ysicists 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