OOLITE AND LIAS.
Subdivisions of the Oolitic or Jurassic group--Physical geography of
the Oolite in England and France--Upper Oolite--Portland stone and
fossils--Lithographic stone of Solenhofen--Middle Oolite, coral
rag--Zoophytes--Nerinaean limestone--Diceras limestone--Oxford clay,
Ammonites and Belemnites--Lower Oolite, Crinoideans--Great Oolite and
Bradford clay--Stonesfield slate--Fossil mammalia, placental and
marsupial--Resemblance to an Australian fauna--Doctrine of progressive
development--Collyweston slates--Yorkshire Oolitic coal-field--Brora
coal--Inferior Oolite and fossils.
_OOLITIC OR JURASSIC GROUP._--Below the freshwater group called the
Wealden, or, where this is wanting, immediately beneath the Cretaceous
formation, a great series of marine strata, commonly called "the Oolite,"
occurs in England and many other parts of Europe. This group has been so
named, because, in the countries where it was first examined, the
limestones belonging to it had an oolitic structure (see p. 12.). These
rocks occupy in England a zone which is nearly 30 miles in average breadth,
and extends across the island, from Yorkshire in the north-east, to
Dorsetshire in the south-west. Their mineral characters are not uniform
throughout this region; but the following are the names of the principal
subdivisions observed in the central and south-eastern parts of England:--
OOLITE.
Upper { _a._ Portland stone and sand.
{ _b._ Kimmeridge clay.
Middle { _c._ Coral rag.
{ _d._ Oxford clay.
Lower { _e._ Cornbrash and Forest marble.
{ _f._ Great Oolite and Stonesfield slate.
{ _g._ Fuller's earth.
{ _h._ Inferior Oolite.
The Lias then succeeds to the Inferior Oolite.
The Upper oolitic system of the above table has usually the Kimmeridge
clay for its base; the Middle oolitic system, the Oxford clay. The Lower
system reposes on the Lias, an argillo-calcareous formation, which some
include in the Lower Oolite, but which will be treated of separately in
the next chapter. Many of these subdivisions are distinguished by
peculiar organic remains; and though varying in thickness, may be traced
in certain directions for great distances, especially if we compare the
part of England to which the above-mentioned type refers with the
north-east of France, and the Jura mountains adjoining. In that country,
distant above 400 geographical miles, the analogy to the English type,
notwithstanding the thinness, or occasional absence of the clays, is
more perfect than in Yorkshire or Normandy.
_Physical geography._--The alternation, on a grand scale, of distinct
formations of clay and limestone, has caused the oolitic and liassic series
to give rise to some marked features in the physical outline of parts of
England and France. Wide valleys can usually be traced throughout the long
bounds of country where the argillaceous strata crop out; and between these
valleys the limestones are observed, composing ranges of hills, or more
elevated grounds. These ranges terminate abruptly on the side on which the
several clays rise up from beneath the calcareous strata.
[Illustration: Fig. 266. Cross section.]
The annexed diagram will give the reader an idea of the configuration of
the surface now alluded to, such as may be seen in passing from London to
Cheltenham, or in other parallel lines, from east to west, in the southern
part of England. It has been necessary, however, in this drawing, greatly
to exaggerate the inclination of the beds, and the height of the several
formations, as compared to their horizontal extent. It will be remarked,
that the lines of cliff, or escarpment, face towards the west in the great
calcareous eminences formed by the Chalk and the Upper, Middle, and Lower
Oolites; and at the base of which we have respectively the Gault,
Kimmeridge clay, Oxford clay, and Lias. This last forms, generally, a broad
vale at the foot of the escarpment of inferior oolite, but where it
acquires considerable thickness, and contains solid beds of marlstone, it
occupies the lower part of the escarpment.
The external outline of the country which the geologist observes in
travelling eastward from Paris to Metz is precisely analogous, and is
caused by a similar succession of rocks intervening between the tertiary
strata and the Lias; with this difference, however, that the escarpments
of Chalk, Upper, Middle, and Lower Oolites, face towards the east
instead of the west.
The Chalk crops out from beneath the tertiary sands and clays of the Paris
basin, near Epernay, and the Gault from beneath the Chalk and Upper
Greensand at Clermont-en-Argonne; and passing from this place by Verdun and
Etain to Metz, we find two limestone ranges, with intervening vales of
clay, precisely resembling those of southern and central England, until we
reach the great plain of Lias at the base of the Inferior Oolite at Metz.
