lowest) as follows:—1. Lias, a set of strata variously composed of
limestone, clay, marl, and shale, clay being predominant; 2. Lower
oolitic formation, including, besides the great oolite bed of central
England, fullers’ earth beds, forest marble, and cornbrash; 3. Middle
oolitic formation, composed of two sub-groups, the Oxford clay and coral
rag, the latter being a mere layer of the works of the coral polype; 4.
Upper oolitic formation, including what are called Kimmeridge clay and
Portland oolite. In Yorkshire there is an additional group above the
lias, and in Sutherlandshire there is another group above that again. In
the wealds (moorlands) of Kent and Sussex, there is, in like manner,
above the fourth of the Bath series, another additional group, to which
the name of the _Wealden_ has been given, from its situation, and which,
composed of sandstones and clays, is subdivided into Purbeck beds,
Hastings sand, and Weald clay.
There are no particular appearances of disturbance between the close of
the new red sandstone and the beginning of the oolite system, as far as
has been observed in England. Yet there is a great change in the
materials of the rocks of the two formations, shewing that while the
bottoms of the seas of the one period had been chiefly arenaceous, those
of the other were chiefly clayey and limy. And there is an equal
difference between the two periods in respect of both botany and zoology.
While the new red sandstone shews comparatively scanty traces of organic
creation, those in the oolite are extremely abundant, particularly in the
department of animals, and more particularly still of sea mollusca,
which, it has been observed, are always the more conspicuous in
proportion to the predominance of calcareous rocks. It is also
remarkable that the animals of the oolitic system are entirely different
in species from those of the preceding age, and that these species cease
before the next. In this system we likewise find that uniformity over
great space which has been remarked of the Faunas of earlier formations.
“In the equivalent deposits in the Himalaya Mountains, at Fernando Po, in
the region north of the Cape of Good Hope, and in the Run of Cutch, and
other parts of Hindostan, fossils have been discovered, which, as far as
English naturalists who have seen them can determine, are
undistinguishable from certain oolite and lias fossils of Europe.” {108a}
The dry land of this age presented cycadeæ, “a beautiful class of plants
between the palms and conifers, having a tall, straight trunk,
terminating in a magnificent crown of foliage.” {108b} There were tree
ferns, but in smaller proportion than in former ages; also equisetaceæ,
lilia, and conifers. The vegetation was generally analogous to that of
the Cape of Good Hope and Australia, which seems to argue a climate (we
must remember, a universal climate) between the tropical and temperate.
It was, however, sufficiently luxuriant in some instances to produce thin
seams of coal, for such are found in the oolite formation of both
Yorkshire and Sutherland. The sea, as for ages before, contained algæ,
of which, however, only a few species have been preserved to our day.
The lower classes of the inhabitants of the ocean were unprecedentedly
abundant. The polypiaria were in such abundance as to form whole strata
of themselves. The crinoidea and echinites were also extremely numerous.
Shell mollusks, in hundreds of new species, occupied the bottoms of the
seas of those ages, while of the swimming shell-fish, ammonites and
belemnites, there were also many scores of varieties. The belemnite here
calls for some particular notice. It commences in the oolite, and
terminates in the next formation. It is an elongated, conical shell,
terminating in a point, and having, at the larger end, a cavity for the
residence of the animal, with a series of air-chambers below. The
animal, placed in the upper cavity, could raise or depress itself in the
water at pleasure by a pneumatic operation upon the entral air tube
pervading its shell. Its tentacula, sent abroad over the summit of the
shell, searched the sea for prey. The creature had an ink-bag, with
which it could muddle the water around it, to protect itself from more
powerful animals, and, strange to say, this has been found so well
preserved that an artist has used it in one instance as a paint,
wherewith to delineate the belemnite itself.
