more abundant, the number of species extended, and important additions
made in certain vestiges of fuci, or sea-plants, and of fishes. This
group of rocks has been called by English geologists, the _Silurian
System_, because largely developed at the surface of a district of
western England, formerly occupied by a people whom the Roman historians
call Silures. It is a series of sandstones, limestones, and beds of
shale (hardened mud), which are classed in the following sub-groups,
beginning with the undermost:—1, Llandillo rocks, (darkish calcareous
flagstones;) 2 and 3, two groups called Caradoc rocks; 4, Wenlock shale;
5, Wenlock limestone; 6, Lower Ludlow rocks, (shales and limestones;) 7,
Aymestry limestone; 8, Upper Ludlow rocks, (shales and limestone, chiefly
micaceous.) From the lowest beds upwards, there are polypiaria, though
most prevalent in the Wenlock limestone; conchifera, a vast number of
genera, but all of the order brachiopoda, (including terebratula,
pentamerus, spirifer, orthis, leptæna;) mollusca, of several orders and
many genera, (including turritella, orthoceras, nautilus, bellerophon;)
crustacea, all of them trilobites, (including trinucleus, asaphus,
calamene.) A little above the Llandillo rocks, there have been
discovered certain convoluted forms, which are now established as
annelids, or sea-worms, a tribe of creatures still existing, (nereidina
and serpulina,) and which may often be found beneath stones on a
sea-beach. One of these, figured by Mr. Murchison, is furnished with
feet in vast numbers all along its body, like a centipede. The
occurrence of annelids is important, on account of their character and
status in the animal kingdom. They are red-blooded and hermaphrodite,
and form a link of connexion between the annulosa (white-blooded worms)
and a humble class of the vertebrata. {62} The Wenlock limestone is most
remarkable amongst all the rocks of the Silurian system, for organic
remains. Many slabs of it are wholly composed of corals, shells, and
trilobites, held together by shale. It contains many genera of crinoidea
and polypiaria, and it is thought that some beds of it are wholly the
production of the latter creatures, or are, in other words, coral reefs
transformed by heat and pressure into rocks. Remains of fishes, of a
very minute size, have been detected by Mr. Philips in the Aymestry
limestone, being apparently the first examples of vertebrated animals
which breathed upon our planet. In the upper Ludlow rocks, remains of
six genera of fish have been for a longer period known; they belong to
the order of cartilaginous fishes, an order of mean organization and
ferocious habits, of which the shark and sturgeon are living specimens.
“Some were furnished with long palates, and squat, firmly-based teeth,
well adapted for crushing the strong-cased zoophytes and shells of the
period, fragments of which occur in the fœcal remains; some with teeth
that, like the fossil sharks of the later formations, resemble lines of
miniature pyramids, larger and smaller alternating; some with teeth
sharp, thin, and so deeply serrated, that every individual tooth
resembles a row of poniards set up against the walls of an armory; and
these last, says Agassiz, furnished with weapons so murderous, must have
been the pirates of the period. Some had their fins guarded with long
spines, hooked like the beak of an eagle; some with spines of straighter
and more slender form, and ribbed and furrowed longitudinally like
columns; some were shielded by an armour of bony points, and some thickly
covered with glistening scales.” {64}
The traces of fuci in this system are all but sufficient to allow of a
distinction of genera. In some parts of North America, extensive though
thin beds of them have been found. A distinguished French geologist, M.
Brogniart, has shewn that all existing marine plants are classifiable
with regard to the zones of climate; some being fitted for the torrid
zone, some for the temperate, some for the frigid. And he establishes
that the fuci of these early rocks speak of a torrid climate, although
they may be found in what are now temperate regions; he also states that
those of the higher rocks betoken, as we ascend, a gradually diminishing
temperature.
We thus early begin to find proofs of the general uniformity of organic
life over the surface of the earth, at the time when each particular
system of rocks was formed. Species identical with the remains in the
Wenlock limestone occur in the corresponding class of rocks in the Eifel,
and partially in the Harz, Norway, Russia, and Brittany. The situations
of the remains in Russia are fifteen hundred miles from the Wenlock beds;
but at the distance of between six and seven thousand from those,—namely,
in the vale of Mississippi, the same species are discovered. Uniformity
in animal life over large geographical areas argues uniformity in the
conditions of animal life; and hence arise some curious inferences.
