diligently collected and studied by Dr. Debey, and as they afford the
only example yet known of a terrestrial flora older than the Eocene, in
which the great divisions of the vegetable kingdom are represented in
nearly the same proportions as in our own times, they deserve
particular attention. Dr. Debey estimates the number of species as
amounting to more than two hundred, of which sixty-seven are
cryptogamous, chiefly ferns, twenty species of which can be well
determined, most of them being in fructification. The scars on the bark
of one or two are supposed to indicate tree-ferns. Of thirteen genera
three are still existing, namely, _Gleichenia,_ now inhabiting the Cape
of Good Hope, and New Holland; Lygodium, now spread extensively through
tropical regions, but having some species which live in Japan and North
America; and _ Asplenium,_ a cosmopolite form. Among the phænogamous
plants, the Conifers are abundant, the most common belonging to a genus
called Cycadopteris by Debey, and hardly separable from Sequoia (or
Wellingtonia), of which both the cones and branches are preserved. When
I visited Aix, I found the silicified wood of this plant very
plentifully dispersed through the white sands in the pits near that
city. In one silicified trunk 200 rings of annual growth could be
counted. Species of Araucaria like those of Australia are also found.
Cycads are extremely rare, and of Monocotyledons there are but few. No
palms have been recognised with certainty, but the genus Pandanus, or
screw pine, has been distinctly made out. The number of the
Dicotyledonous Angiosperms is the most striking feature in so ancient a
flora.[3]
Among them we find the familiar forms of the Oak, Fig, and Walnut
(Quercus, Ficus, and Juglans), of the last both the nuts and leaves;
also several genera of the Myrtaceæ. But the predominant order is the
Proteaceæ, of which there are between sixty and seventy supposed
species, many of extinct genera, but some referred to the following
living forms—Dryandra, Grevillea, Hakea, Banksia, Persoonia—all now
belonging to Australia, and Leucospermum, species of which form small
bushes at the Cape.
Brongniart. Lindley. Cryptogamic. 1. Cryptogamous amphigens,
or cellular cryptogamic. Thallogens. Lichens, sea-weeds,
fungi. 2. Cryptogamous acrogens. Acrogens. Mosses,
equisetums, ferns, lycopodiums,—Lepidodendra.
Phænerogamic. 3. Dicotyledonous
gymnosperms. Gymnogens. Conifers and Cycads. 4. Dicot.
angiosperms. Exogens. Compositæ, leguminosæ, cruciferæ, healths,
etc. All native European trees except conifers. 5.
Monocotyledons. Endogens. Palms, lilies, aloes, rushes, grasses,
etc.
The epidermis of the leaves of many of these Aix plants, especially of
the Proteaceæ, is so perfectly preserved in an envelope of fine clay,
that under the microscope the stomata, or polygonal cellules, can be
detected, and their peculiar arrangement is identical with that known
to characterise some living Proteaceæ (Grevillea, for example).
Although this peculiarity of the structure of stomata is also found in
plants of widely distant orders, it is, on the whole, but rarely met
with, and being thus observed to characterise a foliage previously
suspected to be proteaceous, it adds to the probability that the
botanical evidence had been correctly interpreted.
An occasional admixture at Aix-la-Chapelle of Fucoids and Zosterites
attests, like the shells, the presence of salt-water. Of insects, Dr.
Debey has obtained about ten species of the families Curculionidæ and
Carabidæ.
The resemblance of the flora of Aix-la-Chapelle to the tertiary and
living floras in the proportional number of dicotyledonous angiosperms
as compared to the gymnogens, is a subject of no small theoretical
interest, because we can now affirm that these Aix plants flourished
before the rich reptilian fauna of the secondary rocks had ceased to
exist. The Ichthyosaurus, Pterodactyl, and Mosasaurus were of coeval
date with the oak, the walnut, and the fig. Speculations have often
been hazarded respecting a connection between the rarity of Exogens in
the older rocks and a peculiar state of the atmosphere. A denser air,
it was suggested, had in earlier times been alike adverse to the
well-being of the higher order of flowering plants, and of the
quick-breathing animals, such as mammalia and birds, while it was
favourable to a cryptogamic and gymnospermous flora, and to a
predominance of reptile life. But we now learn that there is no
incompatibility in the co-existence of a vegetation like that of the
present globe, and some of the most remarkable forms of the extinct
reptiles of the age of gymnosperms.
If the passage seem at present to be somewhat sudden from the flora of
the Lower or Neocomian to that of the Upper Cretaceous period, the
abruptness of the change will probably disappear when we are better
acquainted with the fossil vegetation of the uppermost beds of the
Neocomian and that of the lowest strata of the Gault or true Cretaceous
series.
Hippurite limestone.—_Difference between the Chalk of the North and
South of Europe._—By the aid of the three tests, superposition, mineral
character, and fossils, the geologist has been enabled to refer to the
same Cretaceous period certain rocks in the north and south of Europe,
which differ greatly both in their fossil contents and in their mineral
composition and structure.
Fig. 273: Map.
If we attempt to trace the cretaceous deposits from England and France
to the countries bordering the Mediterranean, we perceive, in the first
place, that in the neighbourhood of London and Paris they form one
great continuous mass, the Straits of Dover being a trifling
interruption, a mere valley with chalk cliffs on both sides. We then
observe that the main body of the chalk which surrounds Paris stretches
from Tours to near Poitiers (see Fig. 273, in which the shaded part
represents chalk).
Between Poitiers and La Rochelle, the space marked A on the map
separates two regions of chalk. This space is occupied by the Oolite
and certain other formations older than the Chalk and Neocomian, and
has been supposed by M. E. de Beaumont to have formed an island in the
Cretaceous sea. South of this space we again meet with rocks which we
at once recognise to be cretaceous, partly from the chalky matrix and
partly from the fossils being very similar to those of the white chalk
of the north: especially certain species of the genera _Spatangus,
Ananchytes, Cidarites, Nucula, Ostrea,_ _Gryphæa (Exogyra), Pecten,
Plagiostoma (Lima), Trigonia, Catillus (Inoceramus),_ and
_Terebratula._[4] But Ammonites, as M. d’Archiac observes, of which so
many species are met with in the chalk of the north of France, are
scarcely ever found in the southern region; while the genera _Hamite,
Turrilite,_ and _Scaphite,_ and perhaps _Belemnite,_ are entirely
wanting.
