THE SILURIAN SYSTEM. FIRST TRACES OF LIFE.
The group of rocks on which we next enter are termed fossiliferous, that
is, there is contained in their hard stony substance the impressions and
actual remains of organic bodies. As we proceed upward through the series
in their ascending order, we will find different rocks distinguished by
different classes of fossils, and characterized by distinct lithological
appearances. They are in consequence divided into different formations,
and called by particular names. Hence the origin of SYSTEMS, of which
there are five or six recognized by geologists, separable into their
respective groups of strata. Descending from the primary, the highest as
well as lowest in the series of rocky combinations, the group which first
invites attention is the SILURIAN; so denominated because the strata
are widely spread over the districts in England and Wales, anciently
inhabited by a people called the “Silures.” They are found in various
quarters of the world, and occupy a large area on the southern frontiers
of Scotland.
The rocks of this class consist of a group of argillaceous, calcareous,
and arenaceous deposits, varying in color and texture. They are of
great thickness and severally impressed with their own written story,
the fossil memoranda of the changes and events that occurred betwixt
the formation of each. These are the transition rocks of Werner. The
newly-adopted term of Silurian implies no peculiar theory as to their
origin. It simply expresses the fact that in the district in question a
complete succession of fossiliferous strata is interpolated between the
oldest slaty crystalline rocks and the old red sandstone. The system is
divided by their discoverer and historian, Sir R. I. Murchison, under the
ascending series, into the Cambrian System, Llandeilo-flags, Caradoc
Sandstone, Wenlock Shales and Limestones, and Lower and Upper Ludlow
Rocks.
Do the equivalents of all, or of any, of these groups exist in the
Grampian range? Geologists for the most part have been answering these
questions in the negative. Hitherto no true silurian deposits have been
recognized as existing among the northern Scottish mountains, and no
well-authenticated organism of the system has been detected in any of
their localities. This, however, will hardly be taken as a conclusive
argument after the admission into the family of the Skiddaw slate, in
which the faintest traces of organized matter have only very recently
been observed, while the over-lying series consisting of chlorite-slate,
and alternating beds of porphyry and greenstone, from twenty to thirty
thousand feet thick, have not yet been proved to contain a single fossil.
“Good fossil groups,” Professor Sedgwick argues, “are the foundation of
all geology; and are out of all comparison the most remarkable monuments
of the past physical history of our globe, so far as it is made out in
any separate physical region.”
We are convinced that the clayslates and graywackes which repose on the
southern flank of the Grampians, as well as abundantly in the interior,
will, upon strict examination, have their place assigned among the
Silurian class. Mr. Nicol, who has done so much for the Lammermuir
deposits, will find ample scope for his investigations, and all his
ingenious speculations, in determining the true position of these
argillaceous beds, which are of prodigious thickness and vast extent.
This is not the place to enter into details, but in support of the view
now advanced, the following among other reasons may be given.
First of all, the clayslate of the Grampians resembles in its lithology
the slates of Wales and Cumberland, admitted to be silurian. In hand
specimens they cannot easily be distinguished from each other: practical
men consider the slates of Dunkeld and Glenalmond as softer and less
flinty than those of the south. They pass from extremely coarse into the
finest grained varieties, when the graywacke character is entirely lost
in the homogeneous mass. Their position in reference to the crystalline
rocks, in the next place, is very distinct, never alternating with, nor
lying conformable to, either the gneiss or mica-schists. They form the
outer zone, from east to west, of the Grampian range, where feldspar,
porphyries, and trappean rocks are along the whole line mixed up or
associated with them. Then overlying the clayslate, precisely as in
Cumberland, the old red sandstone is found in immediate succession and
resting unconformably. Shall we add that, even in a topographical point
of view, these beds will be admitted to vindicate their claim to Silurian
origin, constituting, as they do, in extension, a portion of the great
primary belt that encompasses the western shores of Great Britain, and
beyond the channel, stretches through Brittany and Normandy?
