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. 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 any one 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 exist.

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_,