Species, in the same low class of animals, are now much more limited; for
instance, the Red Sea gives different polypiaria, zoophytes, and
shell-fish, from the Mediterranean. It is the opinion of M. Brogniart,
that the uniformity which existed in the primeval times can only be
attributed to the temperature arising from the internal heat, which had
yet, as he supposes, been sufficiently great to overpower the ordinary
meteorological influences, and spread a tropical clime all over the
globe.
ERA OF THE OLD RED SANDSTONE—
FISHES ABUNDANT.
WE advance to a new chapter in this marvellous history—the era of the
_Old Red Sandstone System_. This term has been recently applied to a
series of strata, of enormous thickness in the whole mass, largely
developed in Herefordshire, Shropshire, Worcestershire, and South Wales;
also in the counties of Fife, Forfar, Moray, Cromarty, and Caithness; and
in Russia and North America, if not in many other parts of the world.
The particular strata forming the system are somewhat different in
different countries; but there is a general character to the extent of
these being a mixture of flagstones, marly rocks, and sandstones, usually
of a laminous structure, with conglomerates. There is also a schist
shewing the presence of bitumen; a remarkable new ingredient, since it is
a vegetable production. In the conglomerates, of great extent and
thickness, which form, in at least one district, the basis or leading
feature of the system, inclosing water-worn fragments of quartz and other
rocks, we have evidence of the seas of that period having been subjected
to a violent and long-continued agitation, probably from volcanic causes.
The upper members of the series bear the appearance of having been
deposited in comparatively tranquil seas. The English specimens of this
system shew a remarkable freedom from those disturbances which result in
the interjection of trap; and they are thus defective in mineral ores.
In some parts of England the old red sandstone system has been stated as
10,000 feet in thickness.
In this era, the forms of life which existed in the Silurian are
continued: we have the same orders of marine creatures, zoophyta,
polypiaria, conchifera, crustacea; but to these are added numerous
fishes, some of which are of most extraordinary and surprising forms.
Several of the strata are crowded with remains of fish, shewing that the
seas in which those beds were deposited had swarmed with that class of
inhabitants. The investigation of this system is recent; but already
{68} M. Agassiz has ascertained about twenty genera, and thrice the
number of species. And it is remarkable that the Silurian fishes are
here only represented in genera; the whole of the _species_ of that era
had already passed away. Even throughout the sub-groups of the system
itself, the species are changed; and these are phenomena observed
throughout all the subsequent systems or geological eras; apparently
arguing that, during the deposition of all the rocks, a gradual change of
physical conditions was constantly going on. A varying temperature, or
even a varying depth of sea, would at present be attended with similar
changes in marine life; and by analogy we are entitled to assume that
such variations in the ancient seas might be amongst the causes of that
constant change of genera and species in the inhabitants of those seas to
which the organic contents of the rocks bear witness.
Some of the fossils of this system,—the cephalaspis, coccosteus,
pterichthys, holoptychius—are, in form and structure, entirely different
from any fishes now existing, only the sturgeon family having any trace
of affinity to them in any respect. They seem to form a sort of
connecting link between the crustacea and true fishes.
The _cephalaspis_ may be considered as making the smallest advance from
the crustacean character; it very much resembles in form the asaphus of
lower formations, having a longish tail-like body inserted within the
cusp of a large crescent-shaped head, somewhat like a saddler’s
cutting-knife. The body is covered with strong plates of bone,
enamelled, and the head was protected on the upper side with one large
plate, as with a buckler—hence the name, implying _buckler-head_. A
range of small fins conveys the idea of its having been as weak in motion
as it is strong in structure. The _coccosteus_ may be said to mark the
next advance to fish creation. The outline of its body is of the form of
a short thick coffin, rounded, covered with strong bony plates, and
terminating in a long tail, which seems to have been the sole organ of
motion. It is very remarkable, that, while the tail establishes this
creature among the vertebrata and the fishes, its mouth has been opened
vertically, like those of the crustaceans, but which is contrary to the
mode of vertebrata generally. This seems a pretty strong mark of the
link character of the coccosteus between these two great departments of
the animal kingdom. The _pterichthys_ has also strong bony plates over
its body, arranged much like those of a tortoise, and has a long tail;
but its most remarkable feature, and that which has suggested its name,
is a pair of long and narrow wing-like appendages attached to the
shoulders, which the creature is supposed to have erected for its defence
when attacked by an enemy.
