The Silurian rocks have been estimated by Sir Roderick Murchison to
occupy, altogether, an area of about 7,600 square miles in England and
Wales, 18,420 square miles in Scotland, and nearly 7,000 square miles in
Ireland. Thus, as regards the British Isles, the Silurian rocks rise to
the surface over nearly 33,000 square miles.
The Silurian rocks have been traced from Cumberland to the Land’s End,
at the southern extremity of England. They lie at the base of the
southern Highlands of Scotland, from the North Channel to the North Sea,
and they range along the entire western coast of that country. In a
westerly direction they extended to the sea, where the mountains of
Wales--the Alps of the great chain--would stand out in bold relief, some
of them facing the sea, others in detached groups; some clothed with a
stunted vegetation, others naked and desolate; all of them wild and
picturesque. But an interest surpassing all others belongs to these
mountains. They are amongst the most ancient sedimentary rocks which
exist on our globe, a page of the book in which is written the history
of the antiquities of Great Britain--in fine, of the world.
[Illustration: Fig. 20. Ogygia Guettardi. Natural size.]
In Shropshire and Wales three zones of Silurian life have been
established. In rocks of three different ages _Graptolites_ have left
the trace of their existence. Another fossil characteristic of these
ancient rocks is the _Lingula_. This shell is horny or slightly
calcareous, which has probably been one cause of its preservation. The
family to which the Lingula belongs is so abundant in the rocks of the
Welsh mountains, that Sir R. Murchison has used it to designate a
geological era. These Lingula-flags mark the beginning of the first
Silurian strata.
In the Lower Llandovery beds, which mark the close of the period, other
fossils present themselves, thus greatly augmenting the forms of life in
the Lower Silurian rocks. These are cœlenterata, articulata, and
mollusca. They mark, however, only a very ephemeral passage over the
globe, and soon disappear altogether.
The vertebrated animals are only represented by rare Fishes, and it is
only on reaching the Upper Ludlow rocks, and specially in those beds
which pass upward into the Old Red Sandstone, that the remains have been
found of fishes--the most ancient beings of their class.
The class of Crustaceans, of which the lobster, shrimp, and the crab of
our days are the representatives, was that which predominated in this
epoch of animal life. Their forms were most singular, and different from
those of all existing Crustaceans. They consisted mainly of the
_Trilobites_, a family which became entirely extinct at the close of the
Carboniferous epoch, but in whose nicely-jointed shell the armourer of
the middle ages might have found all his contrivances anticipated, with
not a few besides which he has failed to discover. The head presents, in
general, the form of an oval buckler; the body is composed of a series
of articulations, or rings, as represented in Fig. 20; the anterior
portion carrying the eyes, which in some are reticulated, like those of
many insects (Figs. 18 and 19); the mouth was placed forward and beneath
the head. Many of these Crustaceans could roll themselves into balls,
like the wood-louse (Figs. 23 and 25). They swam on their backs.
[Illustration: Fig. 21.--Lituites cornu-arietis. One-third natural
size.]
[Illustration: Fig. 22.--Hemicosmites pyriformis. One-third natural
size.]
During the middle and later Silurian ages, whole rocks were formed
almost exclusively of their remains; during the Devonian period they
seem to have gradually died out, almost disappearing in the
Carboniferous age, and being only represented by one doubtful species in
the Permian rocks of North America. The Trilobites are unique as a
family, marking with certainty the rocks in which they occur; “and yet,”
says Hugh Miller, “how admirably do they exhibit the articulated type of
being, and illustrate that unity of design which pervades all Nature,
amid its endless diversity!” Among other beings which have left their
traces in the Silurian strata is _Nereites Cambriensis_, a species of
annelide, whose articulations are very distinctly marked in the ancient
rocks.
Besides the Trilobites, many orders of Mollusca were numerously
represented in the Silurian seas. As Sir R. Murchison has observed, no
zoological feature in the Upper Silurian rocks is more striking than the
great increase and profusion of Cephalopods, many of them of great size,
which appear in strata of the age immediately antecedent to the dawn of
vertebrated life. Among the Cephalopods we have _Gyroceras_ and
_Lituites cornu-arietis_ (Fig. 21), whose living representatives are the
Nautilus and Cuttlefish of every sea. The genus _Bellerophon_ (Figs. 54
and 56), with many others, represented the Gasteropods, and like the
living carinaria sailed freely over the sea by means of its fleshy
parts. The Gasteropods, with the Lamellibranchs, of which the Oyster is
a living type, and the Brachiopods, whose congeners may still be
detected in the _Terebratula_ of our Highland lochs and bays, and the
_Lingulæ_ of the southern hemisphere, were all then represented. The
Lamellibranchiata are without a head, and almost entirely destitute of
power of locomotion. Among the Echinodermata we may cite the
_Hemiscosmites_, of which _H. pyriformis_ (Fig. 22) may be considered an
example.
The rocks of the Lower Silurian age in France are found in Languedoc, in
the environs of Neffiez and of Bédarrieux. They occupy, also, great part
of Brittany. They occur in Bohemia, also in Spain, Russia, and in the
New World. Limestones, sandstones, and schists (slates of Angers) form
the chief part of this series. The Cambrian slates are largely
represented in Canada and the United States.