cryptogamic kind, a proportion which would probably be much increased if
we knew the whole Flora of that era. The ascertained dicotyledons, or
higher-class plants, are comparatively few in this formation; but it will
be found that they constantly increased as the globe grew older.
The master-form or type of the era was the _fern_, or breckan, of which
about one hundred and thirty species have already been ascertained as
entering into the composition of coal. {84a} The fern is a plant which
thrives best in warm, shaded, and moist situations. In tropical
countries, where these conditions abound, there are many more species
than in temperate climes, and some of these are arborescent, or of a
tree-like size and luxuriance. {84b} The ferns of the coal strata have
been of this magnitude, and that without regard to the parts of the earth
where they are found. In the coal of Baffin’s Bay, of Newcastle, and of
the torrid zone alike, are the fossil ferns arborescent, shewing clearly
that, in that era, the present tropical temperature, or one even higher,
existed in very high latitudes.
In the swamps and ditches of England there grows a plant called the
horse-tail (_equisetum_), having a succulent, erect, jointed stem, with
slender leaves, and a scaly catkin at the top. A second large section of
the plants of the carboniferous era were of this kind (_equisetaceæ_),
but, like the fern, reaching the magnitudes of trees. While existing
equiseta rarely exceed three feet in height, and the stems are generally
under half an inch in diameter, their kindred, entombed in the coal beds,
seem to have been generally fourteen or fifteen feet high, with stems
from six inches to a foot in thickness. Arborescent plants of this
family, like the arborescent ferns, now grow only in tropical countries,
and their being found in the coal beds in all latitudes is consequently
held as an additional proof, that at this era a warm climate was extended
much farther to the north than at present. It is to be remarked that
plants of this kind (forming two genera, the most abundant of which is
the _calamites_) are only represented on the present surface by plants of
the same _family_: the _species_ which flourished at this era gradually
lessen in number as we advance upwards in the series of rocks, and
disappear before we arrive at the tertiary formation.
The club-moss family (_lycopodiaceæ_) are other plants of the present
surface, usually seen in a lowly and creeping form in temperate
latitudes, but presenting species which rise to a greater magnitude
within the tropics. Many specimens of this family are found in the coal
beds; it is thought they have contributed more to the substance of the
coal than any other family. But, like the ferns and equisetaceæ, they
rise to a prodigious magnitude. The lepidodendra (so the fossil genus is
called) have probably been from sixty-five to eighty feet in height,
having at their base a diameter of about three feet, while their leaves
measured twenty inches in length. In the forests of the coal era, the
lepidodendra would enjoy the rank of firs in our forests, affording shade
to the only less stately ferns and calamites. The internal structure of
the stem, and the character of the seed-vessels, shew them to have been a
link between single-lobed and double-lobed plants, a fact worthy of note,
as it favours the idea that, in vegetable, as well as animal creation, a
progress has been observed, in conformity with advancing conditions. It
is also curious to find a missing link of so much importance in a genus
of plants which has long ceased to have a living place upon earth.
The other leading plants of the coal era are without representatives on
the present surface, and their characters are in general less clearly
ascertained. Amongst the most remarkable are—the _sigillaria_, of which
large stems are very abundant, shewing that the interior has been soft,
and the exterior fluted with separate leaves inserted in vertical rows
along the flutings—and the _stigmaria_, plants apparently calculated to
flourish in marshes or pools, having a short, thick, fleshy stem, with a
dome-shaped top, from which sprung branches of from twenty to thirty feet
long. Amongst monocotyledons were some palms, (_flabellaria_ and
_næggerathia_,) besides a few not distinctly assignable to any class.
The dicotyledons of the coal are comparatively few, though on the present
surface they are the most numerous sub-class. Besides some of doubtful
affinity, (_annularia_, _asterophyllites_, &c.,) there were a few of the
pine family, which seem to have been the highest class of trees of this
era, and are only as yet found in isolated cases, and in sandstone beds.
The first discovered lay in the Craigleith quarry, near Edinburgh, and
consisted of a stem about two feet thick, and forty-seven feet in length.