The _holoptychius_ is of a flat oval form, furnished with fins, and
ending in a long tail; the whole body covered with strong plates which
overlap each other, and the head forming only a slight rounded projection
from the general figure. The specimens in the lower beds are not above
the size of a flounder; but in the higher strata, to judge by the size of
the scales or plates which have been found, the creature attained a
comparatively monstrous size.
The other fishes of the system,—the osteolepis, glyptolepis, dipterus,
&c., are, in general outline, much like fishes still existing, but their
organization has, nevertheless, some striking peculiarities. They have
been entirely covered with bony scales or plates, enamelled externally;
their spines are tipped with bone, and, as one striking and unvarying
feature, the tail is only finned on the lower side. The internal
skeleton, of which no traces have been preserved, is presumed to have
been cartilaginous. They therefore unite the character of cartilaginous
fishes with a character peculiar to themselves, and in which we see
pretty clear vestiges of the pre-existent crustaceous form.
With regard to the link character of these animals, some curious facts
are mentioned. It appears that in the imperfect condition of the
vertebral column, and the inferior situation of the mouth in the
pterichthys, coccosteus, &c., there is an analogy to the form of the
dorsal cord and position of the mouth in the embryo of perfect fishes.
The one-sided form of the tail in the osteolepis &c. finds a similar
analogy in the form of the tail in the embryo of the salmon. It is not
premature to remark how broadly these facts seem to hint at a parity of
law affecting the progress of general creation, and the progress of an
individual fœtus of one of the more perfect animals.
It is equally ascertained of the types of being prevalent in the old red,
as of those of the preceding system, that they are uniform in the
corresponding strata of distant parts of the earth; for instance, Russia
and North America.
In the old red sandstone, the marine plants, of which faint traces are
observable in the Silurians, continue to appear. It would seem as if
less change took place in the vegetation than in the animals of those
early seas; and for this, as Mr. Miller has remarked, it is easy to
imagine reasons. For example, an infusion of lime into the sea would
destroy animal life, but be favourable to vegetation.
As yet there were no land animals or plants, and for this the presumable
reason is, that no dry land as yet existed. We are not left to make this
inference solely from the absence of land animals and plants; in the
arrangement of the primary (stratified) rocks, we have further evidence
of it. That these rocks were formed in a generally horizontal position,
we are as well assured as that they were formed at the bottom of seas.
But they are always found greatly inclined in position, tilted up against
the slopes of the granitic masses which are beneath them in geological
order, though often shooting up to a higher point in the atmosphere. No
doubt can be entertained that these granitic masses, forming our
principal mountain ranges, have been protruded from below, or, at least,
thrust much further up, _since_ the deposition of the primary rocks. The
protrusion was what tilted up the primary rocks; and the inference is, of
course, unavoidable, that these mountains have risen chiefly, at least,
since the primary rocks were laid down. It is remarkable that, while the
primary rocks thus incline towards granitic nuclei or axes, the strata
higher in the series rest against these again, generally at a less
inclination, or none at all, shewing that these strata were laid down
after the swelling mountain eminences had, by their protrusion, tilted up
the primary strata. And thus it may be said an era of local upthrowing
of the primitive and (perhaps) central matter of our planet, is
established as happening about the close of the primary strata, and
beginning of the next ensuing system. It may be called the _Era of the
Oldest Mountains_, or, more boldly, of the formation of the detached
portions of dry land over the hitherto watery surface of the globe—an
important part of the designs of Providence, for which the time was now
apparently come. It may be remarked, that volcanic disturbances and
protrusions of trap took place throughout the whole period of the
deposition of the primary rocks; but they were upon a comparatively
limited scale, and probably all took place under water. It was only now
that the central granitic masses of the great mountain ranges were thrown
up, carrying up with them broken edges of the primary strata; a process
which seems to have had this difference from the other, that it was the
effect of a more tremendous force exerted at a lower depth in the earth,
and generally acting in lines pervading a considerable portion of the
earth’s surface. We shall by-and-by see that the protrusion of some of
the mountain ranges was not completed, or did not stop, at that period.
