PRE-CAMBRIAN FORMATIONS. A term usually applied to all formations which are older than those containing the Olenellus fauna of the Cambrian period. The reason for applying such a general name to this series of rocks is that the Pre-Cambrian strata are often so highly meta morphosed. and devoid of organic remains, that their stratigraphic relations are indistinct, and it is therefore impossible to assign them an exact position in the geologic scale.
The Pre-Cambrian rocks consist in general of: (1) A great series of more or less highly meta morphosed igneous and sedimentary strata, such as gneisses, schi,ts, slates, quartzites, crystalline limestones, etc.; (21 igneous rocks but slightly altered; (3) recognizable sedimentary rock,, which in rare cases contain fossils. In addition to the intense metamorphism which sonic of the rocks have undergone. they are often interfolded and much broken by faults. The mere fact that rocks are highly metamorphosed. however, does not determine them to be of Pre-Cambrian age. for such changes have sometimes taken place in rocks of much later date, as in the Devonian and Carboniferous.
The length of time occupied by the Pre-Cam brian periods must always be largely a matter of speculation, but geologically it extended from the time of formation of a solid crust up to the beginning of the Cambrian. .Judging from the variety of forms that are found developed in as early a period as the Cambrian. and the length of time that has been required to develop the present fauna and flora found on the earth. the length of Pre-Cambrian time must have undoubt edly been very great. The highly altered char acter of the Pre-Cambrian rocks has unfortu nately obliterated many fossil remains that were buried in the sedimentary strata. and it is only in the younger members of the series that dis tinct fossils have been found. Such traces have been discovered at several loeali.ties. notably in Newfoundland, Canada, the Lake Superior dis trict. the Grand Cation of the Colorado, and in Great Britain. The Pre-Cambrian rocks are at the present time divided into a lower system— the Archaean and an upper system termed the Algonkian. The former is sometime- also termed the basement or fundamental complex. and con sists of igneous and highly metamorphosed rocks. so altered that their original condition is a matter of great uncertainty. Under the A]gonkian are included the youneer Pre-Cambrian rocks. which can usually be identified as sedimentary, a] though igneous intrusions also took place in this period. An earlier grouped all the Pre-Cambrian rocks as Archaean, subdividing them into a lower series called the Laurentian, and an upper series. the Iluronian, which corre sponds to the present Algoukian.
The Pre-Cambrian rocks are widely distributed and form a -cries of disconnected areas. sonic of them of considerable extent. Their exposure at the surface may be due to their having remained uncovered since early geologic times. or because the overlying sediments and other rocks of later date have been worn away. The more important
Pre-Cambrian areas found in North America are the following: (1) Laurentian area of Canada, consisting of a basement complex or Arebman system known as the Ottawa gneiss. which grades up into the Grenville series of Algonkian age.
(2) The Hastings district southwest. of Ottawa.
(3) The original Huronian area bordering the north channel of Lake Huron and extending west to Lake Superior. Both the Archaean and Al goukian are recognizable here, the latter being subdivided into an upper and lower Huronian.
(4) Adirondack area of eastern New York. (5) The Lake Superior region. Probably a greater amount of work has been done in this area than any other of Pre-Cambrian age and the strati graphic details have been most carefully worked out. (6) Eastern United States: a belt extend ing from Maine through New Hampshire, Massa chusetts. Connecticut. New York, New Jersey, Pennsylvania. INlaryland, Georgia, North Caro lina to Alabama, and consisting of a complex series of semi-crystalline and holocrystalline rocks. (7) Black Hills area. (8) Southeast Missouri area. (9) Central Texas. Scat tered areas in the Co•dilleran region. Pre-Cam brian rocks are also known in Great Britain, Scandinavia. Central Europe, China, India, and Australasia.
The Pre-Cambrian rocks are rich in mineral deposits. Almost inexhaustible supplies of iron ore are found in the Lake Superior region. and important deposits of the same metal are known in New York and New Jersey. Many valuable quarries of building stone are located within the Pre-Cambrian areas. while the other economic minerals include graphite. garnet. apatite, tale, emery. feldspar. gold, copper, and nickel.
BIBLIOGRAPHY. Van Hise. "Principles of PreBibliography. Van Hise. "Principles of Pre- Cambrian North American Geoloc•," Sixteenth Annual Report United States Geological Surrey (Washington, 1896) ; Geikie. Text-Book of Ge ology (London. 1893) ; Van Disc, "The Iron Ore Deposits of the Lake Superior Region," Twenty first Annual Report United States Geological Surrey (Washington. 1901) ; Adams. "On the Typical Laurentian Area of Canada," Journal of Geology (Chicago. 1893) : Adams. "Origin and Relations of the Grenville and Hastings series in the Canadian Laurentian." American Journal of Science, 4th series, vol. iii. (New Haven, 1897) ; Matthew, "The Effusive and Dike Roeks Near Saint John. New Brunswick." New York Academy of Sciences Transactions, vol. xiv. (New York, 1895) ; Kemp. "Pre-Cambrian Sediments in the Adirondacks." Science, vol. xii. Oh.. 1900) ; Kemp. "Crystalline Limestones. Ophicalcites. and Associated Schists of the Eastern Adirondacks," Geological Society of America Bulletin. vol. vi. (Rochester. 1895) : Van Hise. "Correlation Papers. Arehman and Algonkian." United States Geological Surreal. Bulletin No. 86 (Washington, 1892) : Leith, "Reviews of Pre-Cambrian Litera ture," Journal of Geology. from vol. i. to date (Chicago). See ARCILEAN SYSTEM; ALGONKIAN SYSTEM; LAURENTIAN SYSTEM; etc.