Fig. 274: Radiolites. White chalk of France. Fig. 275: Radiolites
foliaceus. White chalk of France.
Fig. 276: Hippurites organisans. Upper chalk:—chalk marl of Pyrenees?
Fig. 276: Hippurites organisans. Upper chalk:—chalk marl of
Pyrenees?[5]
On the other hand, certain forms are common in the south which are rare
or wholly unknown in the north of France. Among these may be mentioned
many _Hippurites, Sphærulites,_ and other members of that great family
of mollusca called _Rudistes_ by Lamarck, to which nothing analogous
has been discovered in the living creation, but which is quite
characteristic of rocks of the Cretaceous era in the south of France,
Spain, Sicily, Greece, and other countries bordering the Mediterranean.
The species called _ Hippurites organisans_ (Fig. 276) is more abundant
than any other in the south of Europe; and the geologist should make
himself well acquainted with the cast of the interior, _d,_ which is
often the only part preserved in many compact marbles of the Upper
Cretaceous period. The flutings on the interior of the Hippurite, which
are represented on the cast by smooth, rounded longitudinal ribs, and
in some individuals attain a great size and length, are wholly unlike
the markings on the exterior of the shell.
Cretaceous Rocks in the United States.—If we pass to the American
continent, we find in the State of New Jersey a series of sandy and
argillaceous beds wholly unlike in mineral character to our Upper
Cretaceous system; which we can, nevertheless, recognise as referable,
palæontologically, to the same division.
That they were about the same age generally as the European chalk and
Neocomian, was the conclusion to which Dr. Morton and Mr. Conrad came
after their investigation of the fossils in 1834. The strata consist
chiefly of green sand and green marl, with an overlying coralline
limestone of a pale yellow colour, and the fossils, on the whole, agree
most nearly with those of the Upper European series, from the
Maestricht beds to the Gault inclusive. I collected sixty shells from
the New Jersey deposits in 1841, five of which were identical with
European species—_Ostrea larva, O. vesicularis, Gryphæa costata, Pecten
quinque-costatus, Belemnitella mucronata._ As some of these have the
greatest vertical range in Europe, they might be expected more than any
others to recur in distant parts of the globe. Even where the species
were different, the generic forms, such as the Baculite and certain
sections of Ammonites, as also the _Inoceramus_ (see Fig. 252) and
other bivalves, have a decidedly cretaceous aspect. Fifteen out of the
sixty shells above alluded to were regarded by Professor Forbes as good
geographical representatives of well-known cretaceous fossils of
Europe. The correspondence, therefore, is not small, when we reflect
that the part of the United States where these strata occur is between
3000 and 4000 miles distant from the chalk of Central and Northern
Europe, and that there is a difference of ten degrees in the latitude
of the places compared on opposite sides of the Atlantic. Fish of the
genera _Lamna, Galeus,_ and _ Carcharodon_ are common to New Jersey and
the European cretaceous rocks. So also is the genus _Mosasaurus_ among
reptiles.
It appears from the labours of Dr. Newberry and others, that the
Cretaceous strata of the United States east and west of the
Appalachians are characterised by a flora decidedly analogous to that
of Aix-la-Chapelle above-mentioned, and therefore having considerable
resemblance to the vegetation of the Tertiary and Recent Periods.
[1] For particulars of structure see p. 318.
[2] Geol. Trans., 1st Series, vol. iv, p. 413.
[3] In this and subsequent remarks on fossil plants I shall often use
Dr. Lindley’s terms, as most familiar in this country; but as those of
M. A. Brongniart are much cited, it may be useful to geologists to
give a table explaining the corresponding names of groups so much
spoken of in palæontology.
[4] D’Archiac, Sur la form. Crétacée du S.-O. de la France Mém. de la
Soc. Géol. de France, tome ii.
[5] D’Orbigny’s Paléontologie français, pl. 533.
CHAPTER XVIII.
LOWER CRETACEOUS OR NEOCOMIAN FORMATION.
Classification of marine and fresh-water Strata. — Upper Neocomian. —
Folkestone and Hythe Beds. — Atherfield Clay. — Similarity of
Conditions causing Reappearance of Species after short Intervals. —
Upper Speeton Clay. — Middle Neocomian. — Tealby Series. — Middle
Speeton Clay. — Lower Neocomian. — Lower Speeton Clay. — Wealden
Formation. — Fresh-water Character of the Wealden. — Weald Clay. —
Hastings Sands. — Punfield Beds of Purbeck, Dorsetshire. — Fossil
Shells and Fish of the Wealden. — Area of the Wealden. — Flora of the
Wealden.
We now come to the Lower Cretaceous Formation which was formerly called
Lower Greensand, and for which it will be useful for reasons before
explained (p. 282) to use the term “Neocomian.”
LOWER CRETACEOUS OR NEOCOMIAN GROUP.
Marine Fresh-water
Upper Neocomian—Greensand of Folkestone, Sandgate, and Hythe,
Atherfield clay, upper part of Speeton clay.
Middle Neocomian—Punfield Marine bed, Tealby beds, middle part of
Speeton clay.
Lower Neocomian—Lower part of Speeton clay.
Part of Wealden beds of Kent, Surrey, Sussex, Hants, and Dorset.
In Western France, the Alps, the Carpathians, Northern Italy, and the
Apennines, an extensive series of rocks has been described by
Continental geologists under the name of Tithonian. These beds, which
are without any marine equivalent in this country, appear completely to
bridge over the interval between the Neocomian and the Oolites. They
may, perhaps, as suggested by Mr. Judd, be of the same age as part of
the Wealden series.
UPPER NEOCOMIAN.
Folkstone and Hythe Beds.—The sands which crop out beneath the Gault in
Wiltshire, Surrey, and Sussex are sometimes in the uppermost part pure
white, at others of a yellow and ferruginous colour, and some of the
beds contain much green matter. At Folkestone they contain layers of
calcareous matter and chert, and at Hythe, in the neighbourhood, as
also at Maidstone and other parts of Kent, the limestone called Kentish
Rag is intercalated. This somewhat clayey and calcareous stone forms
strata two feet thick, alternating with quartzose sand. The total
thickness of these Folkestone and Hythe beds is less than 300 feet, and
they are seen to rest immediately on a grey clay, to which we shall
presently allude as the Atherfield clay. Among the fossils of the
Folkestone and Hythe beds we may mention _Nautilus plicatus_ (Fig.