From considerations such as these there are sufficient grounds, we
think, for constituting the clayslates and porphyries of the Grampians
into a “physical group,” existing in a “separate physical region.” The
absence of organic remains may be accounted for by the fact of the vast
disturbance prevailing in the seas at the period, and indicated by the
prodigious quantity of igneous matter spread repeatedly over their
bottom. These causes would act in so far in preventing the existence and
increase of living things, over all these parts, and most certainly in
obliterating the traces of their remains, if any were deposited. But as
future explorers may yet detect them in abundance we proceed to consider
the nature and classes of fossils elsewhere discovered in the Silurian
strata.
ANIMAL REMAINS. Here, in this series of rocks, we are carried back to
the beginning of life upon the globe, in which we see the very dawn and
commencement of earthly enjoyment, the first forms and races of creatures
which were privileged to eat at the banquet of creation. As matter of
history, therefore, nothing can be more interesting; as a subject of
mere curiosity concerning ancient relics, the most ardent archæologist
will be amply gratified; and as showing the manner of the divine actings
in replenishing the earth with living things, the word and the works
of Deity are again to the devout inquiring mind brought into pleasing
harmonious comparison.
We find that the creatures belonging to this first epoch of organic
existence are, generally, low in the scale of animated being. The
rocks in which their remains are imbedded are, in some instances almost
entirely composed of organic matter, showing that life at first was
not bestowed sparingly, or, through some hidden mysterious processes,
stealthily introduced upon the stage; it rather appears in an abundance
and variety, speaking of a purpose in obedience to a designing creative
act. As suitable to the condition of the planet, not at once but
by successive arrangements brought into a state of adaptation for
sustaining life, the animals now formed appear to have been chiefly of
the invertebrate division, that is, animals of comparatively simple
structure, destitute of a bony skeleton, suited to live in shallow waters
and muddy bottoms, and to be content with such fare as an infant state of
things over the young earth could produce. Among these ancient families
are graptolites,—many of them zoophytic bodies, allied to the modern
sea-pen; crinoids, or lily-shaped animals, of beautifully-developed
forms; and trilobites, crustacean creatures divided into three dorsal
lobes. There are several species of each. And so accurately has nature
adhered to her plan of operations, that we find the corals of that early
age doing the same offices, and piling up similar submarine reefs, by
which these busy little architects are still distinguished. The mollusca
of the period are very numerous, embracing almost every order and form
of shell that are found in our present seas, though wholly of different
species; conchifera, brachiopoda, gasteropoda, cephalopoda, pteropoda,
beside the heteropoda, of which there are no existing analogues. The
habits of all these orders must have been nearly the same as those of
our modern types. The cephalopoda, embracing the nautilus and orthoceras
tribes, were then as they are now, the tyrants of the deep, furnished
with eyes and ears, and armed with powers that enabled them to roam and
prey at will in the bays and estuaries of the primeval world. There have
been named and catalogued of these first forms of the moving creatures of
the deep about three hundred and fifty distinct species.
But, beside these, there have been discovered in the silurian rocks six
or seven genera, involving a still greater number of species, of fishes
of the order of the Placoids, so denominated from the broad scales or
plates with which they are covered. The probability is, that more of
these higher organisms will yet be brought to light, as all the strata
of the system consist of marine deposits, and only the most limited
sections have anywhere been explored. They constitute the lowest of
the fossiliferous beds; are generally found, except in Russia, in a
vertical or highly inclined position, and consequently but little of
their superficial area is exposed. Here, however, geologists have named
and described an Onchus Murchisoni, a Thelodus parvidens, and other four
genera of equally erudite-sounding names. The onchus type is continued,
and greatly multiplied in species, in the two succeeding formations, when
it dies out, or at least no trace of the genus is found in later times;
while the rest appear to come and to depart within their own geological
epoch. These organisms are all as yet termed Ichthyolites, that is,
simply fossil fragments of fish, as no entire animal has been anywhere
detected, while of their true class M. Agassiz affirms with confidence.