The crustacea discovered in this formation are less numerous. There are
many fishes, some of which (_acrodus_, _psammodus_, &c.,) are presumed
from remains of their palatal bones, to have been of the gigantic
cartilaginous class, now represented by such as the cestraceon. It has
been considered by Professor Owen as worthy of notice, that, the
cestraceon being an inhabitant of the Australian seas, we have, in both
the botany and ichthyology of this period, an analogy to that continent.
The pycnodontes, (thick-toothed,) and lepidoides, (having thick scales,)
are other families described by M. Agassiz as extensively prevalent. In
the shallow waters of the oolitic formation, the ichthyosaurus,
plesiosaurus, and other huge saurian carnivora of the preceding age,
plied, in increased numbers, their destructive vocation. {110} To them
were added new genera, the cetiosaurus, mososaurus, and some others, all
of similar character and habits.
Land reptiles abounded, including species of the pterodactyle of the
preceding age—tortoises, trionyces, crocodilians—and the pliosaurus, a
creature which appears to have formed a link between the plesiosaurus and
the crocodile. We know of at least six species of the flying saurian,
the pterodactyle, in this formation.
Now, for the first time, we find remains of insects, an order of animals
not well calculated for fossil preservation, and which are therefore
amongst the rarest of the animal tribes found in rocks, though they are
the most numerous of all living families. A single libellula
(dragon-fly) was found in the Stonesfield slate, a member of the lower
oolitic group quarried near Oxford; and this was for several years the
only specimen known to exist so early; but now many species have been
found in a corresponding rock at Solenhofen, in Germany. It is
remarkable that the remains of insects are found most plentifully near
the remains of pterodactyles, to which undoubtedly they served as prey.
The first glimpse of the highest class of the vertebrate
sub-kingdom—_mammalia_—is obtained from the Stonesfield slate, where
there has been found the jaw-bone of a quadruped evidently insectivorous,
and inferred, from peculiarities in the structure of that small fragment,
to have belonged to the marsupial family, (pouched animals). It may be
observed, although no specimens of so high a class of animals as mammalia
are found earlier, such may nevertheless have existed: the defect may be
in our not having found them; but, other things considered, the
probability is that heretofore there were no mammifers. It is an
interesting circumstance that the first mammifers found should have
belonged to the marsupialia, when the place of that order in the scale of
creation is considered. In the imperfect structure of their brain,
deficient in the organs connecting the two hemispheres—and in the mode of
gestation, which is only in small part uterine—this family is clearly a
link between the oviparous vertebrata (birds, reptiles, and fishes) and
the higher mammifers. This is further established by their possessing a
faint development of two canals passing from near the anus to the
external surface of the viscera, which are fully possessed in reptiles
and fishes, for the purpose of supplying aerated water to the blood
circulating in particular vessels, but which are unneeded by mammifers.
Such rudiments of organs in certain species which do not require them in
any degree, are common in both the animal and vegetable kingdoms, but are
always most conspicuous in families approaching in character to those
classes to which the full organs are proper. This subject will be more
particularly adverted to in the sequel.
The highest part of the oolitic formation presents some phenomena of an
unusual and interesting character, which demand special notice.
Immediately above the upper oolitic group in Buckinghamshire, in the
vicinity of Weymouth, and other situations, there is a thin stratum,
usually called by workmen the _dirt-bed_, which appears, from
incontestable evidence, to have been a soil, formed, like soils of the
present day, in the course of time, upon a surface which had previously
been the bottom of the sea. The dirt-bed contains exuviæ of tropical
trees, accumulated through time, as the forest shed its honours on the
spot where it grew, and became itself decayed. Near Weymouth there is a
piece of this stratum, in which stumps of trees remain rooted, mostly
erect or slightly inclined, and from one to three feet high; while trunks
of the same forest, also silicified, lie imbedded on the surface of the
soil in which they grew.