277), _ Ancyloceras (Scaphites) gigas_ (Fig. 278), which has been aptly
described as an Ammonite more or less uncoiled; _Trigonia caudata_
(Fig. 280), _Gervillia anceps_ (Fig. 279), a bivalve genus allied to
Avicula, and _Terebratula sella_ (Fig. 281). In ferruginous beds of the
same age in Wiltshire is found a remarkable shell called _Diceras
Lonsdalii_ (Fig. 282), which abounds in the Upper and Middle Neocomian
of Southern Europe. This genus is closely allied to Chama, and the cast
of the interior has been compared to the horns of a goat.
Fig. 277: Nautilus licatus. Fig. 278: Ancyloceras gigas. Fig. 279:
Gervillia anceps. Fig. 280: Trigonia caudata.
Atherfield Clay.—We mentioned before that the Folkstone and Hythe
series rest on a grey clay. This clay is only of slight thickness in
Kent and Surrey, but acquires great dimensions at Atherfield, in the
Isle of Wight. The difference, indeed, in mineral character and
thickness of the Upper Neocomian formation near Folkestone, and the
corresponding beds in the south of the Isle of Wight, about 100 miles
distant, is truly remarkable. In the latter place we find no limestone
answering to the Kentish Rag, and the entire thickness from the bottom
of the Atherfield clay to the top of the Neocomian, instead of being
less than 300 feet as in Kent, is given by the late Professor E. Forbes
as 843 feet, which he divides into sixty-three strata, forming three
groups. The uppermost of these consists of ferruginous sands, the
second of sands and clay, and the third or lowest of a brown clay,
abounding in fossils.
Fig. 281: Terebratula sella. Fig. 282: Diceras Lonsdalii. a. The
bivavle shell, b. Cast of one of the valves enlarged.
Pebbles of quartzose sandstone, jasper, and flinty slate, together with
grains of chlorite and mica, and, as Mr. Godwin-Austen has shown,
fragments and water-worn fossils of the oolitic rocks, speak plainly of
the nature of the pre-existing formations, by the wearing down of which
the Neocomian beds were formed. The land, consisting of such rocks, was
doubtless submerged before the origin of the white chalk, a deposit
which was formed in a more open sea, and in clearer waters.
Fig. 283: Perna mulleti.
Among the shells of the Atherfield clay the biggest and most abundant
shell is the large _Perna Mulleti,_ of which a reduced figure is given
in Fig. 283.
_Similarity of Conditions causing Reappearance of Species._—Some
species of mollusca and other fossils range through the whole series,
while others are confined to particular subdivisions, and Forbes laid
down a law which has since been found of very general application in
regard to estimating the chronological relations of consecutive strata.
Whenever similar conditions, he says, are repeated, the same species
reappear, provided too great a lapse of time has not intervened;
whereas if the length of the interval has been geologically great, the
same genera will reappear represented by distinct species. Changes of
depth, or of the mineral nature of the sea-bottom, the presence or
absence of lime or of peroxide of iron, the occurrence of a muddy, or a
sandy, or a gravelly bottom, are marked by the banishment of certain
species and the predominance of others. But these differences of
conditions being mineral, chemical, and local in their nature, have no
necessary connection with the extinction, throughout a large area, of
certain animals or plants. When the forms proper to loose sand or soft
clay, or to perfectly clear water, or to a sea of moderate or great
depth, recur with all the same species, we may infer that the interval
of time has been, geologically speaking, small, however dense the mass
of matter accumulated. But if, the genera remaining the same, the
species are changed, we have entered upon a new period; and no
similarity of climate, or of geographical and local conditions, can
then recall the old species which a long series of destructive causes
in the animate and inanimate world has gradually annihilated.
Fig. 284: Ammonites Deshayesii.
Speeton Clay, Upper Division.—On the coast, beneath the white chalk of
Flamborough Head, in Yorkshire, an argillaceous formation crops out,
called the Speeton clay, several hundred feet in thickness, the
palæontological relations of which have been ably worked out by Mr.
John W. Judd,[1] and he has shown that it is separable into three
divisions, the uppermost of which, 150 feet thick, and containing 87
species of mollusca, decidedly belongs to the Atherfield clay and
associated strata of Hythe and Folkestone, already described. It is
characterised by the _Perna Mulleti_ (Fig. 283) and _Terebratula sella_
(Fig. 281), and by _ Ammonites Deshayesii_ (Fig. 284), a well-known
Hythe fossil. Fine skeletons of reptiles of the genera Pliosaurus and
Teleosaurus have been obtained from this clay. At the base of this
upper division of the Speeton clay there occurs a layer of large
Septaria, formerly worked for the manufacture of cement. This bed is
crowded with fossils, especially Ammonites, one species of which, three
feet in diameter, was observed by Mr. Judd.
MIDDLE NEOCOMIAN.
Tealby Series.—At Tealby, a village in the Lincolnshire Wolds, there
crop out beneath the white chalk some non-fossiliferous ferruginous
sands about twenty-feet thick, beneath which are beds of clay and
limestone, about fifty feet thick, with an interesting suite of
fossils, among which are _ Pecten cinctus_ (Fig. 285), from 9 to 12
inches in diameter, _ Ancyloceras Duvallei_ (Fig. 286), and some forty
other shells, many of them common to the Middle Speeton clay, about to
be mentioned. Mr. Judd remarks that as _Ammonites clypeiformis_ and
_Terebratula hippopus_ characterise the Middle Neocomian of the
Continent, it is to this stage that the Tealby series containing the
same fossils may be assigned.[2]
Fig. 285: Pecten cinctus. Fig. 286: Ancyloceras (Crioceras) Duvallei.
The middle division of the Speeton clay, occurring at Speeton below the
cement-bed, before alluded to, is 150 feet thick, and contains about 39
species of mollusca, half of which are common to the overlying clay.
Among the peculiar shells, _Pecten cinctus_ (Fig. 285) and _Ancyloceras
(Crioceras) Duvallei_ (Fig. 286) occur.
LOWER NEOCOMIAN.
In the lower division of the Speeton clay, 200 feet thick, 46 species
of mollusca have been found, and three divisions, each characterised by
its peculiar ammonite, have been noticed by Mr. Judd. The central zone
is marked by _Ammonites Noricus_ (see Fig. 287). On the Continent these
beds are well-known by their corresponding fossils, the Hils clay and
conglomerate of the north of Germany agreeing with the Middle and Lower
Speeton, the latter of which, with the same mineral characters and
fossils as in Yorkshire, is also found in the little island of
Heligoland. Yellow limestone, which I have myself seen near Neuchatel,
in Switzerland, represents the Lower Neocomian at Speeton.
