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ARCHEAN SYSTEM (from apXil, beginning)
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ARCHEAN See also:SYSTEM (from apXil, beginning) , in See also:geology. Below the lowest distinctly fossiliferous strata, that is, below those See also:Cambrian rocks which See also:bear the Olenellus See also:fauna, there lies a See also:great See also:mass of stratified, metamorphic and igneous See also:rock, to which the non-committal epithet " pre-Cambrian " is often applied; and indeed in not a few instances this See also:general See also:term is sufficiently precise for the See also:present See also:state of our knowledge. EtncryWaLLrt MC. Nevertheless there are large tracts, both in the Old See also:World and in the New, in which a subdivision of this assemblage of See also:ancient rocks is not only possible but desirable. It is quite clear in certain regions that there is a lowermost See also:group with a prevailing granitoid, gneissic and schistose facies, mainly of igneous origin, above which there are one or several See also:groups bearing a distinctly sedimentary aspect. It is to this lowermost gneissic group that the term " Archean " may be conveniently limited. Thus, while the name " pre-Cambrian " may be used to indicate all these very old rocks whenever there is still any difficulty in subdividing them further, it is an See also:advantage to have a See also:special appellation for the See also:oldest group where this can be distinguished. • It must be pointed out that the term "Archean " has. been used as a synonym for pre-Cambrian; and that the expressions Azoic (from a-, privative; -wil, See also:life), Eozoic (from it ,s, See also:dawn), and Fundamental Complex, have been employed in somewhat the same sense. Archeozoic has been proposed by See also:American writers to apply to the lowest pre-Cambrian rocks with the same significance as " Archean " in the restricted sense employed here; but it is perhaps safer to avoid any reference to the supposed See also:stage of life development where all See also:direct See also:evidence is non-existent. The so-called " Azoic " rocks have already been made to yield evidence of life, and there is no See also:reason to presuppose the impossibility of finding other records of still earlier organisms. The prevailing rocks of the Archean system are igneous, withmetamorphosed varieties of the same; sedimentary rocks, distinctly recognizable as such, are scarce, though highly metamorphosed rocks supposed to be sediments, in some regions, take an important See also:place. There are several features which are peculiarly characteristic of the Archean rocks:—(r) the extraordinary complexity of the assemblage of igneous materials; (2) the extreme See also:metamorphism and deformation which nearly all the rocks have suffered; and (3) the inextricable intermixture of igneous rocks with those for which a sedimentary origin is postulated. Wherever the Archean rocks have been closely examined two great groups of rocks are distinguishable, an older, schistose group and a younger, granitoid and gneissic group. For many years the latter was supposed to be the older, hence the epithets " See also:primitive " or " fundamental " were applied to it. Now, however, it has been shown, both in See also:Europe and in See also:North See also:America, that in certain regions a schistose See also:series is penetrated by a gneissose series and when this occurs the See also:schists must be the older. But bearing in mind the difficulties of See also:interpretation, it is not at all unreasonable to assume that there may yet be regions where the gneissose rocks are the oldest; for where no schistose series is present there may be no criterion for estimating the See also:age of the granites and gneisses. The exceedingly great difficulties which See also:lie in the way of every See also:attempt to unravel the See also:history of an Archean rock-complex cannot be too forcibly emphasized; for to be able to demonstrate the See also:order of events and See also:succession of rocks we should at least know whether we are dealing with sediments, flows of volcanic material, or intrusions, yet in many instances this cannot be done. In some areas the See also:gradual passage of highly foliated and metamorphosed schists may be traced into comparatively unaltered arkoses, greywackes, conglomerates; or into volcanic See also:lava-flows, pyro-clastic rocks or dikes; or again through a gneissose rock into a See also:granite or a See also:gabbro; but the districts wherein these relationships have been thoroughly worked out are very few. This much may be said, that where the Archean - ystem has been most carefully studied, there appears to be (I) a schistose series, of itself by no means See also:simple but containing the foliated equivalents of sedimentary and igneous rocks; into this series a gneissose group (2) has been intruded in the See also:form of batholites, great sheets and sills with accompanying intrusional prolongations into the schists; subsequently, into the gneisses and schists, after they had been further deformed, sheared and foliated, another set (3) of dikes or thin See also:sheet-like intrusions penetrated. All this, namely, the formation of sediments, the outpouring of volcanic rocks, their repeated deformation by powerful dynamic agencies and then their penetration by dikes and sheets had been completed and erosion had been at See also:work upon the hardened and exposed rocks, before the earliest pre-Cambrian sediment was deposited. There has been much premature See also:speculation as to the nature and origin of these very ancient rocks. The prevalence of See also:regular foliation with layers of different See also:mineral See also:composition, producing a See also:close resemblance to bedding, has led some to imagine that the gneisses and schists were themselves the product of the primeval oceans, a supposition that is no longer worthy of further discussion. Others have supposed that the gneisses were largely produced by the resorption and See also:fusion of older sediments in the molten interior of the See also:earth; there is no evidence that this has taken place upon an extended See also:scale, though there is reason to believe that something of this See also:kind has happened in places, and there is in the See also:hypothesis nothing radically untenable. In one way the sedimentary schists have undoubtedly been incorporated within the gneissose mass, namely, by the extremely thorough and intimate penetration of the former by the latter along planes of foliation; and when a complex mass such as this has been further sheared and metamorphosed, a See also:uniform See also:gneiss appears to result from the intermixture. A not uncommon cause of the' apparently bedded arrangement of layers of different mineralogical composition may be traced to the See also:original differentiation of the granitoid magma into different mineral-sheets. When these mineralogically
Efferent layers were forced into other rocks, sometimes before the See also:complete consolidation of the former and sometimes subsequent to it, in the generally metamorphosed See also:condition of the whole, it is easy to see a superficial resemblance to bedding.
