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.
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