Fig. 287: Ammonites Noricus.
WEALDEN FORMATION.
Beneath the Atherfield clay or Upper Neocomian of the S.E. of England,
a fresh-water formation is found, called the Wealden, which, although
it occupies a small horizontal area in Europe, as compared to the White
Chalk and the marine Neocomian beds, is nevertheless of great
geological interest, since the imbedded remains give us some insight
into the nature of the terrestrial fauna and flora of the Lower
Cretaceous epoch. The name of Wealden was given to this group because
it was first studied in parts of Kent, Surrey, and Sussex, called the
Weald; and we are indebted to Dr. Mantell for having shown, in 1822, in
his “Geology of Sussex,” that the whole group was of fluviatile origin.
In proof of this he called attention to the entire absence of
Ammonites, Belemnites, Brachiopoda, Echinodermata, Corals, and other
marine fossils, so characteristic of the Cretaceous rocks above, and of
the Oolitic strata below, and to the presence in the Weald of Paludinæ,
Melaniæ, Cyrenæ, and various fluviatile shells, as well as the bones of
terrestrial reptiles and the trunks and leaves of land-plants.
The evidence of so unexpected a fact as that of a dense mass of purely
fresh-water origin underlying a deep-sea deposit (a phenomenon with
which we have since become familiar) was received, at first, with no
small doubt and incredulity. But the relative position of the beds is
unequivocal; the Weald Clay being distinctly seen to pass beneath the
Atherfield Clay in various parts of Surrey, Kent, and Sussex, and to
reappear in the Isle of Wight at the base of the Cretaceous series,
being, no doubt, continuous far beneath the surface, as indicated by
the dotted lines in Fig. 288. They are also found occupying the same
relative position below the chalk in the peninsula of Purbeck,
Dorsetshire, where, as we shall see in the sequel, they repose on
strata referable to the Upper Oolite.
_Weald Clay._—The Upper division, or Weald Clay, is, in great part, of
fresh-water origin, but in its highest portion contains beds of oysters
and other marine shells which indicate fluvio-marine conditions. The
uppermost beds are not only conformable, as Dr. Fitton observes, to the
inferior strata of the overlying Neocomian, but of similar mineral
composition. To explain this, we may suppose that, as the delta of a
great river was tranquilly subsiding, so as to allow the sea to
encroach upon the space previously occupied by fresh-water, the river
still continued to carry down the same sediment into the sea. In
confirmation of this view it may be stated that the remains of the
_Iguanodon Mantelli,_ a gigantic terrestrial reptile, very
characteristic of the Wealden, has been discovered near Maidstone, in
the overlying Kentish Rag, or marine limestone of the Upper Neocomian.
Hence we may infer that some of the saurians which inhabited the
country of the great river continued to live when part of the district
had become submerged beneath the sea. Thus, in our own times, we may
suppose the bones of large alligators to be frequently entombed in
recent fresh-water strata in the delta of the Ganges. But if part of
that delta should sink down so as to be covered by the sea, marine
formations might begin to accumulate in the same space where
fresh-water beds had previously been formed; and yet the Ganges might
still pour down its turbid waters in the same direction, and carry
seaward the carcasses of the same species of alligator, in which case
their bones might be included in marine as well as in subjacent
fresh-water strata.
Fig. 288
The Iguanodon, first discovered by Dr. Mantell, was an herbivorous
reptile, of which the teeth, though bearing a great analogy, in their
general form and crenated edges (see Figs. 289 _a_ and _b_), to the
modern Iguanas which now frequent the tropical woods of America and the
West Indies, exhibit many important differences. It appears that they
have often been worn by the process of mastication; whereas the
existing herbivorous reptiles clip and gnaw off the vegetable
productions on which they feed, but do not chew them. Their teeth
frequently present an appearance of having been chipped off, but never,
like the fossil teeth of the Iguanodon, have a flat ground surface (see
Fig. 290, _b_) resembling the grinders of herbivorous mammalia. Dr.
Mantell computes that the teeth and bones of this species which passed
under his examination during twenty years must have belonged to no less
than seventy-one distinct individuals, varying in age and magnitude
from the reptile just burst from the egg, to one of which the femur
measured twenty-four inches in circumference. Yet, notwithstanding that
the teeth were more numerous than any other bones, it is remarkable
that it was not until the relics of all these individuals had been
found, that a solitary example of part of a jaw-bone was obtained. Soon
afterwards remains both of the upper and lower jaw were met with in the
Hastings beds in Tilgate Forest, near Cuckfield. In the same sands at
Hastings, Mr. Beckles found large tridactyle impressions which it is
conjectured were made by the hind feet of this animal, on which it is
ascertained that there were only three well-developed toes.
Fig. 289 a, b: Tooth of Iguanodon Mantelli. Fig. 290: a. Partially worn
tooth of young individual of the same; <i>b.</i> Crown of tooth in
adult worn down.
Fig. 291: Cypris spinigera.
Occasionally bands of limestone, called Sussex Marble, occur in the
Weald Clay, almost entirely composed of a species of _ Paludina,_
closely resembling the common _P. vivipara_ of English rivers. Shells
of the _Cypris,_ a genus of Crustaceans mentioned (p. 57) as abounding
in lakes and ponds, are also plentifully scattered through the clays of
the Wealden, sometimes producing, like plates of mica, a thin
lamination (see Fig. 292).
Fig. 292: Weald clay with Cyprides.
Hastings Sands.—This lower division of the Wealden consists of sand,
sandstone, calciferous grit, clay, and shale; the argillaceous strata,
notwithstanding the name, predominating somewhat over the arenaceous,
as will be seen by reference to the following table, drawn up by
Messrs. Drew and Foster, of the Geological Survey of Great Britain:
Names of Subordinate
Formations. Mineral Composition
of the Strata. Thickness
in feet. Hastings Sand Tunbridge Wells Sand Sandstone and
loam 150 Wadhurst Clay Blue and brown shale and clay, with
a little calc-grit 100 Ashdown Sand Hard sand, with some beds of
calc-grit 160 Ashburnham Beds Mottled white and red clay, with
some sandstone 330
The picturesque scenery of the “High Rocks” and other places in the
neighbourhood of Tunbridge Wells is caused by the steep natural cliffs,
to which a hard bed of white sand, occurring in the upper part of the
Tunbridge Wells Sand, mentioned in the above table, gives rise. This
bed of “rock-sand” varies in thickness from 25 to 48 feet. Large masses
of it, which were by no means hard or capable of making a good
building-stone, form, nevertheless, projecting rocks with perpendicular
faces, and resist the degrading action of the river because, says Mr.
