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Pre-Cambrian

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In geology, the enormously long and indistinctly defined period of time anterior to the Cambrian period. In the restricted sense in which it is now often employed it embraces a period or group of periods subsequent to the Archean (q.v.) and anterior to the Cambrian, although some writers still prefer to include the former. The superior limit of pre-Cambrian rocks is fixed by the Olenellus fauna at the base of the Cambrian (some geologists speak of certain pre-Olenellus beds as eo-Cambrian); the lower limit has not yet been generally established, though it is sufficiently clear in certain regions. The rocks of this period are much more obviously of sedimentary origin than those of the Archean; they include conglomerates, sandstones, greywackes, quartzites, slates, limestones and dolomites, which appear to have been formed under conditions similar to those which obtained in later epochs. Although the sediments prevail, they are often very highly metamorphosed and distorted by crustal movements; igneous rocks occur in great bulk in some regions. Fossils are usually extremely rare and very ill-preserved; but indications of protozoa, coelenterates, echinoderms, molluscoids, mollusca, worms and arthropods have been distinguished. The name pre-Cambrian is the equivalent of the "Algonkian" of the United States Geological Survey, and of the "Proterozoic" of other American authorities; the terms eozoic, archaeozoic, agnotozoic, cryptozoic, eparchaic and others have also been applied to the same period.

Three or more great stratigraphical breaks have been recognized within the system of pre-Cambrian rocks; but how far these breaks synchronize in widely separated regions where they are found is difficult to determine in the absence of good palaeontological evidence.

The most striking development of pre-Cambrian rocks in Great Britain is the Torridonian group of the north-west highlands of Scotland, which lies with strong unconformability between the Lewisian gneiss and the basal quartzite of the Cambrian. The Eastern or Dalradian schists of Scotland and their equivalents in Ireland and Anglesey may be, in part at least, of the same age. In Shropshire, in the neighbourhood of the Welsh border, is the remnant of an ancient ridge now forming the Longmynd and the smaller hills to the west, Caer Caradoc, the Wrekin, and the Cardington Hills. The latter are built mainly of much altered porphyries and tuffs which C. Callaway named the Uriconian series; this series is clearly of pre-Cambrian age. The great mass of grits, flags and slates forming the Longmynd cannot yet be definitely assigned to this period, though they may be provisionally retained here under Callaway's name, Londmyndian. Probably contemporaneous with the Uriconian are the volcanic series of Barnt Green, Licky Hill and Caldecote. The micaceous schists of Rushton (Salop) may be placed here. In the Charnwood Forest a group of crystalline rocks, named Charnian by W. W. Watts, rises up in the form of small hills amid the surrounding Trias; they are classed as follows in descending order: The Brand series, including the slates of Swithland and Groby, quartzite and conglomerate and purple and green beds; the Maplewell series, including the olive hornstones of Bradgate, the Woodhouse beds, the slate-agglomerate of Roecliffe, the Beacon Hill hornstones and a felspathic agglomerate; and the Blackbrook series of grits and hornstones. The ancient volcanic rocks of St Davids, Pembrokeshire, were formerly regarded by H. Hicks as of pre-Cambrian age, in which he recognized a lower, "Dimetian," a middle, "Arvonian," and an upper, "Pebidian," series. The preCambrian age of these rocks was for a long time disputed, but J. F. N. Green (Q. J. Geol. Soc., 1908, 64, p. 363) made it clear that there is an Upper Pebidian (Rhyolitic group), and a Lower Pebidian (Trachytic group), and that Hicks's "Dimetian," the St Davids granophyre, is a laccolitic mass intrusive in the Pebidian. Both the Pebidian volcanic rocks and the intruded granophyre are separated from the Cambrian by an unconformity.

