Vestiges of the Natural History of Creation (Chapter 8)
by Robert Chambers
Era of the Oolite.
Commencement of Mammalia.
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.
The series, as shewn in the neighbourhood of Bath, is
(beginning with the 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 subgroups, 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
Rimmeridge 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."[1]
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."[2] 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
central 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.[3] 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 oolitie 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
insectirorous, 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 vertebrate (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 exuviae 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 lanes 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."[4] 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 southeast 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.
Notes
Murchison's Silurian System, p. 583.
Buckland.
In some instances, these fossils are found with
the contents of the stomach faithfully preserved, and even with pieces of
the external skin. The pellets ejected by them (coprolites) are
found in vast numbers, each generally enclosed in a nodule of ironstone, and
sometimes shewing remains of the fishes which had formed their food.
De la Beche's Geological Researches, p. 344.
| |
[ Robert Chambers,
Vestiges of the
Natural History of Creation, 1st edition, 1844; James Secord,
ed., Chicago: University of Chicago Press, 1994, pp. 105-115. ]
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