Drew, they present a solid mass without planes of division. The
calcareous sandstone and grit of Tilgate Forest, near Cuckfield, in
which the remains of the Iguanodon and Hylæosaurus were first found by
Dr. Mantell, constitute an upper member of the Tunbridge Wells Sand,
while the “sand-rock” of the Hastings cliffs, about 100 feet thick, is
one of the lower members of the same. The reptiles, which are very
abundant in this division, consist partly of saurians, referred by Owen
and Mantell to eight genera, among which, besides those already
enumerated, we find the Megalosaurus and Plesiosaurus. The Pterodactyl
also, a flying reptile, is met with in the same strata, and many
remains of Chelonians of the genera _Trionyx_ and _Emys,_ now confined
to tropical regions.
The fishes of the Wealden are chiefly referable to the Ganoid and
Placoid orders. Among them the teeth and scales of _ Lepidotus_ are
most widely diffused (see Fig. 293, next page). These ganoids were
allied to the _Lepidosteus,_ or Gar-pike, of the American rivers. The
whole body was covered with large rhomboidal scales, very thick, and
having the exposed part coated with enamel. Most of the species of this
genus are supposed to have been either river-fish, or inhabitants of
the sea at the mouth of estuaries.
Fig. 293: Lepidotus Mantelli, a. Palate and teeth, b. Side view of
teeth, c. Scale.
Fig. 294: Unio Valdensis. Fig. 295: Under side of slab of sandstone
about one yard in diameter.
At different heights in the Hastings Sands, we find again and again
slabs of sandstone with a strong ripple-mark, and between these slabs
beds of clay many yards thick. In some places, as at Stammerham,
Horsham, near there, are indications of this clay having been exposed
so as to dry and crack before the next layer was thrown down upon it.
The open cracks in the clay have served as moulds, of which casts have
been taken in relief, and which are, therefore, seen on the lower
surface of the sandstone (see Fig. 295).
Near the same place a reddish sandstone occurs in which are innumerable
traces of a fossil vegetable, apparently _ Sphenopteris,_ the stems and
branches of which are disposed as if the plants were standing erect on
the spot where they originally grew, the sand having been gently
deposited upon and around them; and similar appearances have been
remarked in other places in this formation.[3] In the same division
also of the Wealden, at Cuckfield, is a bed of gravel or conglomerate,
consisting of water-worn pebbles of quartz and jasper, with rolled
bones of reptiles. These must have been drifted by a current, probably
in water of no great depth.
Fig. 296: Sphenopteris gracilis.
From such facts we may infer that, notwithstanding the great thickness
of this division of the Wealden, the whole of it was a deposit in water
of a moderate depth, and often extremely shallow. This idea may seem
startling at first, yet such would be the natural consequence of a
gradual and continuous sinking of the ground in an estuary or bay, into
which a great river discharged its turbid waters. By each foot of
subsidence, the fundamental rock would be depressed one foot farther
from the surface; but the bay would not be deepened, if newly-deposited
mud and sand should raise the bottom one foot. On the contrary, such
new strata of sand and mud might be frequently laid dry at low water,
or overgrown for a season by a vegetation proper to marshes.
Punfield Beds, Brackish and Marine.—The shells of the Wealden beds
belong to the genera _Melanopsis, Melania, Paludina, Cyrena, Cyclas,
Unio_ (see Fig. 294), and others, which inhabit rivers or lakes; but
one band has been found at Punfield, in Dorsetshire, indicating a
brackish state of the water, where the genera _Corbula, Mytilus,_ and
_Ostrea_ occur; and in some places this bed becomes purely marine,
containing some well-known Neocomian fossils, among which _Ammonites
Deshayesii_ (Fig. 284) may be mentioned. Others are peculiar as
British, but very characteristic of the Upper and Middle Neocomian of
Spain, and among these the _ Vicarya Lujani_ (Fig. 297), a shell allied
to Nerinea, is conspicuous.
By reference to table (p. 308) it will be seen that the Wealden beds
are given as the fresh-water equivalents of the Marine Neocomian. The
highest part of them in England may, for reasons just given, be
regarded as Upper Neocomian, while some of the inferior portions may
correspond in age to the Middle and Lower divisions of that group. In
favour of this latter view, M. Marcou mentions that a fish called
_Asteracanthus granulosus,_ occurring in the Tilgate beds, is
characteristic of the lowest beds of the Neocomian of the Jura, and it
is well known that _Corbula alata,_ common in the Ashburnham beds, is
found also at the base of the Neocomian of the Continent.
Fig. 297: Vicarya Lujani. Fig. 297: _Vicarya Lujani_, De Verneuil.[4]
Wealden, Punfield.
_Area of the Wealden._—In regard to the geographical extent of the
Wealden, it cannot be accurately laid down, because so much of it is
concealed beneath the newer marine formations. It has been traced about
320 English miles from west to east, from the coast of Dorsetshire to
near Boulogne, in France; and nearly 200 miles from north-west to
south-east, from Surrey and Hampshire to Vassy, in France. If the
formation be continuous throughout this space, which is very doubtful,
it does not follow that the whole was contemporaneous; because, in all
likelihood, the physical geography of the region underwent frequent
changes throughout the whole period, and the estuary may have altered
its form, and even shifted its place. Dr. Dunker, of Cassel, and H. von
Meyer, in an excellent monograph on the Wealdens of Hanover and
Westphalia, have shown that they correspond so closely, not only in
their fossils, but also in their mineral characters, with the English
series, that we can scarcely hesitate to refer the whole to one great
delta. Even then, the magnitude of the deposit may not exceed that of
many modern rivers. Thus, the delta of the Quorra or Niger, in Africa,
stretches into the interior for more than 170 miles, and occupies, it
is supposed, a space of more than 300 miles along the coast, thus
forming a surface of more than 25,000 square miles, or equal to about
one-half of England.[5] Besides, we know not, in such cases, how far
the fluviatile sediment and organic remains of the river and the land
may be carried out from the coast, and spread over the bed of the sea.