There is no part of geological science more clear than that which refers
to the ages of mountains. It is as certain that the Grampian mountains
of Scotland are older than the Alps and Apennines, as it is that
civilization had visited Italy, and had enabled her to subdue the world,
while Scotland was the residence of “roving barbarians.” The Pyrenees,
Carpathians, and other ranges of continental Europe, are all younger than
the Grampians, or even the insignificant Mendip Hills of southern
England. Stratification tells this tale as plainly as Livy tells the
history of the Roman republic. It tells us—to use the words of Professor
Philips—that at the time when the Grampians sent streams and detritus to
straits where now the valleys of the Forth and Clyde meet, the greater
part of Europe was a wide ocean.
The last three systems—called, in England, the Cumbrian, Silurian, and
Devonian, and collectively the palæozoic rocks, from their containing the
remains of the earliest inhabitants of the globe—are of vast thickness;
in England, not much less than 30,000 feet, or nearly six miles. In
other parts of the world, as we have seen, the earliest of these systems
alone is of much greater depth—arguing an enormous profundity in the
ocean in which they were formed.
SECONDARY ROCKS.
ERA OF THE CARBONIFEROUS FORMATION.
LAND FORMED.
COMMENCEMENT OF LAND PLANTS.
WE now enter upon a new great epoch in the history of our globe. There
was now dry land. As a consequence of this fact, there was fresh water,
for rain, instead of immediately returning to the sea, as formerly, was
now gathered in channels of the earth, and became springs, rivers, and
lakes. There was now a theatre for the existence of land plants and
animals, and it remains to be inquired if these accordingly were
produced.
The Secondary Rocks, in which our further researches are to be
prosecuted, consist of a great and varied series, resting, generally
unconformably, against flanks of the upturned primary rocks, sometimes
themselves considerably inclined, at others, forming extensive basin-like
beds, nearly horizontal; in many places, much broken up and shifted by
disturbances from below. They have all been formed out of the materials
of the older rocks, by virtue of the wearing power of air and water,
which is still every day carrying down vast quantities of the elevated
matter of the globe into the sea. But the separate strata are each much
more distinct in the matter of its composition than might be expected.
Some are siliceous or arenaceous (sandstones), composed mainly of fine
grains from the quartz rocks—the most abundant of the primary strata.
Others are argillaceous—clays, shales, &c., chiefly derived, probably,
from the slate beds of the primary series. Others are calcareous,
derived from the early limestone. As a general feature, they are softer
and less crystalline than the primary rocks, as if they had endured less
of both heat and pressure than the senior formation. There are beds
(_coal_) formed solely of vegetable matter, and some others in which the
main ingredient is particles of iron, (_the iron black band_.) The
secondary rocks are quite as communicative with regard to their portion
of the earth’s history as the primitive were.
The first, or lowest, group of the secondary rocks is called the
_Carboniferous Formation_, from the remarkable feature of its numerous
interspersed beds of coal. It commences with the beds of the _mountain
limestone_, which, in some situations, as in Derbyshire and Ireland, are
of great thickness, being alternated with chert (a siliceous sandstone),
sandstones, shales, and beds of coal, generally of the harder and less
bituminous kind (_anthracite_), the whole being covered in some places by
the millstone grit, a siliceous conglomerate composed of the detritus of
the primary rocks. The mountain limestone, attaining in England to a
depth of eight hundred yards, greatly exceeds in volume any of the
primary limestone beds, and shews an enormous addition of power to the
causes formerly suggested as having produced this substance. In fact,
remains of corals, crinoidea, and shells, are so abundant in it, as to
compose three-fourths of the mass in some parts. Above the mountain
limestone commence the more conspicuous _coal beds_, alternating with
sandstones, shales, beds of limestone, and ironstone. Coal is altogether
composed of the matter of a terrestrial vegetation, transmuted by
pressure. Some fresh-water shells have been found in it, but few of
marine origin, and no remains of those zoophytes and crinoidea so
abundant in the mountain limestone and other rocks. Coal beds exist in
Europe, Asia, and America, and have hitherto been esteemed as the most
valuable of mineral productions, from the important services which the
substance renders in manufactures and in domestic economy. It is to be
remarked, that there are some local variations in the arrangement of coal
beds. In France, they rest immediately on the granite and other primary
rocks, the intermediate strata not having been found at those places. In
America, the kind called anthracite occurs among the slate beds, and this
species also abounds more in the mountain limestone than with us. These
last circumstances only shew that different parts of the earth’s surface
did not all witness the same events of a certain fixed series exactly at
the same time. There had been an exhibition of dry land about the site
of America, a little earlier than in Europe.
