Archæan System forms, in Geology (q.v.), the basement division of the stratified series of rocks. The rocks included under archæan system consist principally of crystalline schistose rocks, and attain a thickness of many thousand feet. There is invariably an unconformity between them and the strata which happen to rest upon them. It is quite possible that some of the regions of crystalline schists which are described as belonging to the archæan system may really pertain to a later date—because all that can be asserted of such crystalline schists is that they are older than the strata which immediately overlie them. Sometimes these overlying strata are of older, sometimes of younger Palæozoic, or even of Mesozoic and Cainozoic age. Two groups are recognised in the archæan system—the lower or 'old gneiss formation,' consisting principally of coarsely crystalline gneiss, along with hornblende-schist, quartzite, and crystalline limestone. The upper group, or 'crystalline schist formation,' comprises chiefly mica-schists, chlorite-schists, talc-schists, phyllites, and occasional conglomerates. The archæan system is well developed in many different parts of the globe. It covers considerable areas in the Outer Hebrides and the western parts of Ross and Sutherland, and much more extensive regions in Scandinavia, Finland, and the NW. of Russia. Archæan rocks also form the core or central portion of many mountain-ranges, as the Urals, the Carpathians, the Alps, the Pyrenees, &c. They appear also in many of the hilly tracts of Middle Europe, as in Saxony, Bohemia, and Bavaria. In North America they extend from the region of the great lakes north to the arctic circle, and they also form the nuclei of many mountain-ranges in the same country. They have been recognised in the coast-ranges of Brazil, in Venezuela, and the Andes; and they likewise occur in India and New Zealand. In Central Europe the lower group passes up conformably into the upper group. But in Canada there is a marked unconformity between the two series—the lower (termed Laurentian) attaining a thickness of 30,000 feet, and the upper (Huronian), a thickness of 10,000 to 20,000 feet. In one of the crystalline limestones of the Laurentian group, occur the remains of what is believed by Sir W. Dawson and others to have been a massive reef-building Foraminifer (Eozoön, q.v.). Other observers deny the organic origin of Eozoön, and are of opinion that it is entirely a mineral structure (ophicalcite). By the latter, the archæan system is sometimes spoken of as the Azoic (Gr., 'without life') system or formation. Those who think that Eozoön is really a fossil, occasionally call the great series of rocks in which it occurs the Eozoic formation.
The archæan rocks are often penetrated by mineral veins from which large supplies of various metals have been obtained. Great masses of iron-ore form a marked feature of the archæan system in Canada.