Others have since been found, both in the same situation, and at
Newcastle. Leaves and fruit being wanting, an ingenious mode of
detecting the nature of these trees was hit upon by Mr. Witham of
Lartington. Taking thin polished cross slices of the stem, and
subjecting them to the microscope, he detected the structure of the wood
to be that of a cone-bearing tree, by the presence of certain
“reticulations” which distinguish that family, in addition to the usual
radiating and concentric lines. That particular tree was concluded to be
an araucaria, a species now found in Norfolk Island, in the South Sea,
and in a few other remote situations. The coniferæ of this era form the
dawn of dicotyledenous trees, of which they may be said to be the
simplest type, and to which, it has already been noticed, the
lepidodendra are a link from the monocotyledons. The concentric rings of
the Craigleith and other coniferæ of this era have been mentioned. It is
interesting to find in these a record of the changing seasons of those
early ages, when as yet there were no human beings to observe time or
tide. They are clearly traced; but it is observed that they are more
slightly marked than is the case with their family at the present day, as
if the changes of temperature had been within a narrower range.
Such was the vegetation of the carbonigenous era, composed of forms at
the bottom of the botanical scale, flowerless, fruitless, but luxuriant
and abundant beyond what the most favoured spots on earth can now shew.
The rigidity of the leaves of its plants, and the absence of fleshy
fruits and farinaceous seeds, unfitted it to afford nutriment to animals;
and, monotonous in its forms, and destitute of brilliant colouring, its
sward probably unenlivened by any of the smaller flowering herbs, its
shades uncheered by the hum of insects, or the music of birds, it must
have been but a sombre scene to a human visitant. But neither man nor
any other animals were then in existence to look for such uses or such
beauties in this vegetation. It was serving other and equally important
ends, clearing (probably) the atmosphere of matter noxious to animal
life, and storing up mineral masses which were in long subsequent ages to
prove of the greatest service to the human race, even to the extent of
favouring the progress of its civilization.
The animal remains of this era are not numerous, in comparison with those
which go before, or those which come after. The mountain limestone,
indeed, deposited at the commencement of it, abounds unusually in
polypiaria and crinoidea; but when we ascend to the coal-beds themselves,
the case is altered, and these marine remains altogether disappear. We
have then only a limited variety of conchifers and shell mollusks, with
fragments of a few species of fishes, and these are rarely or never found
in the coal seams, but in the shales alternating with them. Some of the
fishes are of a sauroid character, that is, partake of the nature of the
lizard, a genus of the reptilia, a land class of animals, so that we may
be said here to have the first approach to a kind of animals calculated
to breathe the atmosphere. Such is the Megalichthys Hibbertii, found by
Dr. Hibbert Ware, in a limestone bed of fresh-water origin, underneath
the coal at Burdiehouse, near Edinburgh. Others of the same kind have
been found in the coal measures in Yorkshire, and in the low coal shales
at Manchester. This is no more than might be expected, as collections of
fresh water now existed, and it is presumable that they would be peopled.
The chief other fishes of the coal era are named palæothrissum,
palæoniscus, diperdus.
Coal strata are nearly confined to the group termed the carboniferous
formation. Thin beds are not unknown afterwards, but they occur only as
a rare exception. It is therefore thought that the most important of the
conditions which allowed of so abundant a terrestrial vegetation, had
ceased about the time when this formation was closed. The high
temperature was not one of the conditions which terminated, for there are
evidences of it afterwards; but probably the superabundance of carbonic
acid gas supposed to have existed during this era was expended before its
close. There can be little doubt that the infusion of a large dose of
this gas into the atmosphere at the present day would be attended by
precisely the same circumstances as in the time of the carboniferous
formation. Land animal life would not have a place on earth; vegetation
would be enormous; and coal strata would be formed from the vast
accumulations of woody matter, which would gather in every sea, near the
mouths of great rivers. On the exhaustion of the superabundance of
carbonic acid gas, the coal formation would cease, and the earth might
again become a suitable theatre of being for land animals.