It is evident, therefore, that the denuding causes have acted similarly
over an area several hundred miles in diameter, sweeping away the softer
clays more extensively than the limestones, and undermining these last so
as to cause them to form steep cliffs wherever the harder calcareous rock
was based upon a more yielding and destructible clay. This denudation
probably occurred while the land was slowly rising out of the sea.[259-A]
_Upper Oolite._
The Portland stone has already been mentioned as forming in Dorsetshire
the foundation on which the freshwater limestone of the Lower Purbeck
reposes (see p. 232.). It supplies the well-known building stone of
which St. Paul's and so many of the principal edifices of London are
constructed. This upper member, characterized by peculiar marine
fossils, rests on a dense bed of sand, called the Portland sand, below
which is the Kimmeridge clay. In England these Upper Oolite formations
are almost wholly confined to the southern counties. Corals are rare in
them, although one species is found plentifully at Tisbury, in
Wiltshire, in the Portland sand converted into flint and chert, the
original calcareous matter being replaced by silex (fig. 267.).
[Illustration: Fig. 267. _Columnaria oblonga_, Blainv.
As seen on a polished slab of chert from the sand of the
Upper Oolite, Tisbury.]
Among the characteristic fossils of the Upper Oolite, may be mentioned
the _Ostrea deltoidea_ (fig. 269.), found in the Kimmeridge clay
throughout England and the north of France, and also in Scotland, near
Brora. The _Gryphaea virgula_ (fig. 268.), also met with in the same clay
near Oxford, is so abundant in the Upper Oolite of parts of France as to
have caused the deposit to be termed "marnes a gryphees virgules." Near
Clermont, in Argonne, a few leagues from St. Menehould, where these
indurated marls crop out from beneath the gault, I have seen them, on
decomposing, leave the surface of every ploughed field literally strewed
over with this fossil oyster.
[2 Illustrations: Upper Oolite: Kimmeridge clay. 1/4 nat. size.
Fig. 268. _Gryphaea virgula._
Fig. 269. _Ostrea deltoidea._]
[Illustration: Fig. 270. _Trigonia gibbosa._ 1/2 nat. size. _a._ the hinge.
Portland Oolite, Tisbury.]
The Kimmeridge clay consists, in great part, of a bituminous shale,
sometimes forming an impure coal several hundred feet in thickness. In
some places in Wiltshire it much resembles peat; and the bituminous
matter may have been, in part at least, derived from the decomposition
of vegetables. But as impressions of plants are rare in these shales,
which contain ammonites, oysters, and other marine shells, the bitumen
may perhaps be of animal origin.
The celebrated lithographic stone of Solenhofen, in Bavaria, belongs to one
of the upper divisions of the oolite, and affords a remarkable example of
the variety of fossils which may be preserved under favourable
circumstances, and what delicate impressions of the tender parts of certain
animals and plants may be retained where the sediment is of extreme
fineness. Although the number of testacea in this slate is small, and the
plants few, and those all marine, Count Munster had determined no less than
237 species of fossils when I saw his collection in 1833; and among them no
less than seven _species_ of flying lizards, or pterodactyls, six saurians,
three tortoises, sixty species of fish, forty-six of crustacea, and
twenty-six of insects. These insects, among which is a libellula, or
dragon-fly, must have been blown out to sea, probably from the same land to
which the flying lizards, and other contemporaneous reptiles, resorted.
_Middle Oolite._
_Coral Rag._--One of the limestones of the Middle Oolite has been called
the "Coral Rag," because it consists, in part, of continuous beds of
petrified corals, for the most part retaining the position in which they
grew at the bottom of the sea. They belong chiefly to the genera
_Caryophyllia_ (fig. 271.), _Agaricia_, and _Astrea_, and sometimes form
masses of coral 15 feet thick. In the annexed figure of an _Astrea_, from
this formation, it will be seen that the cup-shaped cavities are deepest on
the right-hand side, and that they grow more and more shallow, till those
on the left side are nearly filled up. The last-named stars are supposed to
be Polyparia of advanced age. These coralline strata extend through the
calcareous hills of the N.W. of Berkshire, and north of Wilts, and again
recur in Yorkshire, near Scarborough.
[Illustration: Fig. 271. _Caryophyllia annularis_, Parkin. Coral
rag, Steeple Ashton.]
[Illustration: Fig 272. _Astrea._ Coral rag.]