Above this bed lie those which have been called the Wealden, from their
full development in the Weald of Sussex; and these as incontestably argue
that the dry land forming the dirt-bed had next afterwards become the
area of brackish estuaries, or lakes partially connected with the sea;
for the Wealden strata contain exuviæ of fresh-water tribes, besides
those of the great saurians and chelonia. The area of this estuary
comprehends the whole south-east province of England. A geologist thus
confidently narrates the subsequent events: “Much calcareous matter was
first deposited [in this estuary], and in it were entombed myriads of
shells, apparently analogous to those of the vivipara. Then came a thick
envelope of sand, sometimes interstratified with mud; and, finally, muddy
matter prevailed. The solid surface beneath the waters would appear to
have suffered a long continued and gradual depression, which was as
gradually filled, or nearly so, with transported matter; in the end,
however, after a depression of several hundred feet, the sea again
entered upon the area, not suddenly or violently—for the Wealden rocks
pass gradually into the superincumbent cretaceous series—but so quietly,
that the mud containing the remains of terrestrial and fresh-water
creatures was tranquilly covered up by sands replete with marine exuviæ.”
{114} A subsequent depression of the same area, to the depth of at least
three hundred fathoms, is believed to have taken place, to admit of the
deposition of the cretaceous beds lying above.
From the scattered way in which remains of the larger terrestrial animals
occur in the Wealden, and the intermixture of pebbles of the special
appearance of those worn in rivers, it is also inferred that the estuary
which once covered the south-east part of England was the mouth of a
river of that far-descending class of which the Mississippi and Amazon
are examples. What part of the earth’s surface presented the dry land
through which that and other similar rivers flowed, no one can tell for
certain. It has been surmised, that the particular one here spoken of
may have flowed from a point not nearer than the site of the present
Newfoundland. Professor Philips has suggested, from the analogy of the
mineral composition, that anciently elevated coal strata may have
composed the dry land from which the sandy matters of these strata were
washed. Such a deposit as the Wealden almost necessarily implies a
local, not a general condition; yet it has been thought that similar
strata and remains exist in the Pays de Bray, near Beauvais. This leads
to the supposition that there may have been, in that age, a series of
river-receiving estuaries along the border of some such great ocean as
the Atlantic, of which that of modern Sussex is only an example.
ERA OF THE CRETACEOUS FORMATION.
THE record of this period consists of a series of strata, in which chalk
beds make a conspicuous appearance, and which is therefore called the
cretaceous system or formation. In England, a long stripe, extending
from Yorkshire to Kent, presents the cretaceous beds upon the surface,
generally lying conformably upon the oolite, and in many instances rising
into bold escarpments towards the west. The celebrated cliffs of Dover
are of this formation. It extends into northern France, and thence
north-westward into Germany, whence it is traced into Scandinavia and
Russia. The same system exists in North America, and probably in other
parts of the earth not yet geologically investigated. Being a marine
deposit, it establishes that seas existed at the time of its formation on
the tracts occupied by it, while some of its organic remains prove that,
in the neighbourhood of those seas, there were tracts of dry land.
The cretaceous formation in England presents beds chiefly sandy in the
lowest part, chiefly clayey in the middle, and chiefly of chalk in the
upper part, the chalk beds being never absent, which some of the lower
are in several places. In the vale of the Mississippi, again, the true
chalk is wholly, or all but wholly absent. In the south of England, the
lower beds are, (reckoning from the lowest upwards), 1. _Shankland_ or
_greensand_, “a triple alternation of sands and sandstones with clay;” 2.
Galt, “a stiff blue or black clay, abounding in shells, which frequently
possess a pearly lustre;” 3. _Hard_ chalk; 4. Chalk with flints; these
two last being generally white, but in some districts red, and in others
yellow. The whole are, in England, about 1200 feet thick, shewing the
considerable depths of the ocean in which the deposits were made.
