Archaean Sentence Examples
They are known to be pre-Jurassic, but whether they are Palaeozoic or Archaean is uncertain.
On the mainland, the north and east shores are of gneisses and granites of archaean age, with a broken and hilly surface rising in places to 600 ft.
The outcrops of these rocks succeed each other in order of age in roughly concentric belts, with the Archaean mass of the island of Anglesey as a centre, but the arrangement in detail is much disturbed and often very irregular.
Here is the margin, often concealed by very modern deposits, of the great mass of Archaean and Palaeozoic rocks which forms nearly the whole of Bohemia and Moravia.
E Arabian Sea Ba Of G A L e Geological information incomplete Desert Deposits Quaternary Tertiary Mesozoic Palaeozoic Archaean and Metamorphic Younger Volcanic Rocks English Miles b iuHi iiiiuiiiiii after llargl,aua Geology The geology of Asia is so complex and over wide areas so little known that it is difficult to give a connected account of either the structure or the development of the continent, and only the broader features can be dealt with here.Advertisement
There are, it is true, mountain ranges which are formed of folded beds; but in many cases the direction of the chains is different from that of the folds, so that the ranges must owe their elevation to other causes; and the folds, moreover, are of ancient date, for the most part Archaean or Palaeozoic. The configuration of the region is largely due to faulting, trough-like or tray-like depressions being formed, and the intervening strips, which have not been depressed, standing up as mountain ridges.
Some of the metamorphic rocks may belong to the older Palaeozoic period, but the greater part of the series is probably Archaean.
The origin of the schistose rocks has long been under discussion, and controversy has centred more particularly around the schistes lustres, which are held by some to be of Triassic age and by others to be pre-Carboniferous and even, perhaps, Archaean.
The oldest rocks, belonging to the Archaean system, occur in the south, forming the Vosges and the Black Forest in the west, and the greater part of the Bohemian massif, including the Erzgebirge, in the east.
In Saxony also the upper Cretaceous beds rest directly upon the Palaeozoic or Archaean rocks.Advertisement
Between the Palaeozoic area near Ottawa, and Georgian Bay to the north of the region just referred to, there is a southward projection of the Archaean protaxis consisting of granite and gneiss of the Laurentian, enclosing bands of crystalline limestone and schists, which are of interest as furnishing the only mines of "Old Ontario."
While all the larger cities and most of the manufacturing and farming districts of the province belong to old Ontario, there is now in process of development a "New Ontario," stretching for hundreds of miles to the north and north-west of the region just described and covering a far larger area, chiefly made up of Laurentian and Huronian rocks of the Archaean protaxis.
From Lake Ontario the St Lawrence emerges through the meshes of the Thousand Islands, where it crosses Archaean rocks, after which follow several rapids separated by quieter stretches before Montreal is reached at the head of ocean navigation.
The low island of Anglesey, which is built tip of the fundamental Archaean rocks, is important as a link in the main line of communication with Ireland, because it is separated from the mainland by a channel narrow enough to be bridged, and lies not far out of the straight line joining London and Dublin.
Around the western and northern edge of the Old Red Sandstone plain the underlying Silurian rocks (and even the Cambrian and Archaean in places) have been bent up so that their edges form hills of singular abruptness and beauty.Advertisement
Wenlock Edge, running from south-west to north-east, is an escarpment of Silurian limestone, while the broad upland of Long Mynd, nearly parallel to it on the north, is a mass of Archaean rock.
The remarkable features of the scenery of South Devon and Cornwall are due to a narrow band of Archaean rock which appears in the south of the peninsulas terminating in Lizard Head and Start Point, and to huge masses of granite and other eruptive rocks which form a series of great bosses and dykes.
Superficially, each is a simple rolling plateau, much broken by erosion (though considerable undissected areas drained by underground channels remain), especially in the east, and dotted with hills; some of these are residual outliers of the eroded Mississippian limestones to the west, and others are the summits of an archaean topography above which sedimentary formations that now constitute the valley-floor about them were deposited and then eroded.
Archaean granite is thus exposed at Yampol and other places in Russia, and this is followed towards the west by Silurian and Devonian beds in regular succession - the Devonian being of the Old Red Sandstone type characteristic of the British Isles and of Northern Russia.
In Bolivia this eastern ridge, separated from the western Cordillera by the longitudinal valley in which Lake Titicaca lies, is formed chiefly of Archaean and Palaeozoic rocks.Advertisement
Archaean terrain and the surrounding Lower Proterozoic terrain.
He wrote Correlation Papers - Archaean and Algonkian (1892), Some Principles Controlling the Deposition of Ores (1901).
South and west of the Fossa Magna the beds are thrown into folds which run approximately parallel to the general direction of the coast, and two zones may be recognizedan outer, consisting of Palaeozoic and Mesozoic beds, and an inner, consisting of Archaean and Palaeozoic rocks, with granitic intrusions.
At the southern extremity of the great table-land, however, in the state of Puebla, there is a considerable mass of crystalline rocks which is believed to be of Archaean age.
The oldest rocks, forming the greater mass of the hinterland, are gneisses, schists and granites of Archaean age.Advertisement
The Mongolian or Great Altai, on the other hand, consists mainly of gneiss and Archaean rocks.
The' four topographic belts of the state correspond very closely to the outcrops of its geological formations; the rocks of the Appalachian belt being of Palaeozoic age; the formation of the Highlands, Archaean; that of the Triassic Lowland, Triassic; that of the irregular hills of the Coastal Plain, Cretaceous and Tertiary.
The Western Division is composed entirely of Archaean and Palaeozoic rocks, embracing the whole range from pre-Cambrian up to Carboniferous.
A few small outcrops occur where still more ancient strata have been raised to the surface, as, for instance, in Charnwood Forest, where the Archaean rocks, with intrusions of granite, create a patch of highland scenery in the very heart of the English plain; and in the Lickey Hills, near Birmingham, where the prominent features are due to volcanic rocks of very ancient date.