One of the limestones of the Jura, referred to the age of the English
coral rag, has been called "Nerinaean limestone" (Calcaire a Nerinees) by
M. Thirria; _Nerinaea_ being an extinct genus of univalve shells, much
resembling the _Cerithium_ in external form. The annexed section (fig.
273.) shows the curious form of the hollow part of each whorl, and also
the perforation which passes up the middle of the columella. _N.
Goodhallii_ (fig. 274.) is another English species of the same genus,
from a formation which seems to form a passage from the Kimmeridge clay
to the coral rag.[261-A]
[Illustration: Fig. 273. _Nerinaea hieroglyphica._ Coral rag.]
[Illustration: Fig. 274. _Nerinaea Goodhallii_, Fitton. Coral rag,
Weymouth. 1/4 nat. size.]
A division of the oolite in the Alps, regarded by most geologists as coeval
with the English coral rag, has been often named "Calcaire a Dicerates," or
"Diceras limestone," from its containing abundantly a bivalve shell (see
fig. 275.) of a genus allied to the _Chama_.
[Illustration: Fig. 275. Cast of _Diceras arietina_. Coral rag, France.]
[Illustration: Fig. 276. _Cidaris coronata._ Coral rag.]
_Oxford Clay._--The coralline limestone, or "coral rag," above described,
and the accompanying sandy beds, called "calcareous grits" of the Middle
Oolite, rests on a thick bed of clay, called the Oxford clay, sometimes not
less than 500 feet thick. In this there are no corals, but great abundance
of cephalopoda of the genera Ammonite and Belemnite. (See fig. 277.) In
some of the clay of very fine texture ammonites are very perfect, although
somewhat compressed, and are seen to be furnished on each side of the
aperture with a single horn-like projection (see fig. 278.). These were
discovered in the cuttings of the Great Western Railway, near Chippenham,
in 1841, and have been described by Mr. Pratt.[262-A]
[Illustration: Fig. 277. _Belemnites hastatus._ Oxford Clay.]
[Illustration: Fig. 278. _Ammonites Jason_, Reinecke. Syn. _A. Elizabethae_,
Pratt. Oxford clay, Christian Malford, Wiltshire.]
[Illustration: Fig. 279. _Belemnites Puzosianus_, D'Orb. Oxford
Clay, Christian Malford.
_a, a._ projecting processes of the shell or phragmocone.
_b, c._ broken exterior of a conical shell called the phragmocone, which
is chambered within, or composed of a series of shallow concave
cells pierced by a siphuncle.
_c, d._ The guard or osselet, which is commonly called the belemnite.]
Similar elongated processes have been also observed to extend from the
shells of some belemnites discovered by Dr. Mantell in the same clay
(see fig. 279.), who, by the aid of this and other specimens, has been
able to throw much light on the structure of this singular extinct
form of cuttle-fish.[263-A]
_Lower Oolite._
The upper division of this series, which is more extensive than the
preceding or Middle Oolite, is called in England the Cornbrash. It
consists of clays and calcareous sandstones, which pass downwards into
the Forest marble, an argillaceous limestone, abounding in marine
fossils. In some places, as at Bradford, this limestone is replaced by a
mass of clay. The sandstones of the Forest Marble of Wiltshire are often
ripple-marked and filled with fragments of broken shells and pieces of
drift-wood, having evidently been formed on a coast. Rippled slabs of
fissile oolite are used for roofing, and have been traced over a broad
band of country from Bradford, in Wilts, to Tetbury, in Gloucestershire.
These calcareous tile-stones are separated from each other by thin seams
of clay, which have been deposited upon them, and have taken their form,
preserving the undulating ridges and furrows of the sand in such
complete integrity, that the impressions of small footsteps, apparently
of crabs, which walked over the soft wet sands, are still visible. In
the same stone the claws of crabs, fragments of echini, and other signs
of a neighbouring beach are observed.[263-B]
_Great Oolite._--Although the name of coral-rag has been appropriated,
as we have seen, to a member of the Upper Oolite before described,
some portions of the Lower Oolite are equally intitled in many places
to be called coralline limestones. Thus the Great Oolite near Bath
contains various corals, among which the _Eunomia radiata_ (fig. 280.)
is very conspicuous, single individuals forming masses several feet
in diameter; and having probably required, like the large existing
brain-coral (_Meandrina_) of the tropics, many centuries before
their growth was completed.