Chalk is a carbonate of lime, and the manner of its production in such
vast quantities was long a subject of speculation among geologists. Some
light seemed to be thrown upon the subject a few years ago, when it was
observed, that the detritus of coral reefs in the present tropical seas
gave a powder, undistinguishable, when dried, from ordinary chalk. It
then appeared likely that the chalk beds were the detritus of the corals
which were in the oceans of that era. Mr. Darwin, who made some curious
inquiries on this point, further suggested, that the matter might have
intermediately passed through the bodies of worms and fish, such as feed
on the corals of the present day, and in whose stomachs he has found
impure chalk. This, however, cannot be a full explanation of the
production of chalk, if we admit some more recent discoveries of
Professor Ehrenberg. That master of microscopic investigation announces,
that chalk is composed partly of “inorganic particles of irregular
elliptical structure and granular slaty disposition,” and partly of
shells of inconceivable minuteness, “varying from the one-twelfth to the
two hundred and eighty-eighth part of a line”—a cubic inch of the
substance containing above ten millions of them! The chalk of the north
of Europe contains, he says, a larger proportion of the inorganic matter;
that of the south, a larger proportion of the organic matter, being in
some instances almost entirely composed of it. He has been able to
classify many of these creatures, some of them being allied to the
nautili, nummuli, cyprides, &c. The shells of some are calcareous, of
others siliceous. M. Ehrenberg has likewise detected microscopic
sea-plants in the chalk.
The distinctive feature of the uppermost chalk beds in England, is the
presence of flint nodules. These are generally disposed in layers
parallel to each other. It was readily presumed by geologists that these
masses were formed by a chemical aggregation of particles of silica,
originally held in solution in the mass of the chalk. But whence the
silica in a substance so different from it? Ehrenberg suggests that it
is composed of the siliceous coverings of a portion of the microscopic
creatures, whose shells he has in other instances detected in their
original condition. It is remarkable that the chalk _with_ flint abounds
in the north of Europe; that _without_ flints in the south; while in the
northern chalk siliceous animalcules are wanting, and in the southern
present in great quantities. The conclusion seems but natural, that in
the one case the siliceous exuviæ have been left in their original form;
in the other dissolved chemically, and aggregated on the common principle
of chemical affinity into nodules of flint, probably concentrating, in
every instance, upon a piece of decaying organic matter, as has been the
case with the nodules of ironstone in the earlier rocks, and the
spherules of the oolite.
What is more remarkable, M. Ehrenberg has ascertained that at least
fifty-seven species of the microscopic animals of the chalk, being
infusoria and calcareous-shelled polythalamia, are still found living in
various parts of the earth. These species are the most abundant in the
rock. Singly they are the most unimportant of all animals, but in the
mass, forming as they do such enormous strata over a large part of the
earth’s surface, they have an importance greatly exceeding that of the
largest and noblest of the beasts of the field. Moreover, these species
have a peculiar interest, as the only specific types of that early age
which are reproduced in the present day. Species of sea mollusks, of
reptiles, and of mammifers, have been changed again and again, since the
cretaceous era; and it is not till a long subsequent age that we find the
first traces of any other of even the humblest species which now exist;
but here have these humble infusoria and polythalamia kept their place on
earth through all its revolutions since that time,—are we to say, safe in
their very humility, which might adapt them to a greater variety of
circumstances than most other animals, or are we required to look for
some other explanation of the phenomenon?
All the ordinary and more observable orders of the inhabitants of the
sea, except the cetacea, have been found in the cretaceous
formation—zoophytes, radiaria, mollusks, crustacea, (in great variety of
species,) and fishes in smaller variety. In Europe, remains of the
marine saurians have been found; they may be presumed to have become
extinct in that part of the globe before this time, their place and
destructive office being perhaps supplied by cartilaginous fishes, of
which the teeth are found in great quantities. In America, however,
remains of the plesiosaurus have been discovered in this part of the
stratified series. The reptiles, too, so numerous in the two preceding
periods, appear to have now much diminished in numbers. One, entitled
the mosæsaurus, seems to have held an intermediate place between the
monitor and iguana, and to have been about twenty-five feet long, with a
tail calculated to assist it powerfully in swimming. Crocodiles and
turtles existed, and amongst the fishes were some of a saurian character.