I have shown, when treating of the Mississippi, that a more ancient
delta, including species of shells such as now inhabit Louisiana, has
been upraised, and made to occupy a wide geographical area, while a
newer delta is forming;[6] and the possibility of such movements and
their effects must not be lost sight of when we speculate on the origin
of the Wealden.
It may be asked where the continent was placed, from the ruins of which
the Wealden strata were derived, and by the drainage of which a great
river was fed. If the Wealden was gradually going downward 1000 feet or
more perpendicularly, a large body of fresh-water would not continue to
be poured into the sea at the same point. The adjoining land, if it
participated in the movement, could not escape being submerged. But we
may suppose such land to have been stationary, or even undergoing
contemporaneous slow upheaval. There may have been an ascending
movement in one region, and a descending one in a contiguous parallel
zone of country. But even if that were the case, it is clear that
finally an extensive depression took place in that part of Europe where
the deep sea of the Cretaceous period was afterwards brought in.
_Thickness of the Wealden._—In the Weald area itself, between the North
and South Downs, fresh-water beds to the thickness of 1600 feet are
known, the base not being reached. Probably the thickness of the whole
Wealden series, as seen in Swanage Bay, cannot be estimated as less
than 2000 feet.
_Wealden Flora._—The flora of the Wealden is characterised by a great
abundance of Coniferæ, Cycadeæ, and Ferns, and by the absence of leaves
and fruits of Dicotyledonous Angiosperms. The discovery in 1855, in the
Hastings beds of the Isle of Wight, of Gyrogonites, or spore-vessels of
the Chara, was the first example of that genus of plants, so common in
the tertiary strata, being found in a Secondary or Mesozoic rock.
[1] Judd, Speeton clay, Quart. Geol. Journ., vol. xxiv, 1868, p. 218.
[2] Judd, Quart. Geol. Journ., 1867, vol. xxiii, p. 249.
[3] Mantell, Geol. of S.E. of England, p. 244.
[4] Foss. de Utrillas.
[5] Fitton, Geol. of Hastings, p. 58, who cites Lander’s Travels.
[6] See p. 102 and Second Visit to the United States, vol. ii, chap.
xxxiv.
CHAPTER XIX.
JURASSIC GROUP.—PURBECK BEDS AND OOLITE.
The Purbeck Beds a Member of the Jurassic Group. — Subdivisions of that
Group. — Physical Geography of the Oolite in England and France. —
Upper Oolite. — Purbeck Beds. — New Genera of fossil Mammalia in the
Middle Purbeck of Dorsetshire. — Dirt-bed or ancient Soil. — Fossils of
the Purbeck Beds. — Portland Stone and Fossils. — Kimmeridge Clay. —
Lithographic Stone of Solenhofen. — Archæopteryx. — Middle Oolite. —
Coral Rag. — Nerinæa Limestone. — Oxford Clay, Ammonites and
Belemnites. — Kelloway Rock. — Lower, or Bath, Oolite. — Great Plants
of the Oolite. — Oolite and Bradford Clay. — Stonesfield Slate. —
Fossil Mammalia. — Fuller’s Earth. — Inferior Oolite and Fossils. —
Northamptonshire Slates. — Yorkshire Oolitic Coal-field. — Brora Coal.
— Palæontological Relations of the several Subdivisions of the Oolitic
group.
Classification of the Oolite.—Immediately below the Hastings Sands we
find in Dorsetshire another remarkable fresh-water formation, called
_the Purbeck,_ because it was first studied in the sea-cliffs of the
peninsula of Purbeck in that county. These beds are for the most part
of fresh-water origin, but the organic remains of some few intercalated
beds are marine, and show that the Purbeck series has a closer affinity
to the Oolitic group, of which it may be considered as the newest or
uppermost member.
In England generally, and in the greater part of Europe, both the
Wealden and Purbeck beds are wanting, and the marine cretaceous group
is followed immediately, in the descending order, by another series
called the Jurassic. In this term, the formations commonly designated
as “the Oolite and Lias” are included, both being found in the Jura
Mountains. The Oolite was so named because in the countries where it
was first examined the limestones belonging to it had an Oolitic
structure (see p. 37). These rocks occupy in England a zone nearly
thirty miles in average breadth, which 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._ Purbeck beds.
_b._ Portland stone and sand.
_c._ Kimmeridge clay. Middle _d._ Coral rag.
_e._ Oxford clay, and Kelloway rock. Lower _f._ Cornbrash and
Forest marble.
_g._ Great Oolite and Stonesfield slate.
_h._ Fuller’s earth.
_i._ 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 accepted 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 bands of country where the argillaceous strata crop
out; and between these valleys the limestones are observed, forming
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.
Fig. 298: Configuration of surface.
Fig. 298 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 steep slope, 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. It is evident, therefore, that the denuding
causes (see p. 105) have acted similarly over an area several hundred
miles in diameter, removing the softer clays more extensively than the
limestones, and causing these last to form steep slopes or escarpments
wherever the harder calcareous rock was based upon a more yielding and
destructible formation.
UPPER OOLITE.
Purbeck Beds.—These strata, which we class as the uppermost member of
the Oolite, are of limited geographical extent in Europe, as already
stated, but they acquire importance when we consider the succession of
three distinct sets of fossil remains which they contain. Such repeated
changes in organic life must have reference to the history of a vast
lapse of ages. The Purbeck beds are finely exposed to view in
Durdlestone Bay, near Swanage, Dorsetshire, and at Lulworth Cove and
the neighbouring bays between Weymouth and Swanage. At Meup’s Bay, in
particular, Professor E. Forbes examined minutely, in 1850, the organic
remains of this group, displayed in a continuous sea-cliff section, and
it appears from his researches that the Upper, Middle, and Lower
Purbecks are each marked by peculiar species of organic remains, these
again being different, so far as a comparison has yet been instituted,
from the fossils of the overlying Hastings Sands and Weald Clay.
_Upper Purbeck._—The highest of the three divisions is purely
fresh-water, the strata, about fifty feet in thickness, containing
shells of the genera _Paludina, Physa, Limnæa, Planorbis, Valvata,
Cyclas,_ and _Unio,_ with _ Cyprides_ and fish. All the species seem
peculiar, and among these the _Cyprides_ are very abundant and
characteristic (see Fig. 299, _a, b, c._)
The stone called “Purbeck Marble,” formerly much used in ornamental
architecture in the old English cathedrals of the southern counties, is
exclusively procured from this division.
