How to use Archean in a sentence
In other words, if there was such a thing as an original crust, which may be looked upon as an open question, the Archean, as now defined, does not appear to represent it.
The meta-sedimentary rocks of the Archean include metamorphosed limestone, and schists which carry carbonaceous matter in the form of graphite.
The marble and graphite, as well as some other indirect evidence of life less susceptible of brief statement, have been thought by many geologists sufficient to warrant the inference that life existed before the close of the era when the Archean rocks were formed.
Most of tie Archean rocks fall into one or the other of two great series, a schistose series and a granitoid series, the latter being in large part intrusive in the former.
The likeness of the Archean of one part of the country to that of another is one of its striking features.Advertisement
The Archean appears at the surface in many parts of the United States, and in still larger areas north of the national boundary.
I.ocally the Archean contains iron ore, as in the Vermilion district of northern Minnesota, and at some points in Ontario.- The ore is mostly in the form of haematite.
Prolerozoic (Algonkian) Systems.The Proterozoic group of rocks (called also Algonkian) includes all formations younger than the Archean and older than the Palaeozoic rocks.
The term Archean was formerly proposed to include these rocks, as well as those now called Archean, btit the subdivision here recognized has come to be widely approved.
They appear at the surface adjacent to most of the outcrops of the Archean, and in some other places.Advertisement
The Proterozoic formations are unconformable on the Archean in most places where their relations are known.
The first of these differences between the two systems is significant of the dynamic changes suffered by the Archean before the beginning of that part of the Proterozoic era represented by known formations.
The system is also exposed in many of the western mountains or about their borders, especially about those the cores of which are of Archean or Proterozoic rock.
The mountain structures originated in three great orogenic periods, the earliest in the Archean, the second at the end of the Palaeozoic and the third at the end of the Mesozoic. The Archean mountain chains, which enclosed the present region of Hudson Bay, were so ancient that they had already been worn down almost to a plain before the early Palaeozoic sediments were laid down.
The great central area of Canada is drained towards Hudson Bay, but its two largest rivers have separate watersheds, the Mackenzie flowing north-west to the Arctic Ocean and the St Lawrence north-east towards the Atlantic, the one to the south-west and the other to the south-east of the Archean protaxis.Advertisement
The uneven carving down of the older mountain systems, especially that of the Archean protaxis, and the disorderly scattering of glacial material provide most of the lake basins so characteristic of Canada.
From the map it will be noticed that the largest and most thickly strewn lakes occur within five hundred or a thousand miles of Hudson Bay, and belong to the Archean protaxis or project beyond its edges into the Palaeozoic sedimentary rocks which lean against it.
Within the Archean protaxis they are of the most varied shapes, since they represent merely portions of the irregular surface inundated by some morainic dam at the lowest point.
The Archean protaxis is sometimes spoken of as a plateau, but probably half of it falls below 1000 ft.
In Quebec the chief portion is south of the St Lawrence on the low plain extending from Montreal to the mountains of the " Eastern Townships," while in Ontario it extends from the Archean on the north to the St Lawrence and Lakes Ontario, Erie and Huron.Advertisement
Georgian Bay and the northern part of Lake Huron with the whole northern margin of Lake Superior bathe the foot of the Laurentian plateau, which rises directly from these lakes; so that the older fertile lands of the country with their numerous cities and largely-developed manufactures are cut off by an elevated, rocky and mostly forest-covered tract of the Archean from the newer and far more extensive farm lands of the west.
This great plain runs north-westward between the border of the Archean protaxis and the line of the Rocky Mountains, including most of Manitoba, the southern part of Saskatchewan and most of Alberta.
This interior plain of the continent represents the area of the ancient sea by which it was occupied in Mesozoic times, with a more ancient margin towards the north-west against the Archean, where undisturbed limestones and other rocks of the Silurian and Devonian rest upon the downward slope of the Laurentian Shield.
Dispersed over all parts of the western Highlands, they are most numerous in the north-west, especially in the Outer Hebrides and in the west of the shires of Ross and Cromarty and Sutherland, where the surface of the Archean gneiss is so thickly sprinkled with them that many tracts consist nearly as much of water as of land.
Archean gneiss, Cambrian sandstone, Silurian quartzite, limestone and schist, Jurassic sandstone and limestone, Cretaceous sandstone, and Tertiary basalts, gabbros, and granitic rocks all enter into the composition of the islands.Advertisement
Eastward of the Archean gneiss in the west of Sutherland the effect of enormous underground pressure has been to upraise masses of the ancient gneiss and Torridonian sandstone and thrust them westward over the younger rocks.