Fuci abounded in the seas of this era. Confervæ are found enclosed in
flints. Of terrestrial vegetation, as of terrestrial animals, the
specimens in the European area are comparatively rare, rendering it
probable that there was no dry land near. The remains are chiefly of
ferns, conifers, and cycadeæ, but in the two former cases we have only
cones and leaves. There have been discovered many pieces of wood,
containing holes drilled by the teredo, and thus shewing that they had
been long drifted about in the ocean before being entombed at the bottom.
The series in America corresponding to this, entitled the ferruginous
sand formation, presents fossils generally identical with those of
Europe, not excepting the fragments of drilled wood; shewing that, in
this, as in earlier ages, there was a parity of conditions for animal
life over a vast tract of the earth’s surface. To European reptiles, the
American formation adds a gigantic one, styled the saurodon, from the
lizard-like character of its teeth.
We have seen that footsteps of birds are considered to have been
discovered in America, in the new red sandstone. Some similar isolated
phenomena occur in the subsequent formations. Mr. Mantell discovered
some bones of birds, apparently waders, in the Wealden. The immediate
connexion of that set of birds with land, may account, of course, for
their containing a terrestrial organic relic, which the marine beds above
and below did not possess. In the slate of Glarus, in Switzerland,
corresponding to the English galt, in the chalk formation, the remains of
a bird have been found. From a chalk bed near Maidstone, have likewise
been extracted some remains of a bird, supposed to have been of the
long-winged swimmer family, and equal in size to the albatross. These,
it must be owned, are less strong traces of the birds than we possess of
the reptiles and other tribes; but it must be remembered, that the
evidence of fossils, as to the absence of any class of animals from a
certain period of the earth’s history, can never be considered as more
than negative. Animals, of which we find no remains in a particular
formation, may, nevertheless, have lived at the time, and it may have
only been from unfavourable circumstances that their remains have not
been preserved for our inspection. The single circumstance of their
being little liable to be carried down into seas, might be the cause of
their non-appearance in our quarries. There is at the same time a limit
to uncertainty on this point. We see, from what remains have been found
in the whole series, a clear progress throughout, from humble to superior
types of being. Hence we derive a light as to what animals may have
existed at particular times, which is in some measure independent of the
specialties of fossilology. The birds are below the mammalia in the
animal scale; and therefore they may be supposed to have existed about
the time of the new red sandstone and oolite, although we find but slight
traces of them in those formations, and, it may be said, till a
considerably later period.
ERA OF THE TERTIARY FORMATION.—
MAMMALIA ABUNDANT.
THE chalk-beds are the highest which extend over a considerable space;
but in hollows of these beds, comparatively limited in extent, there have
been formed series of strata—clays, limestones, marls, alternating—to
which the name of the _Tertiary Formation_ has been applied. London and
Paris alike rest on basins of this formation, and another such basin
extends from near Winchester, under Southampton, and re-appears in the
Isle of Wight. There is a patch, or fragment of the formation in one of
the Hebrides. A stripe of it extends along the east coast of North
America, from Massachusetts to Florida. It is also found in Sicily and
Italy, insensibly blended with formations still in progress. Though
comparatively a local formation, it is not of the less importance as a
record of the condition of the earth during a certain period. As in
other formations, it is marked, in the most distant localities, by
identity of organic remains.
The hollows filled by the tertiary formation must be considered as the
beds of estuaries left at the conclusion of the cretaceous period. We
have seen that an estuary, either by the drifting up of its mouth, or a
change of level in that quarter, may be supposed to have become an inland
sheet of water, and that, by another change, of the reverse kind, it may
be supposed to have become an estuary again. Such changes the Paris
basin appears to have undergone oftener than once, for, first, we have
there a fresh-water formation of clay and limestone beds; then, a
marine-limestone formation; next, a second fresh water formation, in
which the material of the celebrated _plaster of Paris_ (gypsum) is
included; then, a second marine formation of sandy and limy beds; and
finally, a third series of fresh-water strata. Such alternations occur
in other examples of the tertiary formation likewise.