In deference to the opinion of a number of geologists we must glance once more at the alternative view of the planetesimal school. In their opinion the molecules of water were partly attracted to the surface out of the disrupted matter, and partly collected within the porous outer layers of the globe. As the latter quantity grew, it would ooze upwards, fill the smaller depressions in the crust, and at length, with the addition of the attracted water, spread over the irregular surface. There is an even more important difference of opinion in regard to the formation of the atmosphere, but we may defer this until the question of climate interests us. We have now made our globe, and will pass on to that early chapter of its story in which living things make their appearance.
To some it will seem that we ought not to pass from the question of origin without a word on the subject of the age of the earth. All that one can do, however, is to give a number of very divergent estimates. Physicists have tried to calculate the age of the sun from the rate of its dissipation of heat, and have assigned, at the most, a hundred million years to our solar system; but the recent discovery of a source of heat in the disintegration of such metals as radium has made their calculations useless. Geologists have endeavoured, from observation of the action of geological agencies to-day, to estimate how long it will have taken them to form the stratified crust of the earth; but even the best estimates vary between twenty-five and a hundred million years, and we have reason to think that the intensity of these geological agencies may have varied in different ages. Chemists have calculated how long it would take the ocean, which was originally fresh water, to take up from the rocks and rivers the salt which it contains to-day; Professor Joly has on this ground assigned a hundred million years since the waters first descended upon the crust. We must be content to know that the best recent estimates, based on positive data, vary between fifty and a hundred million years for the story which we are now about to narrate. The earlier or astronomical period remains quite incalculable. Sir G. Darwin thinks that it was probably at least a thousand million years since the moon was separated from the earth. Whatever the period of time may be since some cosmic cataclysm scattered the material of our solar system in the form of a nebula, it is only a fraction of that larger and illimitable time which the evolution of the stars dimly suggests to the scientific imagination.
THE GEOLOGICAL SERIES
[The scale of years adopted—50,000,000 for the stratified rocks—is merely an intermediate between conflicting estimates.]
ERA. PERIOD. RELATIVE LENGTH.
Quaternary Holocene 500,000 years
Pleistocene
Tertiary Pliocene 5,500,000 years
or Miocene
Cenozoic Oligocene
Eocene
Secondary Cretaceous 7,200,000 years
or Jurassic 3,600,000 "
Mesozoic Triassic 2,500,000 "
Primary Permian 2,800,000 years
or Carboniferous 6,200,000 "
Palaeozoic Devonian 8,000,000 "
Silurian 5,400,000 "
Ordovician 5,400,000 "
Cambrian 8,000,000 "
Archaean Keweenawan Unknown (probably
Animikie at least
Huronian 50,000,000 years)
Keewatin
Laurentian