Some features of the condition of the earth during the deposition of the
carboniferous group, are made out with a clearness which must satisfy
most minds. First we are told of a time when carbonate of lime was
formed in vast abundance at the bottoms of profound seas, accompanied by
an unusually large population of corals and encrinites; while in some
parts of the earth there were patches of dry land, covered with a
luxuriant vegetation. Next we have a comparatively brief period of
volcanic disturbance, (when the conglomerate was formed.) Then the
causes favourable to the so abundant production of limestone, and the
large population of marine acrita, decline, and we find the masses of dry
land increase in number and extent, and begin to bear an amount of forest
vegetation, far exceeding that of the most sheltered tropical spots of
the present surface. The climate, even in the latitude of Baffin’s Bay,
was torrid, and perhaps the atmosphere contained a larger charge of
carbonic acid gas (the material of vegetation) than it now does. The
forests or thickets of the period, included no species of plants now
known upon earth. They mainly consisted of gigantic shrubs, which are
either not represented by any existing types, or are akin to kinds which
are now only found in small and lowly forms. That these forests grew
upon a Polynesia, or multitude of small islands, is considered probable,
from similar vegetation being now found in such situations within the
tropics. With regard to the circumstances under which the masses of
vegetable matter were transformed into successive coal strata, geologists
are divided. From examples seen at the present day, at the mouths of
such rivers as the Mississippi, which traverse extensive sylvan regions,
and from other circumstances to be adverted to, it is held likely by some
that the vegetable matter, the rubbish of decayed forests, was carried by
rivers into estuaries, and there accumulated in vast natural rafts, until
it sunk to the bottom, where an overlayer of sand or mud would prepare it
for becoming a stratum of coal. Others conceive that the vegetation
first went into the condition of a peat moss, that a sink in the level
then exposed it to be overrun by the sea, and covered with a layer of
sand or mud; that a subsequent uprise made the mud dry land, and fitted
it to bear a new forest, which afterwards, like its predecessor, became a
bed of peat; that, in short, by repetitions of this process, the
alternate layers of coal, sandstone, and shale, constituting the
carboniferous group, were formed. It is favourable to this last view
that marine fossils are scarcely found in the body of the coal itself,
though abundant in the shale layers above and below it; also that in
several places erect stems of trees are found with their roots still
fixed in the shale beds, and crossing the sandstone beds at almost right
angles, shewing that these, at least, had not been drifted from their
original situations. On the other hand, it is not easy to admit such
repeated risings and sinkings of surface as would be required, on this
hypothesis, to form a series of coal strata. Perhaps we may most safely
rest at present with the supposition that coal has been formed under both
classes of circumstances, though in the latter only as an exception to
the former.
Upwards of three hundred species of plants have been ascertained to exist
in the coal formation; but it is not necessary to suppose that the whole
contained in that system are now, or ever will be distinguished.
Experiments shew that some great classes of plants become decomposed in
water in a much less space of time than others, and it is remarkable that
those which decompose soonest, are of the classes found most rare, or not
at all, in the coal strata. It is consequently to be inferred that there
may have been grasses and mosses at this era, and many species of trees,
the remains of which had lost all trace of organic form before their
substance sunk into the mass of which coal was formed. In speaking,
therefore, of the vegetation of this period, we must bear in mind that it
may have comprehended forms of which we have no memorial.
Supposing, nevertheless, that, in the main, the ascertained vegetation of
the coal system is that which grew at the time of its formation, it is
interesting to find that the terrestrial botany of our globe begins with
classes of comparatively simple forms and structure. In the ranks of the
vegetable kingdom, the lowest place is taken by plants of cellular
tissue, and which have no flowers, (_cryptogamia_,) as lichens, mosses,
fungi, ferns, sea-weeds. Above these stand plants of vascular tissue,
and bearing flowers, in which again there are two great subdivisions;
first, plants having one seed-lobe, (_monocotyledons_,) and in which the
new matter is added within, (_endogenous_,) of which the cane and palm
are examples; second, plants having two seed-lobes, (_dicotyledons_,) and
in which the new matter is added on the outside under the bark,
(_exogenous_,) of which the pine, elm, oak, and most of the British
forest-trees are examples; these subdivisions also ranking in the order
in which they are here stated. Now it is clear that a predominance of
these forms in succession marked the successive epochs developed by
fossil geology; the simple abounding first, and the complex afterwards.