Fig. 299: Cyprides from the Upper Purbecks.
_Middle Purbeck._—Next in succession is the Middle Purbeck, about
thirty feet thick, the uppermost part of which consists of fresh-water
limestone, with cyprides, turtles, and fish, of different species from
those in the preceding strata. Below the limestone are brackish-water
beds full of _Cyrena,_ and traversed by bands abounding in _Corbula_
and _ Melania._ These are based on a purely marine deposit, with _
Pecten, Modiola, Avicula,_ and _Thracia._ Below this, again, come
limestones and shales, partly of brackish and partly of fresh-water
origin, in which many fish, especially species of _ Lepidotus_ and
_Microdon radiatus,_ are found, and a crocodilian reptile named
_Macrorhynchus._ Among the mollusks, a remarkable ribbed _Melania,_ of
the section _Chilina,_ occurs.
Fig. 300: Ostrea distorta. Fig. 301: Hemicidaris Purbeckensis.
Immediately below is a great and conspicuous stratum, twelve feet
thick, formed of a vast accumulation of shells of _Ostrea distorta_
(Fig. 300), long familiar to geologists under the local name of
“Cinder-bed.” In the uppermost part of this bed Professor Forbes
discovered the first echinoderm (Fig. 301) as yet known in the Purbeck
series, a species of _ Hemicidaris,_ a genus characteristic of the
Oolitic period, and scarcely, if at all, distinguishable from a
previously known Oolitic fossil. It was accompanied by a species of
_Perna._ Below the Cinder-bed fresh-water strata are again seen, filled
in many places with species of _Cypris_ (Fig. 302, _a, b, c_), and with
_Valvata, Paludina, Planorbis, Limnæa, Physa_ (Fig. 303), and _Cyclas,_
all different from any occurring higher in the series. It will be seen
that _Cypris fasciculata_ (Fig. 302, _b_) has tubercles at the end only
of each valve, a character by which it can be immediately recognised.
In fact, these minute crustaceans, almost as frequent in some of the
shales as plates of mica in a micaceous sandstone, enable geologists at
once to identify the Middle Purbeck in places far from the Dorsetshire
cliffs, as, for example, in the Vale of Wardour in Wiltshire. Thick
beds of chert occur in the Middle Purbeck filled with mollusca and
cyprides of the genera already enumerated, in a beautiful state of
preservation, often converted into chalcedony. Among these Professor
Forbes met with gyrogonites (the spore-vessels of _Chara_), plants
never until 1851 discovered in rocks older than the Eocene. About
twenty feet below the “Cinder-bed” is a stratum two or three inches
thick, in which fossil mammalia presently to be mentioned occur, and
beneath this a thin band of greenish shales, with marine shells and
impressions of leaves like those of a large _Zostera,_ forming the base
of the Middle Purbeck.
Fig. 302: Cyprides from the Middle Purbecks.
Fig. 303: Physa Bristovii
_Fossil Mammalia of the Middle Purbeck._—In 1852,[1] after alluding to
the discovery of numerous insects and air-breathing mollusca in the
Purbeck strata, I remarked that, although no mammalia had then been
found, “it was too soon to infer their non-existence on mere negative
evidence.” Only two years after this remark was in print, Mr. W. R.
Brodie found in the Middle Purbeck, about twenty feet below the
“Cinder-bed” above alluded to, in Durdlestone Bay, portions of several
small jaws with teeth, which Professor Owen recognised as belonging to
a small mammifer of the insectivorous class, more closely allied in its
dentition to the _ Amphitherium_ (or _Thylacotherium_) than to any
existing type.
Four years later (in 1856) the remains of several other species of
warm-blooded quadrupeds were exhumed by Mr. S. H. Beckles, F.R.S., from
the same thin bed of marl near the base of the Middle Purbeck. In this
marly stratum many reptiles, several insects, and some fresh-water
shells of the genera _Paludina, Planorbis,_ and _Cyclas,_ were found.
Mr. Beckles had determined thoroughly to explore the thin layer of
calcareous mud from which in the suburbs of Swanage the bones of the
Spalacotherium had already been obtained, and in three weeks he brought
to light from an area forty feet long and ten wide, and from a layer
the average thickness of which was only five inches, portions of the
skeletons of six new species of mammalia, as interpreted by Dr.
Falconer, who first examined them. Before these interesting inquiries
were brought to a close, the joint labours of Professor Owen and Dr.
Falconer had made it clear that twelve or more species of mammalia
characterised this portion of the Middle Purbeck, most of them
insectivorous or predaceous, varying in size from that of a mole to
that of the common polecat, _Mustela putorius._ While the majority had
the character of insectivorous marsupials, Dr. Falconer selected one as
differing widely from the rest, and pointed out that in certain
characters it was allied to the living Kangaroo-rat, or _Hypsiprymnus,_
ten species of which now inhabit the prairies and scrub-jungle of
Australia, feeding on plants, and gnawing scratched-up roots. A
striking peculiarity of their dentition, one in which they differ from
all other quadrupeds, consists in their having a single large
pre-molar, the enamel of which is furrowed with vertical grooves,
usually seven in number.
The largest pre-molar (see Fig. 305) in the fossil genus exhibits in
like manner seven parallel grooves, producing by their termination a
similar serrated edge in the crown; but their direction is diagonal—a
distinction, says Dr. Falconer, which is “trivial, not typical.” As
these oblique furrows form so marked a character of the majority of the
teeth, Dr. Falconer gave to the fossil the generic name of _
Plagiaulax._ The shape and relative size of the incisor, _ a,_ Fig.
306, exhibit a no less striking similarity to Hypsiprymnus.
Nevertheless, the more sudden upward curve of this incisor, as well as
other characters of the jaw, indicate a great deviation in the form of
Plagiaulax from that of the living kangaroo-rats.
Fig. 304: Pre-molar of the recent Australian Hypsiprymnus Gaimardi,
showing 7 grooves at right angles to the length of the jaw. Fig. 305:
Third and largest pre-molar (lower jaw) of Plagiaulax Becklesii,
showing 7 diagonal grooves.
Fig. 306: Plagiaulax Becklessi. Right ramus of lower jaw.
