed species has been called eozoon Canadense (q.v.).