In course of time, periods of which include millions of years,
these earths hardened in layers, and enclosed under a thick
carapace of pudding-stone, schist, compact or friable sandstone,
gravel and stones, the whole of the massive forests.

And what went on in this gigantic crucible, where all this
vegetable matter had accumulated, sunk to various depths?
A regular chemical operation, a sort of distillation.
All the carbon contained in these vegetables had agglomerated,
and little by little coal was forming under the double influence
of enormous pressure and the high temperature maintained by
the internal fires, at this time so close to it.

Thus there was one kingdom substituted for another in this
slow but irresistible reaction. The vegetable was transformed
into a mineral. Plants which had lived the vegeta-tive
life in all the vigor of first creation became petrified.
Some of the substances enclosed in this vast herbal left their
impression on the other more rapidly mineralized products,
which pressed them as an hydraulic press of incalculable power
would have done.

Thus also shells, zoophytes, star-fish, polypi, spirifores, even fish
and lizards brought by the water, left on the yet soft coal their
exact likeness, "admirably taken off."

Pressure seems to have played a considerable part in the formation
of carboniferous strata. In fact, it is to its degree of power that
are due the different sorts of coal, of which industry makes use.
Thus in the lowest layers of the coal ground appears the anthracite,