Teeth, fins, spines, occur so abundantly in a stratum of the Upper Ludlow
series in Wales as now to be termed “the bone-bed,” giving assurance that
the seas were thus early stocked with the finny tribes. The families of
most of these fishes have yet to be determined. But nature, though in her
operations “simpler than man’s wit would make her,” was still pretending
enough to be shaping out thus early the higher types of life.
The science which introduces to such sights and studies, occupies no mean
place among the various branches of human inquiry. To neglect to decipher
what is so indelibly recorded on these pages of creation, is willfully to
shut oneself out from what has been actually preserved for information—a
voice from the past, which speaks in the same distinct articulate
language as the present of the fiat of Omnipotence. No object is mean or
contemptible which divine wisdom has formed, and no subject is unworthy
of investigation which illustrates His ways and works during any period
of creation.
The mind, at this starting point of life, is curious to know what
amount of information can be obtained as to the organic structure and
specific characters of these first denizens of earth, so as to compare
them with the forms and species of the analogous families now existing.
The information derived from this first chapter in palæontology, we
believe is, that the earliest specimens of organization are as perfect
as the latest, each after its kind; and that, in these morning-days of
existence, nature at once stamped, with her plastic hand, her lineaments
of beauty and adaptation on everything she made. There is nothing omitted
to be afterward supplied—nothing formed defective in a single part or
organ that requires to be corrected. The first discoveries in geology
at once speak conclusively of a plan or course of creation derived from
the beginning—a power, not delegated, but linked forever with the first
intelligent cause—a world, through all its changes, continually presided
over and ruled by Him who made it.
VEGETABLE REMAINS were long wanting, and sought for in vain, to complete
during this period the picture of the ancient world, as described in
the pages of revelation. Geology, indeed, had everywhere sternly held
back the required evidence, and animals were announced to be the first
of living things. This, though contrary to all analogy with regard to
the conditions of animal subsistence, was generally received as a well
established dogma; and the earliest book of history was laid aside, or
its statements in these circumstances regarded as irrelevant. Vegetable
remains, however, have been detected in the oldest fossiliferous group
of rocks, and this apparent discrepancy has been forevermore disproved.
Fucoid plants are found in great abundance in the transition series of
Scandinavia as well as in the silurian strata of our own island. That
they are not more widely distributed is satisfactorily accounted for by
experiments which show that some species of plants entirely disappear in
water. A productive flora, therefore, may have existed from the earliest
period, but, unable to resist decomposition, all traces thereof have long
disappeared from the tablets of the earth.
Nay, so abundant in some quarters of the globe has vegetable matter been
at this period, that there are traces of beds, approximating to coal,
entirely composed of it, and the rocks inclosing these beds so charged
with bitumen and carbon as to be used as fuel. “The silurian strata of
the Scandinavian peninsula and the Island of Bornholm, contain,” says
Professor Forchhammer, “in their oldest parts, large beds of aluminous
slate, which is used in a great number of manufactories for making alum;
and this aluminous slate has the great advantage over those slates of the
carboniferous system of Germany and a part of France, that it contains
the sufficient quantity of potash which is required to make alum.” It
is well known that potash constitutes an ingredient in most vegetable
bodies; and that when a plant is burned there remains a skeleton of
this substance. Hence, possibly, the origin of the potash in the alum
slate. But the argument does not rest upon inference. The same authority
relates, that in Bornholm and in Scania, the southernmost part of Sweden,
this slate contains a great number of impressions of a fucoidal plant, of
which Liebmann has given minute botanical descriptions. Then, pursuing
his interesting tale of this first flora of creation, he says,—“According
to Professor Keilhau, Professor Bock, and M. Esmark, the same ceramites
occurs frequently in the aluminous silurian slate of Southern Norway.