The termination of the carboniferous formation is marked by symptoms of
volcanic violence, which some geologists have considered to denote the
close of one system of things and the beginning of another. Coal beds
generally lie in basins, as if following the curve of the bottom of seas.
But there is no such basin which is not broken up into pieces, some of
which have been tossed up on edge, others allowed to sink, causing the
ends of strata to be in some instances many yards, and in a few several
hundred feet, removed from the corresponding ends of neighbouring
fragments. These are held to be results of volcanic movements below, the
operation of which is further seen in numerous upbursts and intrusions of
volcanic rock (trap). That these disturbances took place about the close
of the formation, and not later, is shewn in the fact of the next higher
group of strata being comparatively undisturbed. Other symptoms of this
time of violence are seen in the beds of conglomerate which occur amongst
the first strata above the coal. These, as usual, consist of fragments
of the elder rocks, more or less worn from being tumbled about in
agitated water, and laid down in a mud paste, afterwards hardened.
Volcanic disturbances break up the rocks; the pieces are worn in seas;
and a deposit of conglomerate is the consequence. Of porphyry, there are
some such pieces in the conglomerate of Devonshire, three or four tons in
weight. It is to be admitted for strict truth that, in some parts of
Europe, the carboniferous formation is followed by superior deposits,
without the appearance of such disturbances between their respective
periods; but apparently this case belongs to the class of exceptions
already noticed. {93} That disturbance was general, is supported by the
further and important fact of the destruction of many forms of organic
being previously flourishing, particularly of the vegetable kingdom.
ERA OF THE NEW RED SANDSTONE.
TERRESTRIAL ZOOLOGY COMMENCES
WITH REPTILES.
FIRST TRACES OF BIRDS.
THE next volume of the rock series refers to an era distinguished by an
event of no less importance than the commencement of land animals. The
_New Red Sandstone System_ is subdivided into groups, some of which are
wanting in some places; they are pretty fully developed in the north of
England, in the following ascending order:—1. Lower red sandstone; 2.
Magnesian limestone; 3. Red and white sandstones and conglomerate; 4.
Variegated marls. Between the third and fourth there is, in Germany,
another group, called the Muschelkalk, a word expressing a limestone full
of shells.
The first group, containing the conglomerates already adverted to, seems
to have been produced during the time of disturbance which occurred so
generally after the carbonigenous era. This new era is distinguished by
a paucity of organic remains, as might partly be expected from the
appearances of disturbance, and the red tint of the rocks, the latter
being communicated by a solution of oxide of iron, a substance
unfavourable to animal life.
The second group is a limestone with an infusion of magnesia. It is
developed less generally than some others, but occurs conspicuously in
England and Germany. Its place, above the red sandstone, shews the
recurrence of circumstances favourable to animal life, and we accordingly
find in it not only zoophytes, conchifera, and a few tribes of fish, but
some faint traces of land plants, and a new and startling appearance—a
reptile of saurian (lizard) character, analogous to the now existing
family called monitors. Remains of this creature are found in
cupriferous (copper-bearing) slate connected with the mountain limestone,
at Mansfield and Glucksbrunn, in Germany, which may be taken as evidence
that dry land existed in that age near those places. The magnesia
limestone is also remarkable as the last rock in which appears the
leptæna, or producta, a conchifer of numerous species which makes a
conspicuous appearance in all previous seas. It is likewise to be
observed, that the fishes of this age, to the genera of which the names
palæoniscus, catopterus, platysomus, &c., have been applied, vanish, and
henceforth appear no more.
The third group, chiefly sandstones, variously coloured according to the
amount and nature of the metallic oxide infused into them, shews a
recurrence of agitation, and a consequent diminution of the amount of
animal life. In the upper part, however, of this group, there are
abundant symptoms of a revival of proper conditions for such life. There
are marl beds, the origin of which substance in decomposed shells is
obvious; and in Germany, though not in England, here occurs the
muschelkalk, containing numerous organic remains, (generally different
from those of the magnesian limestone,) and noted for the specimens of
land animals, which it is the first to present in any considerable
abundance to our notice.
