FOUR PERIODS OF THE EARTH’S DEVELOPMENT
A Postscript to Professor Sollas’s Chapter on the Wonderful Story of the World’s Birth, beginning onpage 79
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THE earth was once “a fluid haze of light.” The whole solar system once formed a vast nebula, consisting of glowing gas, or a swarm of meteoroids. Our planet was slowly shaped into a globe out of this primitive nebula.
This globe was at first intensely hot, and probably liquid. A solid crust formed on the surface as heat was lost by radiation, and this crust consisted of the oldest rocks of igneous formation like the granites and gneisses. During this Archæan or Eozoic Period, the earth acquired its atmosphere and its oceans, and it is probable that the mysterious origin of life took place.
The later history of the earth since the stratified rocks began to appear, and life existed, is divided into four main periods, of which the first is known as Primary, or Palæozoic.
The First Period of the Earth
CAMBRIANSYSTEM. The rocks formed in the Cambrian Age are mainly grits, quartzites, and conglomerates, with shales, schists, and limestones. The earth was then mostly covered by seas, and the first well-defined forms of life were of marine origin.
SILURIANSYSTEM. The Silurian rocks are mostly sandstones, shales, and slates deposited in the seas. The first vertebrates made their appearance as fishes, whilst insects began to flutter in the air, and occasionally to alight on the emerging land.
DEVONIANSYSTEM. This was the age of the old red sandstone. Fishes reached a high state of development, whilst the first traces appeared of land vegetation, ferns and lycopods.
CARBONIFEROUSSYSTEM. This system is exceptionally important, because its chief rock is coal, the fossilised remains of the luxuriant vegetation which grew in tropical swamps. The first terrestrial animals, true air breathers, now appeared.
PERMIANSYSTEM. The last of the primary systems gave us the new red sandstone, distinguished from the old by lying above the coal measures. The Permian Age was apparently unfavourable to life, and is only notable for the first appearance of the land reptiles into which the amphibians developed.
The Second Period of the Earth
The Secondary Period marks the emergence of the dry land into importance greater than that of the sea.
TRIASSICSYSTEM. The Triassic rocks chiefly consist of sandstones and hardened clays laid down in shallow sea basins. Land vegetation now first began to assume a modern type, with conifers and cycads. The seas were still richly peopled, and the land first gave a home to huge reptiles, or dinosaurs.
JURASSICSYSTEM. This system is marked by a great variety of limestones, the product of dead sea creatures. It is essentially the age of reptiles. The ichthyosaurus disputed the seas with the plesiosaurus; the pterodactyl ruled the air; whilst on land, huge monsters like the brontosaur and diplodocus browsed on tropical vegetation. From these reptiles the birds were developing, whilst small marsupials, the oldest of the great mammalian race, skipped under the branches.
CRETACEOUSSYSTEM. This was the age of the great chalk deposits. The birds, now emerging from their reptilian ancestry, dominated its life, and the first modern plants appeared on the land.
The Third Period of the Earth
The Tertiary Period marks the true beginning of modern geological history, when the great outlines of geography were laid down, and the first representatives of modern plants and animals made their appearance.
EOCENESYSTEM. The Eocene rocks are mainly limestones, with sandstone and hardened clays. We owe them to the sea and its organisms. Modern evergreen trees now first appeared. The mammals come to the front, with the tapir-like palæotherium and the first recognisable ancestor of the horse.
MIOCENESYSTEM. The Miocene Age was a mountain-building period, when the great chain which runs from the Alps into Central Asia received its final uplift. Deciduous trees, like the beech and elm, now made their appearance. The giant mastodon and the formidable sabre-toothed tiger roamed the Miocene forest, and true apes—man’s first forerunners—mopped and mowed in the boughs.
PLIOCENESYSTEM. The last of the Tertiary ages set the final stamp on the geological moulding of the earth’s crust. Its plants were transitional to the flora of modern Europe. Great herds of herbivora now appeared.
The Fourth Period of the Earth
The Quaternary Period is that in which we are still living. Its outstanding feature is the appearance of man.
PLEISTOCENE ORGLACIALSYSTEM. Its essential feature was the appearance of glacial conditions over most of the northern hemisphere, when great ice sheets rubbed our land into shape. The vegetation was Arctic, and only animals like the reindeer and the hairy mammoth could endure the cold.
HUMAN ORRECENTSYSTEM. The precise antiquity of man is still uncertain, but it was only after the close of the Glacial Period that he made his home in Europe, where he shared a precarious existence with mammoth, cave-bear, and rhinoceros. Man developed through thePalæolithicandNeolithicages of stone implements to theBronzeandIronages, when metal was first worked. In the last of these we live.