[Illustration: Fig. 280. _Eunomia radiata_, Lamouroux.
_a._ section transverse to the tubes.
_b._ vertical section, showing the radiation of the tubes.
_c._ portion of interior of tubes magnified, showing striated surface.]
[Illustration: Fig. 281. _Apiocrinites rotundus_, or Pear Encrinite;
Miller. Fossil at Bradford, Wilts.
_a._ Stem of _Apiocrinites_, and one of the articulations, natural size.
_b._ Section at Bradford of great oolite and overlying clay, containing
the fossil encrinites. See text.
_c._ Three perfect individuals of Apiocrinites, represented as they grew
on the surface of the Great Oolite.
_d._ Body of the _Apiocrinites rotundus_.]
Different species of _Crinoideans_, or stone-lilies, are also common in the
same rocks with corals; and, like them, must have enjoyed a firm bottom,
where their root, or base of attachment, remained undisturbed for years
(_c_, fig. 281.). Such fossils, therefore, are almost confined to the
limestones; but an exception occurs at Bradford, near Bath, where they are
enveloped in clay. In this case, however, it appears that the solid upper
surface of the "Great Oolite" had supported, for a time, a thick submarine
forest of these beautiful zoophytes, until the clear and still water was
invaded by a current charged with mud, which threw down the stone-lilies,
and broke most of their stems short off near the point of attachment. The
stumps still remain in their original position; but the numerous
articulations once composing the stem, arms, and body of the zoophyte, were
scattered at random through the argillaceous deposit in which some now lie
prostrate. These appearances are represented in the section _b_, fig. 281.,
where the darker strata represent the Bradford clay, which some geologists
class with the Forest marble, others with the Great Oolite. The upper
surface of the calcareous stone below is completely incrusted over with a
continuous pavement, formed by the stony roots or attachments of the
Crinoidea; and besides this evidence of the length of time they had lived
on the spot, we find great numbers of single joints, or circular plates of
the stem and body of the encrinite, covered over with _serpulae_. Now these
_serpulae_ could only have begun to grow after the death of some of the
stone-lilies, parts of whose skeletons had been strewed over the floor of
the ocean before the irruption of argillaceous mud. In some instances we
find that, after the parasitic _serpulae_ were full grown, they had become
incrusted over with a coral, called _Berenicea diluviana_; and many
generations of these polyps had succeeded each other in the pure water
before they became fossil.
[Illustration: Fig. 282.
_a._ Single plate or articulation of an Encrinite overgrown with
_serpulae_ and corals. Natural size Bradford clay.
_b._ Portion of the same magnified, showing the coral _Berenicea_
_diluviana_ covering one of the _serpulae_.]
We may, therefore, perceive distinctly that, as the pines and cycadeous
plants of the ancient "dirt bed," or fossil forest, of the Lower Purbeck
were killed by submergence under fresh water, and soon buried beneath
muddy sediment, so an invasion of argillaceous matter put a sudden stop
to the growth of the Bradford Encrinites, and led to their preservation
in marine strata.[265-A]
Such differences in the fossils as distinguish the calcareous and
argillaceous deposits from each other, would be described by naturalists
as arising out of a difference in the _stations_ of species; but besides
these, there are variations in the fossils of the higher, middle, and
lower part of the oolitic series, which must be ascribed to that great
law of change in organic life by which distinct assemblages of species
have been adapted, at successive geological periods, to the varying
conditions of the habitable surface. In a single district it is
difficult to decide how far the limitation of species to certain minor
formations has been due to the local influence of _stations_, or how
far it has been caused by time or the creative and destroying law above
alluded to. But we recognize the reality of the last-mentioned
influence, when we contrast the whole oolitic series of England with
that of parts of the Jura, Alps, and other distant regions, where there
is scarcely any lithological resemblance; and yet some of the same
fossils remain peculiar in each country to the Upper, Middle, and Lower
Oolite formations respectively. Mr. Thurmann has shown how remarkably
this fact holds true in the Bernese Jura, although the argillaceous
divisions, so conspicuous in England, are feebly represented there,
and some entirely wanting.
[Illustration: Fig. 283. _Terebratula digona._ Bradford clay. Nat. size.]
The Bradford clay above alluded to is sometimes 60 feet thick, but, in many
places, it is wanting; and, in others, where there are no limestones, it
cannot easily be separated from the clays of the overlying "forest marble"
and underlying "fuller's earth."