These animals are of the vertebrate sub-kingdom, but of its lowest class
next after fishes,—namely, reptiles,—a portion of the terrestrial tribes
whose imperfect respiratory system perhaps fitted them for enduring an
atmosphere not yet quite suitable for birds or mammifers. {97} The
specimens found in the muschelkalk are allied to the crocodile and lizard
tribes of the present day, but in the latter instance are upon a scale of
magnitude as much superior to present forms as the lepidodendron of the
coal era was superior to the dwarf club-mosses of our time. These
saurians also combine some peculiarities of structure of a most
extraordinary character.
The animal to which the name _ichthyosaurus_ has been given, was as long
as a young whale, and it was fitted for living in the water, though
breathing the atmosphere. It had the vertebral column and general bodily
form of a fish, but to that were added the head and breast-bone of a
lizard, and the paddles of the whale tribes. The beak, moreover, was
that of a porpoise, and the teeth were those of a crocodile. It must
have been a most destructive creature to the fish of those early seas.
The _plesiosaurus_ was of similar bulk, with a turtle-like body and
paddles, shewing that the sea was its element, but with a long
serpent-like neck, terminating in a saurian head, calculated to reach
prey at a considerable distance. These two animals, of which many
varieties have been discovered, constituting distinct species, are
supposed to have lived in the shallow borders of the seas of this and
subsequent formations, devouring immense quantities of the finny tribes.
It was at first thought that no creatures approaching them in character
now inhabit the earth; but latterly Mr. Darwin has discovered, in the
reptile-peopled Galapagos Islands, in the South Sea, a marine saurian
from three to four feet long.
The _megalosaurus_ was an enormous lizard—a land creature, also
carnivorous. The _pterodactyle_ was another lizard, but furnished with
wings to pursue its prey in the air, and varying in size between a
cormorant and a snipe. Crocodiles abounded, and some of these were
herbivorous. Such was the iguanodon, a creature of the character of the
iguana of the Ganges, but reaching a hundred feet in length, or twenty
times that of its modern representative.
There were also numerous _tortoises_, some of them reaching a great size;
and Professor Owen has found in Warwickshire some remains of an animal of
the batrachian order, {99} to which, from the peculiar form of the teeth,
he has given the name of labyrinthidon. Thus, three of Cuvier’s four
orders of reptilia (_sauria_, _chelonia_, and _batrachia_) are
represented in this formation, the serpent order (_ophidia_) being alone
wanting.
The variegated marl beds which constitute the uppermost group of the
formation, present two additional genera of huge saurians,—the
phytosaurus and mastodonsaurus.
It is in the upper beds of the red sandstone that beds of salt first
occur. These are sometimes of such thickness, that the mine from which
the material has been excavated looks like a lofty church. We see in the
present world no circumstances calculated to produce the formation of a
bed of rock salt; yet it is not difficult to understand how such strata
were formed in an age marked by ultra-tropical heat and frequent volcanic
disturbances. An estuary, cut off by an upthrow of trap, or a change of
level, and left to dry up under the heat of the sun, would quickly become
the bed of a dense layer of rock salt. A second shift of level, or some
other volcanic disturbance, connecting it again with the sea, would
expose this stratum to being covered over with a layer of sand or mud,
destined in time to form the next stratum of rock above it.
The plants of this era are few and unobtrusive. Equiseta, calamites,
ferns, Voltzia, and a few of the other families found so abundantly in
the preceding formation, here present themselves, but in diminished size
and quantity.
