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.
There were, therefore, two principal epochs of folding in the island, one at the close of the Palaeozoic era which affected the whole of the island, and one at the close of the Mesozoic which was felt only in the western region.
The Palaeozoic rocks form two extensive masses, one in the south-east and the other in the south-west.
Outside the northern circle lie on the west the folded Palaeozoic rocks of Brittany.
And on the north the Palaeozoic massif of the Ardennes.
The first of these occurred towards the close of the Palaeozoic era, when a great mountain system was raised in the north running approximately from E.
The Central Plateau has probably been a land mass ever since this period, but the rest of the country was flooded by the Palaeozoic sea.
Towards the close of the Palaeozoic era France had become a part of a great continent; in the north the Coal Measures of the Boulonnais and the Nord were laid down in direct connection with those of Belgium and England, while in the Central Plateau the Coal Measures were deposited in isolated and scattered basins.
In France, as in Great Britain, volcanic eruptions occurred during several of the Palaeozoic periods, but during the Mesozoic era the // /
The Coal Measures become narrower in the south, until, owing to the eastward projection of the highlands, the Lower Palaeozoic rocks reach the coast.
The Permian may be represented, but the Trias is absent, and in general the older Palaeozoic rocks are overlaid directly by the Rhaetic and Lias.
- This order has been constituted for a peculiar group of palaeozoic fossils, which have been interpreted as the remains of the skeletons of Hydrozoa of an extinct type.
Since no graptolites are known living, or, indeed, since palaeozoic times, the interpretation of their structure and affinities must of necessity be extremely conjectural, and it is by no means certain that they are Hydrozoa at all.
The vegetation of the Palaeozoic era, till towards its close, was apparently remarkably homogeneous all over the world.
In the southern hemisphere the Palaeozoic flora appears ultimately to have been profoundly modified by a lowering of temperature and the existence of glacial conditions over a wide area.
The Glossopteris flora gradually spread to the northern hemisphere and intermingled with the later Palaeozoic flora which still persisted.
In the early part of the Palaeozoic era only the gneissic region of Finland and Olonets and probably the Archean mass of S.
The Falkland Islands consist entirely, so far as is known, of the older Palaeozoic rocks, Lower Devonian or Upper Silurian, slightly metamorphosed and a good deal crumpled and distorted, in the low grounds clay slate and soft sandstone, and on the ridges hardened sandstone passing into the conspicuous white quartzites.
In the south, in Syria, Arabia and the peninsula of India, none but the oldest rocks are folded, and the Upper Palaeozoic, the Mesozoic and the Tertiary beds lie almost horizontally upon them.
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.
But there is positive evidence that much of the north and east of Asia has been land since the Palaeozoic era, and it has been conclusively proved that the peninsula of India has never been beneath the sea since the Carboniferous period at least.
Excepting in the extreme north, where marine Jurassic and Cretaceous fossils have been found, there is no evidence that this part of Siberia has been beneath the sea since the early part of the Palaeozoic era.
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.
Farther south, in the Chinese provinces of Shansi and Shensi, the geological succession is similar in some respects to that of the Siberian Palaeozoic plateau, but the sequence is more complete.
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.
Freshwater and terrestrial deposits of Mesozoic age occur in many places, and the conclusion is irresistible that the greater part of this area has been land since the close of the Palaeozoic era.
Palaeozoic Era >>
The latter has established, for all the Palaeozoic insects, an order Palaeodictyoptera, there being a closer similarity between the fore-wings and the hind-wings than is to be seen in most living orders of Hexapoda, while affinities are shown to several of these orders - notably the Orthoptera, Ephemeroptera, Odonata and Hemiptera.
None of the groups of existing Endopterygota have been traced with certainty farther back than the Mesozoic epoch, and all the numerous Palaeozoic insect-fossils seem to belong to forms that possessed only imperfect metamorphosis.
Endopterygota - of insects of the present epoch are descended from the predominant - if not the sole - group that existed in the Palaeozoic epoch, viz.
The first hypothesis is not negatived by direct evidence, for we do not actually know the ontogeny of any of the Palaeozoic insects; it is, however, rendered highly improbable by the modern views as to the nature and origin of wings in insects, and by the fact that the Endopterygota include none of the lower existing forms of insects.
It appears to be composed chiefly of Palaeozoic rocks, concealed, in the plains, by Quaternary, and possibly Tertiary, deposits.
While igneous and metamorphic crystalline rocks form the bulk of the Adirondack area, it is surrounded by a ring of ancient Palaeozoic sediments in which these peripheral lowlands have been developed.
Here the rocks are all essentially horizontal and of Palaeozoic age, mainly Devonian.
The Coal Measures which form part of the Palaeozoic or oldest of the three great geological divisions are mainly confined to the countries north of the equator.
Overlying these amongst the Palaeozoic rocks, we meet in many parts of Japan with slates and other rocks possibly of Cambrian or Silurian age.
There is also amongst the Palaeozoic group an interesting series of red slates containing Radiolaria.
They are, first, plutonic rocks, especially granite; secondly, volcanic rocks, chiefly trachyte and dolerite; and thirdly, palaeozoic schists.
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.
This foundation consists of Archean, Palaeozoic and Mesozoic beds folded together, the direction of the folds being N.
The Trepostomata are in the main Palaeozoic, although Heteropora, of which recent species exist, is placed by Gregory in this division.
The Cryptostomata are also Palaeozoic, and include the abundant and widely-distributed genus Fenestella.