The calcareous portion of the Great Oolite consists of several shelly
limestones, one of which, called the Bath Oolite, is much celebrated as
a building stone. In parts of Gloucestershire, especially near
Minchinhampton, the Great Oolite, says Mr. Lycett, "must have been
deposited in a shallow sea, where strong currents prevailed, for there
are frequent changes in the mineral character of the deposit, and some
beds exhibit false stratification. In others, heaps of broken shells are
mingled with pebbles of rocks foreign to the neighbourhood, and with
fragments of abraded madrepores, dicotyledonous wood, and crabs' claws.
The shelly strata, also, have occasionally suffered denudation, and the
removed portions have been replaced by clay."[266-A] In such
shallow-water beds cephalopoda are rare, and, instead of ammonites and
belemnites, numerous genera of carnivorous trachelipods appear. Out of
one hundred and forty-two species of univalves obtained from the
Minchinhampton beds, Mr. Lycett found no less than forty-one to be
carnivorous. They belong principally to the genera _Buccinum_,
_Pleurotoma_, _Rostellaria_, _Murex_, and _Fusus_, and exhibit a
proportion of zoophagous species not very different from that which
obtains in warm seas of the recent period. These conchological results
are curious and unexpected, since it was imagined that we might look
in vain for the carnivorous trachelipods in rocks of such high
antiquity as the Great Oolite, and it was a received doctrine that
they did not begin to appear in considerable numbers till the Eocene
period when those two great families of cephalopoda, the ammonites
and belemnites, had become extinct.
_Stonesfield slate._--The slate of Stonesfield has been shown by Mr.
Lonsdale to lie at the base of the Great Oolite.[266-B] It is a slightly
oolitic shelly limestone, forming large spheroidal masses imbedded in
sand, only 6 feet thick, but very rich in organic remains. It contains some
pebbles of a rock very similar to itself, and which may be portions of the
deposit, broken up on a shore at low water or during storms, and
redeposited. The remains of belemnites, trigoniae, and other marine shells,
with fragments of wood, are common, and impressions of ferns, cycadeae, and
other plants. Several insects, also, and, among the rest, the wing-covers
of beetles, are perfectly preserved (see fig. 284.), some of them
approaching nearly to the genus _Buprestis_.[267-A] The remains, also, of
many genera of reptiles, such as _Plesiosaur_, _Crocodile_, and
_Pterodactyl_, have been discovered in the same limestone.
[Illustration: Fig. 284. Elytron of _Buprestis_? Stonesfield.]
[Illustration: Fig. 285. Bone of a reptile, formerly supposed to be the
ulna of a Cetacean; from the Great Oolite of Enstone, near Woodstock.]
But the remarkable fossils for which the Stonesfield slate is most
celebrated, are those referred to the mammiferous class. The student should
be reminded that in all the rocks described in the preceding chapters as
older than the Eocene, no bones of any land quadruped, or of any cetacean,
have been discovered. Yet we have seen that terrestrial plants were not
rare in the lower cretaceous formation, and that in the Wealden there was
evidence of freshwater sediment on a large scale, containing various
plants, and even ancient vegetable soils with the roots and erect stumps of
trees. We had also in the same Wealden many land-reptiles and
winged-insects, which renders the absence of terrestrial quadrupeds the
more striking. The want, however, of any bones of whales, seals, dolphins,
and other aquatic mammalia, whether in the chalk or in the upper or middle
oolite, is certainly still more remarkable. Formerly, indeed, a bone from
the great oolite of Enstone, near Woodstock, in Oxfordshire, was cited, on
the authority of Cuvier, as referable to this class. Dr. Buckland, who
stated this in his Bridgewater Treatise[267-B], had the kindness to send me
the supposed ulna of a whale, that Mr. Owen might examine into its claims
to be considered as cetaceous. It is the opinion of that eminent
comparative anatomist that it cannot have belonged to the cetacea, because
the fore-arm in these marine mammalia is invariably much flatter, and
devoid of all muscular depressions and ridges, one of which is so prominent
in the middle of this bone, represented in the above cut (fig. 285.). In
saurians, on the contrary, such ridges exist for the attachment of muscles;
and to some animal of that class the bone is probably referable.
[Illustration: Fig. 286. _Amphitherium Prevostii_. Stonesfield
Slate. Natural size.
_a_. coronoid process.
_b_. condyle.
_c_. angle of jaw.
