not be traced. In the one the general appearance was such as
might be produced by compressed and tangled masses of _Chorda filium_,
in which the linear and even tubular character of the plant could be
determined, but not its continuous, cord-like aspect; in the other, the
fragments seemed well nigh as slim as hairs, and the appearance was such
as might be produced by branches of that common ectocarpus, _E.
littoralis_, which may be seen on our rocky coasts roughening at low
water the stems of laminaria. When highly magnified, a mesial groove
might be detected running along each of the hair-like lines. With these
marine plants we occasionally find large rectilinear stems, resolved
into a true coal, but retaining no organic character by which to
distinguish them. As I have seen some of these more than three inches in
diameter, and, though existing as mere fragments, several feet in
length, they must, if they were also plants of the sea, have exceeded in
size our largest laminaria.[46] And such are the few vegetable
organisms, of apparently aquatic origin, which I have hitherto succeeded
in detecting in the Lower Old Red Sandstone of Scotland.[47] Their
individual numbers, however, must have been very great, though, from the
destructible character of their tissues, their forms have perished in
the stone. The immensely developed flagstones of Caithness seem to owe
their dark color to organic matter mainly of vegetable origin. So
strongly bituminous, indeed, are some of the beds of dingier tint, that
they flame in the fire like slates steeped in oil.

[Illustration: Fig. 120.]

The remains of a terrestrial vegetation in this deposit are greatly
scantier than those of its marine plants; but they must be regarded as
possessing a peculiar interest, as, with the exception of the spore
cases of the Ludlow rocks, the oldest of their class, in at least the
British islands, whose true place in the scale can be satisfactorily
established. In the flagstones of Orkney there occurs, though very
rarely, a minute vegetable organism, which I have elsewhere described as
having much the appearance of one of our smaller ferns, such as the
maidenhair-spleenwort, or dwarf moonwort. It consists of a minute stem,
partially covered by what seems to be a small sheath or hollow bract,
and bifurcates into two fronds or pinnae, fringed by from ten to twelve
leaflets, that nearly impinge on each other, and somewhat resemble in
their mode of arrangeme