r in the background, clothing with its
shaggy covering of deep green the lower hill-slopes. And as we found in
the Thallogens of that littoral zone over which we have just passed,
representatives of the marine flora of the Silurian System, from the
first appearance of organisms in its nether beds, to its bone-bed of the
Upper Ludlow rocks, in which the Lycopodites first appear, so in the
Acrogens of that moor, with its solitary coniferous tree, we may
recognize an equally striking representative of the terrestrial flora
which existed during the deposition of these Ludlow rocks, and of the
various formations of the Old Red Sandstone, Lower, Middle, and Upper.

[Illustration: Fig. 10.

PINUS SYLVESTRIS. (Scotch Fir.)]

[Illustration: Fig. 11.

CALAMITE? Of the Lower Old Red Sandstone. Shetland. (One eighth nat.
size.)]

[Illustration: Fig. 12.

LYCOPODITE? Of the Lower Old Red Sandstone. Thurso. (Mag. two
diameters.)]

In the upper beds of the Upper Silurian, as has been already remarked,
Lycopodites are the only terrestrial plants yet found. In the Lower Old
Red Sandstone we find added to these, with Thallogens that bear at least
the same _general_ character as in the system beneath, minute ferns, and
a greatly larger plant, allied to the horse tails. The Old Red flora
seems to have been prevailingly an acrogenic flora; and yet with almost
its first beginnings,--contemporary with at least the earlier fossils of
the system in Scotland, we find a true polycotyledonous tree, not lower
in the scale than the araucarites of the Coal Measures,--which in
structure it greatly resembles,--or than the pines or cedars of our own
times (see Fig. 3). In the Middle Old Red Sandstone there occurs, with
plants representative apparently of the ferns and their allies, a
somewhat equivocal and doubtful organism, which may have been the
panicle or compound fruit of some aquatic rush; while in the Upper Old
Red, just ere the gorgeous flora of the Coal Measures began to be, there
existed in considerable abundance a stately fern, the _Cyclopteris
Hibernicus_ (see Fig. 2), of mayhap not smaller proportions than our
monarch of the British ferns, _Osmunda regalis_, associated with a
peculiar lepidodendron, and what seems to be a lepidostrobus,--possibly
the fructiferous spike or cone of the latter, mingled with carbonaceous
stems, which, in the simplicity of their texture, and their abundance,
give evidence of a low but not scanty vegetat