_d_. double-fanged molars.]
These observations are made to prepare the reader to appreciate more justly
the interest felt by every geologist in the discovery in the Stonesfield
slate of no less than seven specimens of lower jaws of mammiferous
quadrupeds, belonging to three different species and to two distinct
genera, for which the names of _Amphitherium_ and _Phascolotherium_ have
been adopted. When Cuvier was first shown one of these fossils in 1818, he
pronounced it to belong to a small ferine mammal, with a jaw much
resembling that of an opossum, but differing from all known ferine genera,
in the great number of the molar teeth, of which it had at least ten in a
row. Since that period, a much more perfect specimen of the same fossil,
obtained by Dr. Buckland (see fig. 286.), has been examined by Mr. Owen,
who finds that the jaw contained on the whole twelve molar teeth, with the
socket of a small canine, and three small incisors, which are _in situ_,
altogether amounting to sixteen teeth on each side of the lower jaw.
[Illustration: Fig. 287. _Amphitherium Broderipii_. Natural size.
Stonesfield Slate.]
The only question which could be raised respecting the nature of these
fossils was, whether they belonged to a mammifer, a reptile, or a fish. Now
on this head the osteologist observes that each of the seven half jaws is
composed of but one single piece, and not of two or more separate bones, as
in fishes and most reptiles, or of two bones, united by a suture, as in
some few species belonging to those classes. The condyle, moreover (_b_,
fig. 286.), or articular surface, by which the lower jaw unites with the
upper, is convex in the Stonesfield specimens, and not concave as in fishes
and reptiles. The coronoid process (_a_, fig. 286.) is well developed,
whereas it is wanting or very small, in the inferior classes of vertebrata.
Lastly, the molar teeth in the _Amphitherium_ and _Phascolotherium_ have
complicated crowns, and two roots (see _d_, fig. 286.), instead of being
simple and with single fangs.[269-A]
[Illustration: Fig. 288. _Tupaia Tana._ Right ramus of lower jaw, natural
size. A recent insectivorous mammal from Sumatra.]
[2 Illustrations: Part of lower jaw of _Tupaia Tana_; twice natural size.
Fig. 289. End view seen from behind, showing the very slight inflection of
the angle at _c_.
Fig. 290. Side view of same.]
[2 Illustrations: Part of lower jaw of _Didelphis Azarae_; recent,
Brazil. Natural size.
Fig. 291. End view seen from behind, showing the inflection of the angle of
the jaw, _c. d._
Fig. 292. Side view of same.]
The only question, therefore, which could fairly admit of controversy
was limited to this point, whether the fossil mammalia found in the
lower oolite of Oxfordshire ought to be referred to the marsupial
quadrupeds, or to the ordinary placental series. Cuvier had long ago
pointed out a peculiarity in the form of the angular process (_c_, figs.
291. and 292.) of the lower jaw, as a character of the genus
_Didelphys_; and Mr. Owen has since established its generality in the
entire marsupial series. In all these pouched quadrupeds, this process
is turned inwards, as at _c d_, fig. 291. in the Brazilian opossum,
whereas in the placental series, as at _c_, figs. 290. and 289. there is
an almost entire absence of such inflection. The _Tupaia Tana_ of
Sumatra has been selected by my friend Mr. Waterhouse, for this
illustration, because that small insectivorous quadruped bears a great
resemblance to those of the Stonesfield _Amphitherium_. By clearing away
the matrix from the specimen of _Amphitherium Prevostii_ above
represented (fig. 286.), Mr. Owen ascertained that the angular process
(_c_) bent inwards in a slighter degree than in any of the known
marsupialia; in short, the inflection does not exceed that of the mole
or hedgehog. This fact turns the scale in favour of its affinities to
the placental insectivora. Nevertheless, the _Amphitherium_ offers some
points of approximation in its osteology to the marsupials, especially
to the _Myrmecobius_, a small insectivorous quadruped of Australia,
which has nine molars on each side of the lower jaw, besides a canine
and three incisors.[269-B]
Another species of _Amphitherium_ has been found at Stonesfield (fig.
287. p. 268.), which differs from the former (fig. 286.) principally
in being larger.
[Illustration: Fig. 293. _Phascolotherium Bucklandi_, Owen.
_a._ natural size.
_b._ molar of same magnified.]
