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
CHAPTER V.
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