lowish brown colour,
embedded in the dark brown ground substance. On the average, these little
brown bodies may have a diameter of about one-twentieth of an inch. They
lie with their flat surfaces nearly parallel with the two smooth faces of
the block in which they are contained; and, on one side of each, there
may be discerned a figure, consisting of three straight linear marks,
which radiate from the centre of the disk, but do not quite reach its
circumference. In the horizontal section these disks are often converted
into more or less complete rings; while in the vertical sections they
appear like thick hoops, the sides of which have been pressed together.
The disks are, therefore, flattened bags; and favourable sections show
that the three-rayed marking is the expression of three clefts, which
penetrate one wall of the bag.

The sides of the bags are sometimes closely approximated; but, when the
bags are less flattened, their cavities are, usually, filled with
numerous, irregularly rounded, hollow bodies, having the same kind of
wall as the large ones, but not more than one seven-hundredth of an inch
in diameter.

In favourable specimens, again, almost the whole ground substance appears
to be made up of similar bodies--more or less carbonized or blackened--
and, in these, there can be no doubt that, with the exception of patches
of mineral charcoal, here and there, the whole mass of the coal is made
up of an accumulation of the larger and of the smaller sacs.

But, in one and the same slice, every transition can be observed from
this structure to that which has been described as characteristic of
ordinary coal. The latter appears to rise out of the former, by the
breaking-up and increasing carbonization of the larger and the smaller
sacs. And, in the anthracitic coals, this process appears to have gone to
such a length, as to destroy the original structure altogether, and to
replace it by a completely carbonized substance.

Thus coal may be said, speaking broadly, to be composed of two
constituents: firstly, mineral charcoal; and, secondly, coal proper. The
nature of the mineral charcoal has long since been determined. Its
structure shows it to consist of the remains of the stems and leaves of
plants, reduced a little more than their carbon. Again, some of the coal
is made up of the crushed and flattened bark, or outer coat, of the stems
of plants, the inner wood of which has completely decayed away. But what
I