The second mammiferous genus discovered in the same slates was named
originally by Mr. Broderip _Didelphys Bucklandi_ (see fig. 293.), and
has since been called _Phascolotherium_ by Owen. It manifests a much
stronger likeness to the marsupials in the general form of the jaw, and
in the extent and position of its inflected angle, while the agreement
with the living genus _Didelphys_ in the number of the premolar and
molar teeth, is complete.[270-A]
On reviewing, therefore, the whole of the osteological evidence, it will be
seen that we have every reason to presume that the _Amphitherium_ and
_Phascolotherium_ of Stonesfield represent both the placental and marsupial
classes of mammalia; and if so, they warn us in a most emphatic manner, not
to found rash generalizations respecting the non-existence of certain
classes of animals at particular periods of the past, on mere negative
evidence. The singular accident of our having as yet found nothing but the
lower jaws of seven individuals, and no other bones of their skeletons, is
alone sufficient to demonstrate the fragmentary manner in which the
memorials of an ancient terrestrial fauna are handed down to us. We can
scarcely avoid suspecting that the two genera above described, may have
borne a like insignificant proportion to the entire assemblage of
warm-blooded quadrupeds which flourished in the islands of the oolitic sea.
Mr. Owen has remarked that as the marsupial genera, to which the
_Phascolotherium_ is most nearly allied, are now confined to New South
Wales and Van Diemen's Land, so also is it in the Australian seas, that we
find the _Cestracion_, a cartilaginous fish which has a bony palate, allied
to those called _Acrodus_ and _Psammodus_ (see figs. 307, 308. p. 275.), so
common in the oolite and lias. In the same Australian seas, also, near the
shore, we find the living _Trigonia_, a genus of mollusca so frequently met
with in the Stonesfield slate. So, also, the Araucarian pines are now
abundant, together with ferns, in Australia and its islands, as they were
in Europe in the oolitic period. Many botanists incline to the opinion,
that the _Thuja_, _Pine_, _Cycas_, _Zamia_, in short, all the gymnogens,
belong to a less highly developed type of flowering plants than do the
exogens; but even if this be admitted, no naturalist can ascribe a low
standard of organization to the oolitic flora, since we meet with endogens
of the most perfect structure in oolitic rocks, both above and below the
Stonesfield slate, as, for example, the _Podocarya_ of Buckland, a fruit
allied to the _Pandanus_, found in the Inferior Oolite (see fig. 294.), and
the _Carpolithes conica_ of the Coral rag. The doctrine, therefore, of a
regular series of progressive development at successive eras in the animal
and vegetable kingdoms, from beings of a more simple to those of a more
complex organization, receives a check, if not a refutation, from the facts
revealed to us by the study of the Lower Oolites.
[Illustration: Fig. 294. Portion of a fossil fruit of _Podocarya_
magnified. (Buckland's Bridgew. Treat. Pl. 63.) Inferior Oolite,
Charmouth, Dorset.]
The Stonesfield slate, in its range from Oxfordshire to the north-east,
is represented by flaggy and fissile sandstones, as at Collyweston in
Northamptonshire, where, according to the researches of Messrs. Ibbetson
and Morris, it contains many shells, such as _Trigonia angulata_, also
found at Stonesfield. But the Northamptonshire strata of this age assume
a more marine character, or appear at least to have been formed farther
from land. They inclose, however, some fossil ferns, such as _Pecopteris
polypodioides_, of species common to the oolites of the Yorkshire
coast[271-A], where rocks of this age put on all the aspect of a true
coal-field; thin seams of coal having actually been worked in them
for more than a century.
[Illustration: Fig. 295. _Pterophyllum comptum._ (Syn. _Cycadites
comptus_.) Upper sandstone and shale, Gristhorpe, near Scarborough.]
In the north-west of Yorkshire, the formation alluded to consists of an
upper and a lower carbonaceous shale, abounding in impressions of
plants, divided by a limestone considered by many geologists as the
representative of the Great Oolite; but the scarcity of marine fossils
makes all comparisons with the subdivisions adopted in the south
extremely difficult. A rich harvest of fossil ferns has been obtained
from the upper carbonaceous shales and sandstones at Gristhorpe, near
Scarborough (see figs. 295, 296.). The lower shales are well exposed in
the sea-cliffs at Whitby, and are chiefly characterized by ferns and
cycadeae. They contain, also, a species of calamite, and a fossil called
_Equisetum columnare_, which maintains an upright position in sandstone
strata over a wide area. Shells of the genus _Cypris_ and _Unio_,
collected by Mr. Bean from these Yorkshire coal-bearing beds, point to
the estuary or fluviatile origin of the deposit.
