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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
CAMBRIAN SYSTEM. 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.
SILURIAN SYSTEM. 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.
DEVONIAN SYSTEM. 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.
CARBONIFEROUS SYSTEM. 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.
PERMIAN SYSTEM. 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.