The Archean rocks have frequently been spoken of as the original crust of the earth; but even granting a cooling molten globe with a first-formed stony See also:surface, it is tolerably clear that such a crust has nowhere yet been found, nor is it ever likely to be discovered. The very earliest recognizable sediments are the result of the destruction of still earlier exposures of rock; the oldest known volcanic rocks were poured upon a surface we can no longer distinguish, and as for the great granitoid masses, they could only have been formed under the pressure of superincumbent masses of material. The earliest known sediments must have been deep in the zones of shearing and rock flowage before the first pre-Cambrian denudation. The See also:time required for these changes is difficult to conceive.
As regards the life of the Archean, or, as some See also:call it, the " Archeozoic " See also:period, we know nothing. The presence of carbonaceous shale and graphitic schists as well as of the altered sedimentary See also:iron ores has been taken as indicative of See also:vegetable life. Similarly, the occurrence of limestones suggests the existence of organic activity, but direct evidence is wanting. Much See also:interest naturally attaches to this remote period, and when See also:Sir See also: Naturally, the See also:area of its outcrop is limited, for, directly or indirectly, all the younger rock groups must See also:rest upon it. It has been estimated that Archean rocks appear at the surface over one-fifth of the See also:land area (omitting coverings of superficial drifts). This estimate is no more than the roughest approximation, and is liable at any time to revision as our knowledge of little-known regions is increased. It must ever be See also:borne in mind that the presence of a gneissose or schistose complex does not in itself imply the Archean age of such a set of rocks. See also:Local manifestations of a similar petrological facies may and do appear which are of vastly inferior See also:geological age; and unless there is unequivocal evidence that such rocks lie beneath the oldest fossil-bearing strata, there can be no See also:absolute certainty as to their antiquity. It is more than likely that certain occurrences of gneiss and schist, at present regarded as, Archean, may prove on See also:fuller examination to be metamorphosed representatives of younger periods. See also:Britain.—The most important exposure of Archean rocks in Britain is in the north-See also:west of See also:Scotland, where they form the mainland in See also:Sutherland and See also:Ross-See also:shire, and appear also in the See also:outer See also:Hebrides. Their great development in the isle of See also:Lewis has given rise to the term " Lewisian " (Hebridean), by which the gneisses of this region are now generally known. The Lewisian series comprises two great groups of rocks, (r) the so-called " fundamental complex," an assemblage of See also:acid, basic and intermediate irruptive rocks, associated together in a complex of extraordinary intricacy, and (2) a series of dikes, which like the rocks they See also:traverse, show every gradation from ultra-basic to ultra-acid types. But the above bald statement conveys no See also:idea of the complexity of the series, for before the " fundamental complex " had been pierced by the later See also:dike system it had been subjected to severe See also:dynamo-metamorphism and many of the massive rocks had been folded, thrust and sheared, and a very general state of foliation had been produced. Nor was this all, for after the intrusion of the dikes, great movements brought about See also:vertical dislocations, and thrust planes, which traversed the rocks at all angles, accompanied by still further See also:internal shearing and superinduced foliation. In the valley of See also:Loch See also:Maree and thence See also:south-westward into See also:Glenelg, a series of See also:mica-schists, See also:quartz-schists, saccharoid limestones and graphitic schists has been regarded as a group of sedimentary origin through which the Lewisian rocks have been irrupted. In See also:England several small masses of gneiss, notably at See also:Primrose See also: Into these two formations a series of granites have been intruded. Europe.—In Scandinavia, as in Scotland, the pre-Cambrian is represented by an earlier and a later series of rocks of which the former (Grundfjeldet, Urberget) may be taken to be the See also:equivalent of the Lewisian gneisses. This assemblage of coarse red and See also:grey banded gneisses, with associated granulites and many varieties of acid, basic and intermediate rocks in a gneissose condition, is intimately related to a highly metamorphosed sedimentary series comprising limestones, quartzites and schists, which, as in Scotland, is apparently older than the gneisses. Similar rocks occur in. See also:Sweden and See also:Finland. In See also:Bavaria and Bohemia the Archean is divisible into a lower red gneiss, a comparatively simple series, called by C. W. von See also:Gumbel the " gneiss of Bojan "; and an upper, grey gneiss with other schistose rocks, serpentine and graphitic See also:limestone, termed by the same author the " Hercynian gneiss." In See also:Brittany a gneissose and schistose igneous series lies at the base of the pre-Cambrian. The pre-Cambrian cores of the eastern and central See also:Pyrenees, consisting of gneiss, schists and altered limestones, are presumably of Archean age. A sia, See also:Australia, &c.—In northern See also:China, mica-gneisses and granitegneisses with associated schists may be regarded as Archean. In See also:India the system is represented by the See also:Bundelkhand gneiss and the central older gneisses of the Himalayas. In See also:Japan, in the Abukuma See also:plateau, there is much granite, gneiss and schist which may be of this age. In Australia, similar rocks are recognized as Archean in South Australia and Westralia, and they are estimated to See also:cover an area of no less than 20,000 sq. m.; in See also:Tasmania they are well See also:developed on the western side. Although a great area is occupied by crystalline rocks in New See also:Zealand, the Archean age of any portion of the series is not yet satisfactorily established; the lower granites and gneisses may belong to this period. See also:Africa contains enormous tracts of crystalline gneisses, granites and schists, and some of these are almost certainly of Archean age; but in the present state of our knowledge it is impossible to speak more exactly. Additional information and CommentsThere are no comments yet for this article.
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