[Illustration: Fig. 296. _Hemitelites Brownii_, Goepp. Syn.
_Phlebopteris contigua_, Lind. & Hutt. Upper carbonaceous strata, Lower
Oolite, Gristhorpe, Yorkshire.]
At Brora, in Sutherlandshire, a coal formation, probably coeval with the
above, or belonging to some of the lower divisions of the Oolitic period,
has been mined extensively for a century or more. It affords the thickest
stratum of pure vegetable matter hitherto detected in any secondary rock in
England. One seam of coal of good quality has been worked 3-1/2 feet thick,
and there are several feet more of pyritous coal resting upon it.
_Inferior Oolite._--Between the Great and Inferior Oolite, near Bath, an
argillaceous deposit called "the fuller's earth," occurs, but is wanting in
the north of England. The Inferior Oolite is a calcareous freestone,
usually of small thickness, which sometimes rests upon, or is replaced by,
yellow sands, called the sands of the Inferior Oolite. These last, in their
turn, repose upon the lias in the south and west of England.
Among the characteristic shells of the Inferior Oolite, I may instance
_Terebratula spinosa_ (fig. 297.), and _Pholadomya fidicula_ (fig. 298.).
The extinct genus _Pleurotomaria_ is also a form very common in this
division as well as in the Oolitic system generally. It resembles the
_Trochus_ in form, but is marked by a singular cleft (_a_, fig. 299.) on
the right side of the mouth.
[Illustration: Fig. 297. _Terebratula spinosa._ Inferior Oolite.]
[Illustration: Fig. 298.
_a._ _Pholadomya fidicula_, 1/3 nat. size. Inf. Ool.
_b._ Heart-shaped anterior termination of the same.]
[Illustration: Fig. 299. _Pleurotomaria ornata._ Ferruginous Oolite,
Normandy. Inferior Oolite, England.]
As illustrations of shells having a great vertical range, I may allude to
_Trigonia clavellata_, found in the Upper and Inferior Oolite, and _T.
costata_, common to the Upper, Middle, and Lower Oolite; also _Ostrea
Marshii_ (fig. 300.), common to the Cornbrash of Wilts and the Inferior
Oolite of Yorkshire; and _Ammonites striatulus_ (fig. 301.) common to the
Inferior Oolite and Lias.
[Illustration: Fig. 300. _Ostrea Marshii._ 1/2 nat. size. Middle
and Lower Oolite.]
[Illustration: Fig. 301. _Ammonites striatulus_, Sow. 1/3 nat. size.
Inferior Oolite and Lias.]
Such facts by no means invalidate the general rule, that certain fossils
are good chronological tests of geological periods; but they serve to
caution us against attaching too much importance to single species, some
of which may have a wider, others a more confined vertical range. We
have before seen that, in the successive tertiary formations, there are
species common to older and newer groups, yet these groups are
distinguishable from one another by a comparison of the whole assemblage
of fossil shells proper to each.
FOOTNOTES:
[259-A] See Chapters VI. and XIX.
[261-A] Fitton, Geol. Trans., Second Series, vol. iv. pl. 23. fig. 12.
[262-A] S. P. Pratt, Annals of Nat. Hist., November, 1841.
[263-A] See Phil. Trans. 1850, p. 393.
[263-B] P. Scrope, Geol. Proceed., March, 1831.
[265-A] For a fuller account of these Encrinites, see Buckland's
Bridgewater Treatise, vol. i. p. 429.
[266-A] Lycett, Quart. Geol. Journ. vol. iv. p. 183.
[266-B] Proceedings Geol. Soc. vol. i. p. 414.
[267-A] See Buckland's Bridgewater Treatise; and Brodie's Fossil Insects,
where it is suggested that these elytra may belong to _Priomus_.
[267-B] Vol. i. p. 115.
[269-A] I have given a figure in the Principles of Geology, chap. ix., of
another Stonesfield specimen of _Amphitherium Prevostii_, in which the
sockets and roots of the teeth are finely exposed.
[269-B] A figure of this recent _Myrmecobius_ will be found in the
Principles, chap. ix.
[270-A] Owen's British Fossil Mammals, p. 62.
[271-A] Ibbetson and Morris, Report of Brit. Ass., 1847, p. 131.