sometimes be separated from the stone in the form of a carbonaceous film.

Experiments were made many years ago by M. Goppert to illustrate the
process of fossilisation of ferns. Having placed some living ferns in a
mass of clay and dried them, he exposed them to a red heat, and obtained
thereby striking resemblances to fossil plants. According to the degree
of heat to which they were subjected, the plants were found to be either
brown, a shining black, or entirely lost. In the last mentioned case,
only the impression remained, but the carbonaceous matter had gone to
stain the surrounding clay black, thus indicating that the dark colour of
the coal-shales is due to the carbon derived from the plants which they
included.

Another very prominent member of the vegetation of the coal period, was
that order of plants known as the _Calamites_. The generic distinctions
between fossil and living ferns were so slight in many cases as to be
almost indistinguishable. This resemblance between the ancient and the
modern is not found so apparent in other plants. The Calamites of the
coal-measures bore indeed a very striking resemblance, and were closely
related, to our modern horse-tails, as the _equiseta_ are popularly
called; but in some respects they differed considerably.

Most people are acquainted with the horse-tail (_equisetum fluviatile)_
of our marshes and ditches. It is a somewhat graceful plant, and stands
erect with a jointed stem. The foliage is arranged in whorls around the
joints, and, unlike its fossil representatives, its joints are protected
by striated sheaths. The stem of the largest living species rarely
exceeds half-an-inch in diameter, whilst that of the calamite attained a
thickness of five inches. But the great point which is noticeable in the
fossil calamites and _equisetites_ is that they grew to a far greater