From facts presented by the deposits of coal-coal which, in all its varieties, has originated from the decay of plants--it not only demon strates the changes that have taken place in the earth's atmosphere, but also universal changes of climate.

From other facts it proves that there have been oscillations of temperature, periods in which the mean heat has risen, and periods in which the polar ices and snows have covered large portions of the existing continents--glacial periods, as they are termed.
One school of geologists, resting its argument on very imposing evidence, teaches that the whole mass of the earth, from being in a molten, or perhaps a vaporous condition, has cooled by radiation in the lapse of millions of ages, until it has reached its present equilibrium of temperature.

Astronomical observations give great weight to this interpretation, especially so far as the planetary bodies of the solar system are concerned.

It is also supported by such facts as the small mean density of the earth, the increasing temperature at increasing depths, the phenomena of volcanoes and injected veins, and those of igneous and metamorphic rocks.

To satisfy the physical changes which this school of geologists contemplates, myriads of centuries are required.
But, with the views that the adoption of the Copernican system has given us, it is plain that we cannot consider the origin and biography of the earth in an isolated way; we must include with her all the other members of the system or family to which she belongs.