This seems to be the proper place to advert to certain memorials of a
peculiar and unexpected character respecting these early ages in the
sandstones. So low as the bottom of the carboniferous system, slabs are
found marked over a great extent of surface with that peculiar
corrugation or wrinkling which the receding tide leaves upon a sandy
beach when the sea is but slightly agitated; and not only are these
ripple-marks, as they are called, found on the surfaces, but casts of
them are found on the under sides of slabs lying above. The phenomena
suggests the time when the sand ultimately formed into these stone slabs,
was part of the beach of a sea of the carbonigenous era; when, left wavy
by one tide, it was covered over with a thin layer of fresh sand by the
next, and so on, precisely as such circumstances might be expected to
take place at the present day. Sandstone surfaces, ripple-marked, are
found throughout the subsequent formations: in those of the new red, at
more than one place in England, they further bear impressions of
rain-drops which have fallen upon them—the rain, of course, of the
inconceivably remote age in which the sandstones were formed. In the
Greensill sandstone, near Shrewsbury, it has even been possible to tell
from what direction the shower came which impressed the sandy surface,
the rims of the marks being somewhat raised on one side, exactly as might
be expected from a slanting shower falling at this day upon one of our
beaches. These facts have the same sort of interest as the season rings
of the Craigleith conifers, as speaking of a parity between some of the
familiar processes of nature in those early ages and our own.
In the new red sandstone, impressions still more important in the
inferences to which they tend, have been observed,—namely, the footmarks
of various animals. In a quarry of this formation, at Corncockle Muir,
in Dumfriesshire, where the slabs incline at an angle of thirty-eight
degrees, the vestiges of an animal supposed to have been a tortoise are
distinctly traced up and down the slope, as if the creature had had
occasion to pass backwards and forwards in that direction only, possibly
in its daily visits to the sea. Some slabs similarly impressed, in the
Stourton quarries in Cheshire, are further marked with a shower of rain
which we know must have fallen _afterwards_, for its little hollows are
impressed in the footmarks also, though more slightly than on the rest of
the surface, the comparative hardness of a trodden place having
apparently prevented so deep an impression being made. At Hessberg, in
Saxony, the vestiges of four distinct animals have been traced, one of
them a web-footed animal of small size, considered as a congener of the
crocodile; another, whose footsteps having a resemblance to an impression
of a swelled human hand, has caused it to be named the _cheirotherium_.
The footsteps of the cheirotherium have been found also in the Stourton
quarries above mentioned. Professor Owen, who stands at the head of
comparative anatomy in the present day, has expressed his belief that
this last animal was the same batrachian of which he has found fragments
in the new red sandstone of Warwickshire. At Runcorn, near Manchester,
and elsewhere, have been discovered the tracks of an animal which Mr.
Owen calls the rynchosaurus, uniting with the body of a reptile the beak
and feet of a bird, and which clearly had been a _link_ between these two
classes.
If geologists shall ultimately give their approbation to the inferences
made from a recent discovery in America, we shall have the addition of
perfect birds, though probably of a low type, to the animal forms of this
era. It is stated to be in quarries of this rock, in the valley of
Connecticut, that footprints have been found, apparently produced by
birds of the order grallæ, or waders. “The footsteps appear in regular
succession on the continuous track of an animal, in the act of walking or
running, with the right and left foot always in their relative places.
The distance of the intervals between each footstep on the same track is
occasionally varied, but to no greater amount than may be explained by
the bird having altered its pace. Many tracks of different individuals
and different species are often found crossing each other, and crowded,
like impressions of feet upon the shores of a muddy stream, where ducks
and geese resort.” {103} Some of these prints indicate small animals,
but others denote birds of what would now be an unusually large size.
One animal, having a foot fifteen inches in length, (one-half more than
that of the ostrich,) and a stride of from four to six feet, has been
appropriately entitled, _ornithichnites giganteus_.
ERA OF THE OOLITE.
COMMENCEMENT OF MAMMALIA.
THE chronicles of this period consist of a series of beds, mostly
calcareous, taking their general name (_Oolite System_) from a
conspicuous member of them—the oolite—a limestone composed of an
aggregation of small round grains or spherules, and so called from its
fancied resemblance to a cluster of eggs, or the roe of a fish. This
texture of stone is novel and striking. It is supposed to be of chemical
origin, each spherule being an aggregation of particles round a central
nucleus. The oolite system is largely developed in England, France,
Westphalia, and Northern Italy; it appears in Northern India and Africa,
and patches of it exist in Scotland, and in the vale of the Mississippi.
It may of course be yet discovered in many other parts of the world.