epoch, two or three considerations suggest themselves.

In the first place, the great phantom of geological time rises before the
student of this, as of all other, fragments of the history of our earth--
springing irrepressibly out of the facts, like the Djin from the jar
which the fishermen so incautiously opened; and like the Djin again,
being vaporous, shifting, and indefinable, but unmistakably gigantic.
However modest the bases of one's calculation may be, the minimum of time
assignable to the coal period remains something stupendous.

Principal Dawson is the last person likely to be guilty of exaggeration
in this matter, and it will be well to consider what he has to say about
it:--

"The rate of accumulation of coal was very slow. The climate of the
period, in the northern temperate zone, was of such a character that the
true conifers show rings of growth, not larger, nor much less distinct,
than those of many of their modern congeners. The _Sigillarioe_ and
_Calamites_ were not, as often supposed, composed wholly, or even
principally, of lax and soft tissues, or necessarily short-lived. The
former had, it is true, a very thick inner bark; but their dense woody
axis, their thick and nearly imperishable outer bark, and their scanty
and rigid foliage, would indicate no very rapid growth or decay. In the
case of the _Sigillarioe_, the variations in the leaf-scars in different
parts of the trunk, the intercalation of new ridges at the surface
representing that of new woody wedges in the axis, the transverse marks
left by the stages of upward growth, all indicate that several years must
have been required for the growth of stems of moderate size. The enormous
roots of these trees, and the condition of the coal-swamps, must have
exempted them from the danger of being overthrown by violence. They
probably fell in successive generations from natural decay; and making
every allowance for other materials, we may safely assert that every foot
of thickness of pure bituminous coal implies the quiet growth and fall of
at least fifty generations of _Sigillarioe_, and therefore an undisturbed
condition of forest growth enduring through many centuries. Further,
there is evidence that an immense amount of loose parenchymatous tissue,
and even of wood, perished by decay, and we do not know to what extent
even the most durable tissues may have disappeared in this way; so that,
in many coal-seams, we may have only a very small part