This is the name proposed by the United States Geological Survey for the great series of sedimentary and metamorphic rocks which lie between the basal Archaean complex and the oldest Palaeozoic strata; it is but little used outside of this country and is not universally employed even here, but it is beginning to make its way in Europe, and serves a useful, though possibly a temporary, purpose. While it is possible, though not very likely, that more advanced knowledge may lead us to distribute these rocks partly into the Archaean and partly into the Palaeozoic, yet for the present, at least, it is better to form a separate grand division for them.

The Algonkian rocks, which are widely distributed in North America, form an immensely thick mass of strata and of metamorphic rocks which are believed to represent those strata in other regions. These metamorphic rocks were long generally referred to as the Huronian, which was regarded as the upper portion of the Archaean, but, so far as can be learned, they occupy the same stratigraphical position as certain little changed sediments, between the fundamental complex below and the Cambrian above. At the base of the magnificent section exposed in the Grand Canon of the Colorado is a very thick mass of strata, separated by great unconformities from the Archaean gneiss below and from the overlying Cambrian. This mass is again subdivided by minor unconformities into three series. The lower series, at least 1000 feet thick, and perhaps more, is made up of stratified quartzites and semi-crystalline schists, cut by intrusive granite. Above this come nearly 7000 feet of sandstones, with included lava sheets, and at the top more than 5000 feet of shales and limestones, in which a few fossils have been found. The two upper series are not at all metamorphic.

All these strata are steeply inclined, and upon their truncated edges rests the sandstone referred by Mr. Walcott to the Middle Cambrian.

In central Montana is a very extensive exposure of Algonkian rocks, 12,000 feet thick, composed of immense bodies of sandstones, quartzites, limestones, and hard arenaceous shales. These beds, called the Belt series, are upturned in the Belt Mountains, and un-conformably overlaid by a Middle Cambrian sandstone. " In late Algonkian times an orographic movement raised the indurated sediments of the Belt terrane above sea-level, . . . [and] folding of the Belt rocks formed ridges of considerable elevation, and areal [aerial] erosion and the Cambrian sea cut away in places from 3000 to 4000 feet of the upper formations of the Belt terrane before the sands that now form the Middle Cambrian sandstones were deposited." (Walcott).

A very similar succession of rocks to that of the Grand Canon is found in the Lake Superior region, intervening between the Archaean complex and the Upper Cambrian, from both of which they are separated by great unconformities. As in the Grand Cation section, these rocks are divisible into four series by unconformities. The lower two series, with a maximum thickness probably exceeding 5000 feet, are much crumpled, metamorphosed, and semi-crystalline. They comprise limestones, quartzites, mica schists, etc., cut by igneous dykes, also much volcanic tuff and agglomerate. Next follows a series of 12,000 feet of less intensely folded but still metamorphic rocks, quartzites, shales, slates, mica schists, with dykes and interbedded sheets of diorite. A few fossils have been found in the quartzites of this series. The fourth series has a maximum thickness of 50,000 feet, though usually much less. The lower part of this series is formed by thick lava sheets, interbedded with sandstone and conglomerate, and above is a mass of sedimentary rocks largely derived from the volcanic materials.

This uppermost series is by some authorities referred to the Cambrian, but, in the absence of fossils, there seems to be no way of definitely deciding the question.

Over the great Archaean area of Canada occur many districts of metamorphic rocks which are plainly of sedimentary origin, such as crystalline limestones, schistose conglomerates, as well as volcanic tuffs and agglomerates. In the Archaean region of Canada and in New England the Algonkian metamorphics seem to grade into the Archaean complex without unconformity. This apparent conformity may, however, very well be due to subsequent dynamic metamorphism, which, as has been proved, may obliterate nearly all traces of a great unconformity. Through the Rocky Mountain region and the Pacific coast mountains, the Archaean is in very many places overlaid by great thicknesses of metamorphic Algonkian rocks, such as quartzites, sandstones, and schists, which are sometimes as much as 12,000 feet thick, as in the Wasatch and Uinta mountains. Other isolated areas are found, as in the Black Hills, where a great mass of schists, slates, and quartzites is separated by a very marked unconformity from the overlying Cambrian; also in Missouri and Texas. The Algonkian rocks of the West have not been subjected to such extreme folding as have most of those of the East, and hence their distinctness from the Archaean is more clearly marked.

In the southern Appalachians are some little changed strata which are referred to the Algonkian.

In other continents the distinction between the Archaean and the Algonkian is beginning to demand recognition. In Great Britain, for instance, are found very interesting parallels with the Algonkian of this country. In Scotland the Torridon sandstones, 8000 to 10,000 feet thick, which are nearly horizontal and almost unchanged, lie unconformably between the oldest Cambrian and the basal Archaean; and in other areas, metamorphic rocks of sedimentary origin occupy a similar position. In Finland and Sweden 10,000 feet of sedimentary and igneous rocks and schists occur between the Archaean and the Cambrian. Many of the crystalline schists of the European pre-Cambrian areas appear to correspond in character and position to the metamorphic Algonkian.

Lately the surprising announcement has been made of extensive glaciation in the early Algonkian of North America The conglomerate at the base of the Lower Huronian of Canada is regarded as of glacial origin, "since it contains angular and sub-angular boulders of all sizes up to cubic yards, enclosed in an unstratified matrix. These boulders are often miles from any possible source. Recently, striated stones have been broken out of their matrix in the Lower Huronian of the Cobalt Silver region, giving still stronger proofs that the formation is ancient boulder clay." (Coleman).

In South Africa we find clear evidence of three distinct ice ages, before the close of the Palaeozoic Era, and the most ancient of these, which cannot be definitely dated, may correspond in time to that above-mentioned in Canada.