Above the Archean gneiss lies a series of red and chocolate-coloured sandstone (Torridon sandstone), which form a number of detached areas from Cape Wrath down the seaboard of the shires of Sutherland and Ross and Cromarty, across Skye, and as far as the island of Rum.
All of these are pre-Carboniferous in age and most of them probably belong to the Archean period.
The Red Sea is formed by a line of fracture, probably dating from Pliocene times, crossing the centre of a dome of Archean rocks, on both flanks of which, in Egypt and Arabia., rest Secondary and Tertiary deposits.
Outside the arc of the mountain chain no sign of this crumpling is to be detected except in the Salt Range, and the Peninsula of India has been entirely free from folding of any importance since early Palaeozoic times, if not since the Archean period itself.
By far the greater part of Portugal is occupied by ancient rocks of Archean and Palaeozoic age, and by eruptive masses which probably belong to various periods.
The highlands of New South Wales consist, geographically, of a series of tablelands, now in the condition of dissected peneplains; geologically, they are built of a foundation of Archean and folded Lower Palaeozoic rocks, covered in places by sheets of more horizontal Upper Palaeozoic and Mesozoic rocks; these deposits occur along the edge of the highlands, and are widely distributed on the floor of the coastal districts.
The western plains contain isolated ridges of the old Archean and Lower Palaeozoic rocks; but in the main, they consist of plains of Cretaceous beds covered by Cainozoic drifts.
Most of these foliated rocks, however, are doubtless of Archean age.
The oldest rocks in New South Wales are referrable to the Archean system, and consist of gneisses and schists, including the glaucophane-schists in the New England tableland, and hornblendeschists of Berthong.
The Archean rocks are comparatively sparsely exposed in New South Wales.
They enter the state from the south, being continuous with the Archean block of north-eastern Victoria.
They occupy a large area in the western districts of New South Wales, where a projection from the Archean plateau of central Australia crosses into the state from South Australia; it is best exposed in the Barrier Ranges around Broken Hill.
The mines there occur in gneiss and schists, which are probably of Archean age; the lode has in places been worked for a width of over 200 ft.
Lake Superior lies in a deep rift in rocks principally of Archean and Cambrian age, of the Laurentian, Huronian and Keweenaw formations, rich in minerals that have been extensively worked.
The Archean rocks produce a picturesque coast-line, the north shore particularly being indented by deep bays surrounded by high cliffs, mostly burnt off and somewhat desolate; the islands also rise abruptly to considerable heights, the north shore furnish= ing the boldest scenery of the Great Lakes.
Regarding now the outcrops of bed-rock, there are exposures of Algonkian (doubtful, and at most a mere patch on Pilot Knob), Archean, Cambrian, Ordovician, Silurian, Devonian, sub-Carboniferous and Carboniferous.
The St Francois Mountains and the neighbouring portion of the Ozark region are capped with Archean rocks.
Thus the state is to be conceived, in geological history, as gradually built up around an Archean island in successive seas, the whole of the state becoming dry land after the post-Carboniferous uplift.
The haematites are found not only in the archean porphyries but in Cambrian limestone and sandstone, and in the sub-Carboniferous formations; while the limonites are confined almost exclusively to the Cambrian.
The great difference in character, however, between the Silurian strata at Pomeroy in county Tyrone and the adjacent metamorphic series makes it highly probable that the latter masses are truly Archean.
Their rocks have been variously held to be Archean, Cambrian and Silurian, and their general trend has undoubtedly been determined by post-Silurian earth-movements.
In the apparent absence of any Cambrian formation above them, there is little doubt that these rocks are Archean, although this cannot be absolutely proved.
In the Archean region the gneiss is very often found passing into granite, but certain granitic masses have a sufficiently distinct character.
In the Archean region these are very noticeable near Lake Itasy, in the massif of Ankaratra (an ancient volcano) and in Vakinankaratra (at Betafo, Antsirabe, &c.); while there are numerous outflows of doleritic rocks, probably from faults, along the eastern side of the island and almost parallel with the coast line.
Archean rocks are exposed in the north of the Peninsula, particu larly along the great Pyrenean axis, in Galicia, Estremadura, tb Sierra Morena, the Sierra Nevada and Serrania de Ronda.
No Archean rocks are exposed in Nebraska, and the sedimentary formations are undisturbed in situ.
The nucleus of the island is a block of Archean rocks, which are not, so far as is known, extensively exposed.
The most certain representatives of the Archean are the gneiss and schists of the Dove river and the upper Forth, and the hornblende-schists, which are exposed in the river valleys on the margins of the central plateau.
The Mount Lyell schists which underlie the West Coast Range, and the quartzites of Port Davey on the western coast, have also been regarded as Archean.
