When, again, we find that the fossils of one group of rocks differ
entirely from those of a group lying above them, we learn that one race
of creatures died out and was supplanted by a new assemblage of animal
forms.

These general remarks will, I trust, give some notion of the evidence
which is available for reconstructing the history of those remote
periods with which geology deals, and of the kind of reasoning which the
geologist employs for interpreting the records that are submitted to
him.

We will now briefly examine, by aid of these methods, the group of rocks
in which coal occurs in Great Britain, and see how far we can read the
story they have to tell.

The group with which we have to deal is called the carboniferous or
coal bearing system, and it includes four classes of rocks, viz.: 1,
sandstone; 2, shale or bind; 3, limestone; 4, coal and underclay.

We will take the sandstones and shales first. They are grains of sand
known to mineralogists as quartz, and consisting of a substance called
silica by chemists. The grains of sand are bound together by a cement
which in some few cases is identical in composition with themselves, and
consists of pure silica, but usually is a mixture of sandy, clayey, and
other substances. The shales are made up very largely of clay, mixed,
however, usually with sand and other substances, forming a conglomerate.
Both sandstones and shales are divided into layers or beds, and are said
to be stratified. It is this stratified or bedded structure that gives
us the first clew to the way in which these rocks were formed. Rivers
are constantly carrying down sand and mud into the sea or lakes, and