The position of the beds which constitute the coal-measures is
infinitely diverse. Sometimes they are tilted up vertically, sometimes
they are horizontal, sometimes curved into great basins; sometimes they
come to the surface, sometimes they are covered up by thousands of feet
of rock. But, whatever then-present position, there is abundant and
conclusive evidence that every under-clay was once a surface soil. Not
only do carbonized root-fibres frequently abound in these under-clays;
but the stools of trees, the trunks of which are broken off and
confounded with the bed of coal, have been repeatedly found passing into
radiating roots, still embedded in the under-clay. On many parts of the
coast of England, what are commonly known as "submarine forests" are to
be seen at low water. They consist, for the most part, of short stools
of oak, beech, and fir-trees, still fixed by their long roots in the bed
of blue clay in which they originally grew. If one of these submarine
forest beds should be gradually depressed and covered up by new
deposits, it would present just the same characters as an under-clay of
the coal, if the _Sigillaria_ and _Lepidodendron_ of the ancient world
were substituted for the oak, or the beech, of our own times.
In a tropical forest, at the present day, the trunks of fallen trees,
and the stools of such trees as may have been broken by the violence of
storms, remain entire for but a short time. Contrary to what might
be expected, the dense wood of the tree decays, and suffers from the
ravages of insects, more swiftly than the bark. And the traveller,
setting his foot on a prostrate trunk, finds that it is a mere shell,
which breaks under his weight, and lands his foot amidst the insects, or
the reptiles, which have sought food or refuge within.