The broad geological and geographical relationships of the country have already been outlined, but the more important sub-divisions may now be taken up with more detail, and for that purpose five areas may be distinguished, much the largest being the Archean protaxis, covering about 2,000,000 sq.
Most of the pine that formerly grew on the Archean at the northern fringe of the settlements has been cut, but the lumberman is still advancing northwards and approaching the northern limit of the famous Canadian white pine forests, beyond which spruces, tamarack (larch) and poplar are the prevalent trees.
Both to the east and to the west of this depression the Archean and Palaeozoic rocks which form the greater part of the island are strongly folded, with the exception of the uppermost beds, which belong to the Permian system.
Granite and Archean schists form nearly the whole of the eastern hills from the Strait of Bonifacio southwards to the Flumendosa river, culminating in Monti del Gennargentu.
In the geological history of France there have been two great periods of folding since Archean times.
The oldest rocks, the gneisses and schists of the Archean period, form nearly the whole of the Central Plateau, and are also exposed in the axes of the folds in Brittany.
Australia is essentially the fragment of a great plateau land of Archean rocks.
It consists in the main of an Archean block or " coign,"which still occupies nearly the whole of the western half of the continent, outcrops in north-eastern Queensland, forms the foundation of southern New South Wales and eastern Victoria, and is exposed in western Victoria, in Tasmania, and in the western flank of the Southern Alps of New Zealand.
These areas of Archean rocks were doubtless once continuous.
But they have been separated by the foundering of the Coral Sea and the Tasman Sea, which divided the continent of Australia from the islands of the Australasian festoon; and the foundering of the band across Australia, from the Gulf of Carpentaria, through western Queensland and western New South Wales, to the lower basin of the Murray, has separated the Archean areas of eastern and western Australia.
The breaking up of the old Archean foundation block began in Cambrian and Ordovician times.
The sea does not appear to have extended completely across Australia, breaking it into halves, for a projection from the Archean plateau of Western Australia extended as far east as the South Australian highlands, and thence probably continued eastward, till it joined the Victorian highlands.
Ungava includes much of the lower portion of Labrador, with a rim of recent marine deposits along its western coast, but the interior has the usual character of low rocky hills of Archean rocks, especially granite and gneiss, with a long band of little disturbed iron-bearing rocks, resembling the Animikie, or Upper Huronian of the Lake Superior region, near its eastern side.
The Ordovician and Silurian systems are widely developed, and it is most probable that, with the exception of the Archean continents of Finland and the S, the sea covered the whole of Russia.
Here the folded Archean rocks are overlaid by Cambrian and Ordovician beds, which still lie for the most part flat and undisturbed.
South and east of the Palaeozoic plateau is an extensive area consisting chiefly of Archean rocks, and including the greater part of Mongolia north of the Tian-shan.
There is again a floor of folded Archean rocks overlaid by nearly horizontal strata of Lower Palaeozoic age; but these are followed by marine beds belonging to the Carboniferous period.
Bitumen is, in its various forms, one of the most widel y -distributed of substances, occurring in strata of every geological age, from the lowest Archean rocks to those now in process of deposition, and in greater or less quantity throughout both hemispheres, from Spitzbergen to New Zealand, and from California to Japan.
In the southern region, which is by far the better known, the oldest rocks are granites, crystalline schists and other rocks of Archean aspect.
The plateau is built up of granites, gneisses and crystalline schists of Archean and probably Primary age.
Archean rocks - gneiss, schist and granite - cover large areas through which the Nile cuts its way in alternate narrow gorges and open reaches.
In Ankole and Koki rocks consisting of granular quartzite, schistose sandstone, red and brown sandstone, and shales with cleaved killas rest on the Archean platform and possibly represent the Lower Witwatersrand beds of the Transvaal.
The oldest rocks are Archean, represented by the band of gneisses and schists exposed along the western foot of the Southern Alps.
It has been suggested that the, jasperoids and diabases of the Tarawera Mountains on the North Island may be of Upper Archean age, from their resemblance to the Heathcotian rocks of Australia.
The higher elevations are mostly either Archean or Paleozoic formations projecting above Tertiary deposits.
Archean rocks form the cores of the ancient crystalline masses within the littoral zone from Algiers to Bona.
Archeozoic (Archean) Group.The oldest group of rocks, called the Archean, was formerly looked upon, at least in a tentative way, as the original crtist of the earth or its downward extension, much altered by the processes of metamorphism.
This view of its origin is now known not to be applicable to the Archean as a whole, since this system contains some metamorphosed sedimentary rocks.
Timor consists of a core of ancient rocks (Archean?) upon which rest Permian and later deposits of sedimentary origin.