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
_Sigillariae_, 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 of the vegetable matter produced."

Undoubtedly the force of these reflections is not diminished when the
bituminous coal, as in Britain, consists of accumulated spores and
spore-cases, rather than of stems. But, suppose we adopt Principal
Dawson's assumption, that one foot of coal represents fifty
generations of coal plants; and, further, make the moderate
supposition that each generation of coal plants took ten years to come
to maturity--then, each foot-thickness of coal represents five hundred
years. The superimposed beds of coal in one coal-field may amount to
a thickness of fifty or sixty feet, and therefore the coal alone, in
that field, represents 500 x 50 = 25,000 years. But the actual coal is
but an insignificant portion of the total deposit, which, as has been
seen, may amount to between two and three miles of vertical thickness.