In Finno-Scandinavia pre-Cambrian rocks are well developed. In the Scandinavian mountain ranges are the Seve and Sparagmite formations; the latter, a coarse-grained felspathic sandstone, is very similar to the Torridonian of Scotland; it occurs also in Enontekis in Finland. Next in descending order come the Jotnian sandstones (2000 metres), which retain ripple-marks; they are associated with conglomerates and slates and intrusive diabase and the Rapakiwi granite. The Jotnian group rests unconformably upon the Jatulian quartzites and schists, with slates, dolomite and carbonaceous beds (north of Lake Onega is a bed of anthracite 2 metres thick). Outflows of diabase and gabbro occur in this series, which is from 1600 to 2000 metres in thickness. Below the Jatulian is another group of schistose sediments, the Kalevian, more strongly folded than the former and separated from the groups above and below by unconformable junctions. These rocks are regarded by J. J. Sederholm as older than the Huronian of North America (possibly analogous to the Keewatin formation), and yet several groups of sediments in this region (Botnian schists, &c.) lie between the Kalevian series and the granitic (Archean) complex.

Pre-Cambrian rocks occupy large areas and attain an enormous thickness in North America; all types of sediment are represented in various stages of metamorphism, and with these are igneous rocks, often developed upon a vast scale. They have been subdivided into the following groups or formations: an upper Keweenawan and a lower Huronian group; the latter is subdivided into an upper Animikean (north-east Minnesota) or Penokean (north-west Wisconsin); a middle and a lower division. Each of these four groups is separated by marked unconformity from the rocks above and below. Huronian rocks are well developed in the following districts: the Marquette region of northern Michigan, comprising quartzites, slates and conglomerates, with important iron-bearing slates and schists and ferruginous cherts; in the Menominee district of Michigan and Wisconsin similar rocks occur; the Penokee-Gogebic district of Wisconsin and Michigan comprises quartzites, shales and limestones, with beds and dikes of diabase and olivine-gabbro; the same rocks occur in the Crystal Falls, north Michigan; the Mesabi and Vermilion districts, Minnesota, and north of Lake Michigan rock groups of this age take an important place. The valuable iron ores of Mesabi, Penokee-Gogeblc and Menominee belong mainly to the Animikean group; in the Penokee rocks of this age vast thicknesses of igneous rocks constitute the greater part of the formation. The Keweenawan rocks are said to attain the enormous thickness of 50,000 ft.; the higher beds are mainly sandy sediments and conglomerates; in the lower portions are great igneous masses, gabbros, diabase and porphyries; thus in the St Croix valley, northwest Wisconsin and Minnesota, no fewer than 65 lava flows and 5 conglomeratic beds have been counted, which together aggregate some 20,000 ft. in thickness. Some of these lava flows appear to have been due to fissure eruptions. The native copper deposits of this age in north Michigan are the most extensive known.

Pre-Cambrian rocks occupy large areas and reach great thicknesses in the eastern provinces of Canada; in Newfoundland 10,000 ft. of strata lie between the Archean and Cambrian (the Terranovian series of South Hunt; Avalon group of others); similar rocks occur also north of the Great Lakes and in the Hudson Bay region. They are found also in great force in the Colorado Canyon, in the Adirondack Mountains, and Black Hills of S. Dakota and elsewhere.

Turning to Europe, we find pre-Cambrian rocks in Brittany, the "phyllades de Saint LO," or Briovbrian of Chas. Barrois; and along the western border of France and south-west of the central massif. In the Fichtelgebirge, the Silesian mountains and east Thuringia similar rocks occur; the Przibramer Schiefer of Lipoid and rocks in J. Barrande's stage A are of this age. Probably the metamorphosed eruptive rocks on the southern border of the Hunsriick and Taunus are pre-Cambrian. Large tracts of metamorphic sedimentary rocks that may be classed here are found in Shantung and north China, and probably also in Brazil, India and Australia. In South Africa the gold-bearing Witwatersrand beds of the Transvaal and the overlying Ventersdorp and Potchefstroom systems; the Griqualand system and Cango and Ibeques systems of Cape Colony, all occur above Archean rocks and below those of Devonian age; they cannot as yet, therefore, be classed as pre-Cambrian and their age is still uncertain.

Little can be said of the climatic conditions of this remote period, the fossil evidence being so poor; but it is of interest to note that in certain regions, viz. in the Lake Huron region, in the Gaisa series of Varanger Fjord, Norway, and in the Yangtse district in China, conglomerate beds are found in which many of the boulders are scratched like those of the Dwyka beds of South Africa, and thus suggest the possibility of glacial conditions at some stages of the period.

For literature see Geological Literature added to the Geological Society's Library (annual). (J. A. H.)