Recently M. Hisinger has figured an imperfect specimen of it from Berg,
in the province of Ostergothland, in Sweden. Thus this fucus appears to
be characteristic of the alum slate of Scandinavia: and I can scarcely
doubt that the most characteristic properties of the alum slate, as
depending upon its carbon, its sulphur, and its potash, are derived from
the great quantity of sea-weed which has been mixed up with the clay,
and whose carbonaceous matter so affects the whole rock, that the slate
is used as fuel for boiling the aluminous liquor, and burning lime; and
in some parts of the province of Westergothland in Sweden, even small
courses of true coal occur. There can hardly remain any doubt that this
coal is derived from sea-weeds, of which fossil parts have been found,
for not the slightest trace of land plants has ever been discovered.”
These are instructive facts, yet greatly to be extended, when, we
question not, the land will also contribute of its flora to complete
our knowledge of the most ancient fossiliferous strata.—But recently,
bands of true coal have been discovered completely inclosed in this
group of rocks near Oporto, the town of which stands on a ridge of
granite, four or five miles wide, with mica-slate and gneiss resting on
both sides. To the eastward, these again are overlaid by sedimentary
rocks, chiefly clayslate; which, commencing on the coast about thirty
miles north of Oporto, run down and cross the Douro, about sixteen
miles above that town.—To the south of Vallango, the strata overlie a
deposit of anthracite in several beds, some of them from four to six feet
thick.—This coal is now worked in several pits, and principally sent to
Oporto. Along with it are beds of red sandstone and black carbonaceous
slates, with vegetable impressions too indistinct to be determined, but
strongly resembling ferns of the coal measures. In the shales above
this coal Mr. Sharpe, the discoverer, found many fossils, as orthides,
trilobites, and graptolites, most of them new species, but others well
known in the lower silurian rocks of Northern Europe. It would thus
appear that the coal deposits of Oporto are included in the silurian
formation, and are far below the usual level of the coal.
We cannot overvalue the theoretic importance of these discoveries, which
do not indeed bring to light any exuberant variety of the vegetable
tribes, such as the earth afterward threw out of her affluent bosom.
But they mark sufficiently the period when plants, according to the
geological reading of the history, first make their appearance on these
lithological pages: fucoids and algæ are there in abundance, to give
the vegetable portion of the narrative, as trilobites and molluscs form
unquestionably the predominating features of the animal department.
The coal-beds of Oporto—should their position turn out to be truly
defined—show the dawning of a terrestrial flora, not sparingly but
luxuriantly developed: and thus the silurian period may be regarded
throughout as sufficiently characterized by well-marked types of
vegetation, more doubtful in the higher forms, but determinate in the
acotyledonous and cryptogamic tribes which prevail indifferently from the
lower to the upper beds of the system. Nor do we require to overstrain
the statement, by questioning nature or revelation as to the species,
genera, orders, and classes of vegetables referred to in their respective
pages. They are coincident as to the great truth itself, that PLANTS
did exist in the earliest “days” of the earth’s history. As a science,
nothing is taught in the Sacred Record. None of the technicalities
of physical inquiry are employed. But a beautiful progression, and
elimination of one thing after another, are intimated. The light is
separated from the darkness. A firmament is set in the midst of the
waters. The first plant that burst from the soil had thus every element
provided which its nature and habits required—the light, to which it
turns and ever yearns after—the air, in which to perform its respiratory
functions—the water, from which to secrete the juices of circulation—and
a dry land, out of which to elaborate materials for its structure. This
is a Wisdom which is above all philosophy, instructing in the elements
and principles of things, long before botanical arrangements were dreamed
of, or “bushy dell” there was, where
“hoary-headed frosts
Fall in the fresh lap of the crimson rose.”
The silurian group of rocks is very widely extended, as in Britain,
France, Russia, the north-west of Asia; in South Africa, North and South
America, the Falkland Islands, and Australia. The most ancient physical
features of the Old World can almost be recalled, as we thus trace the
outline of the deposit, marking out, by its geographical distribution,
the primary islands and mountain peaks of the aboriginal land. How
changed the very face of things—continuity between states and kingdoms
where seas now roll—and all the great continents occupying the sites over
which the waters held unbounded sway!
[Illustration: Trilobites of the Silurian System.]