The next period of the Primary Epoch is the _Silurian_, a system of
rocks universal in extent, overspreading the whole earth more or less
completely, and covering up the rocks of older age. The term “Silurian”
was given by the illustrious Murchison to the epoch which now occupies
our attention, because the system of rocks formed by the marine
sediments, during the period in question, form large tracts of country
in Shropshire and Wales, a region formerly peopled by the _Silures_, a
Celtic race who fought gloriously against the Romans, under Caractacus
or Caradoc, the British king of those tracts. The reader may find the
nomenclature strange, as applied to the vast range of rocks which it
represents in all parts of the Old and New World, but it indicates, with
sufficient exactness, the particular region in our own country in which
the system typically prevails--reasons which led to the term being
adopted, even at a time when its vast geographical extent was not
suspected.
On this subject, and on the principles which have guided geologists in
their classification of rocks, Professor Sedgwick remarks in one of his
papers in the _Quarterly Journal of the Geological Society_: “In every
country,” he says,[36] “which is not made out by reference to a
pre-existing type, our first labour is that of determining the physical
groups, and establishing their relations by natural sections. The labour
next in order is the determination of the fossils found in the
successive physical groups; and, as a matter of fact, the natural groups
of fossils are generally found to be nearly co-ordinate with the
physical groups--each successive group resulting from certain conditions
which have modified the distribution of organic types. In the third
place comes the collective arrangement of the groups into systems, or
groups of a higher order. The establishment of the Silurian system is an
admirable example of this whole process. The groups called Caradoc,
Wenlock, Ludlow, &c., were physical groups determined by good natural
sections. The successive groups of fossils were determined by the
sections; and the sections, as the representatives of physical groups,
were hardly at all modified by any consideration of the fossils, for
these two distinct views of the natural history of such groups led to
co-ordinate results. Then followed the collective view of the whole
series, and the establishment of a nomenclature. Not only the whole
series (considered as a distinct system), but every subordinate group
was defined by a geographical name, referring us to a local type within
the limits of Siluria; in this respect adopting the principle of
grouping and nomenclature applied by W. Smith to our secondary rocks. At
the same time, the older slate rocks of Wales (inferior to the system of
Siluria), were called _Cambrian_, and soon afterwards the next great
collective group of rocks (superior to the system of Siluria) was called
_Devonian_. In this way was established a perfect congruity of language.
It was geographical in principle, and it represented the actual
development of all our older rocks, which gave to it its true value and
meaning.” The period, then, for the purposes of scientific description,
may be divided into three sub-periods--the Upper and Lower Silurian, and
the Cambrian.
[36] _Quart. Jour. Geol. Soc_., vol. iii., p. 159.
[Illustration: VIII.--Ideal Landscape of the Silurian Period.]
[Illustration: Fig. 18.--Back of Asaphus caudatus (Dudley, Mus. Stokes),
with the eyes well preserved. (Buckland.)]
[Illustration: Fig. 19.--_a_, Side view of the left eye of the above,
magnified, (Buckland.) _b_, Magnified view of a portion of the eye of
Calymene macrophthalmus. (Hœninghaus.)]
The characteristics of the Silurian period, of which we give an ideal
view opposite (PLATE VIII.), are supposed to have been shallow seas of
great extent, with barren submarine reefs and isolated rocks rising here
and there out of the water, covered with Algæ, and frequented by various
Mollusca and articulated animals. The earliest traces of vegetation
belong to the _Thallogens_, flowerless plants of the class Algæ (Fig.
28), without leaves or stems, which are found among the Lower Silurian
rocks. To these succeed other plants, according to Dr. Hooker, belonging
to the Lycopodiaceæ (Fig. 28), the seeds of which are found sparingly in
the Upper Ludlow beds. Among animals, the _Orthoceratites_ led a
predacious life in the Silurian seas. Their organisation indicates that
they preyed upon other animals, pursuing them into the deepest abysses,
and strangling them in the embrace of their long arms. The _Trilobites_,
a remarkable group of Crustacea, possessing simple and reticulated
compound eyes, also highly characterise this period (Figs. 17 to 20);
presenting at one period or other of their existence 1,677 species, 224
of which are met with in Great Britain and Ireland, as we are taught by
the “Thesaurus Siluricus.”[37] Add to this a sun, struggling to
penetrate the dense atmosphere of the primitive world, and yielding a
dim and imperfect light to the first created beings as they left the
hand of the Creator, organisms often rudimentary, but at other times
sufficiently advanced to indicate a progress towards more perfect
creations. Such is the picture which the artist has attempted to
portray.
[37] “The Flora and Fauna of the Silurian Period,” by John T. Bigsby,
M.A., F.G.S. 4to, 1868.
The elaborate and highly valuable “Thesaurus Siluricus” contains the
names of 8,997 species of fossil remains, but it probably does not tell
us of one-tenth part of the Silurian life still lying buried in rocks of
that age in various parts of the world. A rich field is here offered to
the geological explorer.[38]
[38] Ibid, p. vi.