Coal is a vegetable substance.  The wide coal-fields of Britain and other lands are the fossil remains of vast forests.

Long ages ago, as it seems, broad and luxuriant forests flourished over the earth.  In many parts generation after generation of trees lived and died and decayed, leaving no trace of their existence, beyond a little layer of black mould, soon to be carried away by wind and water.  Coal could only be formed where there were bogs and quagmires.

But in bogs and quagmires, and in shallow lakes of low-lying lands, there were great gatherings of slowly-decaying vegetable remains, trees, plants, and ferns all mingling together.  Then after a while the low lands would sink and the ocean pouring in would cover them with layers of protecting sand or mud; and sometimes the land would rise again, and fresh forests would spring into life, only to be in their turn overwhelmed anew, and covered by fresh sandy or earthy deposits.

These buried forests lay through the ages following, slowly hardening into the black and shining coal, so useful now to man.

The coal is found thus in thin or thick seams, with other rock-layers between, telling each its history of centuries long past.  In one place no less than sixteen such beds of coal are found, one below another, each divided from the next above and the next underneath by beds of clay or sand or shale.  The forests could not have grown in the sea, and the earth-layers could not have been formed on land, therefore many land-risings and sinkings must have taken place.  Each bed probably tells the tale of a succession of forests....

* * * * *

Before going on to a sketch of the early ages of the Earth’s history—­ages stretching back long long before the time of Adam—­it is needful to think yet for a little longer about the manner in which that history is written, and the way in which it has to be read.

For the record is one difficult to make out, and its style of expression is often dark and mysterious.  There is scarcely any other volume in the great Book of Nature, which the student is so likely to misread as this one.  It is very needful, therefore, to hold the conclusions of geologists with a light grasp, guarding each with a “perhaps” or a “may be.”  Many an imposing edifice has been built, in geology, upon a rickety foundation which has speedily given way.

In all ages of the world’s history up to the present day, rock-making has taken place—­fire-made rocks being fashioned underground, and water-made rocks being fashioned above ground though under water.

Also in all ages different kinds of rocks have been fashioned side by side—­limestone in one part of the world, sandstone in another, chalk in another, clay in another, and so on.  There have, it is true, been ages when one kind seems to have been the chief kind—­an age of limestone, or an age of chalk.  But even then there were doubtless more rock-buildings going on, though not to so great an extent.  On the other hand, there may have been ages during which no limestone was made, or no chalk, or no clay.  As a general rule, however, the various sorts of rock-building have probably gone on together.  This was not so well understood by early geologists as it is now.