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.
In the early part of the Palaeozoic era only the gneissic region of Finland and Olonets and probably the Archean mass of S.
The Archean rocks have a broad extension in Finland, N.
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.
Large ore-bodies of granular and compact magnetite occur as beds and lenticular masses in Archean gneiss and crystalline schists, in various parts of Norway, Sweden, Finland and the Urals; as also in the states of New York, New Jersey, Pennsylvania and Michigan, as well as in Canada.
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.
- Geologically Venezuela consists of three distinct regions: (1) South of the Orinoco a great mass of granite, gneiss, pyroxenite and other crystalline rocks, continuous with that of Guiana and probably of Archean age.
- Speaking generally, New Caledonia may be described as a band of Palaeozoic and probably Lower Palaeozoic rocks, associated doubtless with some Archean beds; this band runs from north-west to south-east, through the whole length of the island.
This foundation consists of Archean, Palaeozoic and Mesozoic beds folded together, the direction of the folds being N.
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.
Rocks of Archean and Palaeozoic ages contribute only a small share, but there is a Scale, 1:7,700,000 English Miles o 60 80 too 200 ï¿½-' 4,, ,% 4s o,r^ ° o ?
Archean rocks form the cores of the ancient crystalline masses within the littoral zone from Algiers to Bona.
Archean Great Schist Series (Mona, Kitchi, Keewatin, Quinnissec; Lower Huronian of some L authors).
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.
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.
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.
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.
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.