Suppose it be 12,000 feet--which is 240 times the thickness of the
actual coal--is there any reason why we should believe it may not have
taken 240 times as long to form? I know of none. But, in this case,
the time which the coal-field represents would be 25,000 x 240
=6,000,000 years. As affording a definite chronology, of course such
calculations as these are of no value; but they have much use in
fixing one's attention upon a possible minimum. A man may be puzzled
if he is asked how long Rome took a-building; but he is proverbially
safe if he affirms it not to have been built in a day; and our
geological calculations are all, at present, pretty much on that
footing.

A second consideration which the study of the coal brings prominently
before the mind of anyone who is familiar with palaeontology is, that
the coal Flora, viewed in relation to the enormous period of time
which it lasted, and to the still vaster period which has elapsed
since it flourished, underwent little change while it endured, and in
its peculiar characters, differs strangely little from that which at
present exists.

The same species of plants are to be met with throughout the whole
thickness of a coal-field, and the youngest are not sensibly different
from the oldest. But more than this. Notwithstanding that the
carboniferous period is separated from us by more than the whole time
represented by the secondary and tertiary formations, the great types
of vegetation were as distinct then as now. The structure of the
modern club-moss furnishes a complete explanation of the fossil
remains of the _Lepidodendra_, and the fronds of some of the ancient
ferns are hard to distinguish from existing ones. At the same time,
it must be remembered, that there is nowhere in the world, at present,