There are two fossil specimens of lower jaws of this genus evidently
referable to two distinct species extremely unequal in size and
otherwise distinguishable. The _Plagiaulax Becklesii_ (Fig. 306) was
about as big as the English squirrel or the flying phalanger of
Australia (_Petaurus Australis,_ Waterhouse). The smaller fossil,
having only half the linear dimensions of the other, was probably only
one-twelfth of its bulk. It is of peculiar geological interest,
because, as shown by Dr. Falconer, its two back molars bear a decided
resemblance to those of the Triassic _ Microlestes_ (Fig. 389), the
most ancient of known mammalia, of which an account will be given in
Chapter XXI.
Up to 1857 all the mammalian remains discovered in secondary rocks had
consisted solely of single branches of the lower jaw, but in that year
Mr. Beckles obtained the upper portion of a skull, and on the same slab
the lower jaw of another quadruped with eight molars, a large canine,
and a broad and thick incisor. It has been named Triconodon from its
bicuspid teeth, and is supposed to have been a small insectivorous
marsupial, about the size of a hedgehog. Other jaws have since been
found indicating a larger species of the same genus.
Professor Owen has proposed the name of _Galestes_ for the largest of
the mammalia discovered in 1858 in Purbeck, equalling the polecat
(_Mustela putorius_) in size. It is supposed to have been predaceous
and marsupial.
Between forty and fifty pieces or sides of lower jaws with teeth have
been found in oolitic strata in Purbeck; only five upper maxillaries,
together with one portion of a separate cranium, occur at Stonesfield,
and it is remarkable that with these there were no examples in Purbeck
of an entire skeleton, nor of any considerable number of bones in
juxtaposition. In several portions of the matrix there were detached
bones, often much decomposed, and fragments of others apparently
mammalian; but if all of them were restored, they would scarcely
suffice to complete the five skeletons to which the five upper
maxillaries above alluded to belonged. As the average number of pieces
in each mammalian skeleton is about 250, there must be many thousands
of missing bones; and when we endeavour to account for their absence,
we are almost tempted to indulge in speculations like those once
suggested to me by Dr. Buckland, when he tried to solve the enigma in
reference to Stonesfield; “The corpses,” he said, “of drowned animals,
when they float in a river, distended by gases during putrefaction,
have often their lower jaw hanging loose, and sometimes it has dropped
off. The rest of the body may then be drifted elsewhere, and sometimes
may be swallowed entire by a predaceous reptile or fish, such as an
ichthyosaur or a shark.”
As all the above-mentioned Purbeck marsupials, belonging to eight or
nine genera and to about fourteen species, insectivorous, predaceous,
and herbivorous, have been obtained from an area less than 500 square
yards in extent, and from a single stratum no more than a few inches
thick, we may safely conclude that the whole lived together in the same
region, and in all likelihood they constituted a mere fraction of the
mammalia which inhabited the lands drained by one river and its
tributaries. They afford the first positive proof as yet obtained of
the co-existence of a varied fauna of the highest class of vertebrata
with that ample development of reptile life which marks all the periods
from the Trias to the Lower Cretaceous inclusive, and with a
gymnospermous flora, or that state of the vegetable kingdom when cycads
and conifers predominated over all kinds of plants, except the ferns,
so far, at least, as our present imperfect knowledge of fossil botany
entitles us to speak.
The following table will enable the reader to see at a glance how
conspicuous a part, numerically considered, the mammalian species of
the Middle Purbeck now play when compared with those of other
formations more ancient than the Paris gypsum, and, at the same time,
it will help him to appreciate the enormous hiatus in the history of
fossil mammalia which at present occurs between the Eocene and Purbeck
periods, and between the latter and the Stonesfield Oolite, and between
this again and the Trias.
_Number and Distribution of all the known Species of Fossil Mammalia
from Strata older than the Paris Gypsum, or than the Bembridge Series
of the Isle of Wight._
TERTIARY Headon Series and beds between the Paris Gypsum and the
Grès de Beauchamp 14 10 English
4 French Barton Clay and Sables de Beauchamp 0 Bagshot Beds,
Calcaire Grossier, and Upper Soissonnais of
Cuisse-Lamotte 20 16 French
1 English
3 U. States[2] London Clay, including the Kyson Sand 7 English
Plastic Clay and Lignite 9 7 French
2 English Sables de Bracheux 1 French Thanet Sands and Lower
Landenian of Belgium 0
SECONDARY Maestricht Chalk 0 White Chalk 0 Chalk Marl 0
Chloritic Series (Upper Greensand) 0 Gault 0 Neocomian (Lower
Greensand) 0 Wealden 0 Upper Purbeck Oolite 0 Middle Purbeck
Oolite 14 Swanage Lower Purbeck Oolite 0 Portland Oolite 0
Kimmeridge Clay 0 Coral Rag 0 Oxford Clay 0 Great
Oolite 4 Stonesfield Inferior Oolite 0 Lias 0 Upper
Trias 4 Wurtemberg
Somersetshire
N. Carolina Middle Trias 0 Lower Trias 0
PRIMARY Permian 0 Carboniferous 0 Devonian 0 Silurian 0
Cambrian 0 Laurentian 0
The Sables de Bracheux, enumerated in the Tertiary division of the
table, supposed by Mr. Prestwich to be somewhat newer than the Thanet
Sands, and by M. Hébert to be of about that age, have yielded at La
Fere the _Arctocyon (Palæocyon) primævus,_ the oldest known tertiary
mammal.
It is worthy of notice, that in the Hastings Sands there are certain
layers of clay and sandstone in which numerous footprints of quadrupeds
have been found by Mr. Beckles, and traced by him in the same set of
rocks through Sussex and the Isle of Wight. They appear to belong to
three or four species of reptiles, and no one of them to any
warm-blooded quadruped. They ought, therefore, to serve as a warning to
us, when we fail in like manner to detect mammalian footprints in older
rocks (such as the New Red Sandstone), to refrain from inferring that
quadrupeds, other than reptilian, did not exist or pre-exist.
But the most instructive lesson read to us by the Purbeck strata
consists in this: They are all, with the exception of a few
intercalated brackish and marine layers, of fresh-water origin; they
are 160 feet in thickness, have been well searched by skillful
collectors, and by the late Edward Forbes in particular, who studied
them for months consecutively. They have been numbered, and the
contents of each stratum recorded separately, by the officers of the
Geological Survey of Great Britain. They have been divided into three
distinct groups by Forbes, each characterised by the same genera of
pulmoniferous mollusca and cyprides, these genera being represented in
each group by different species; they have yielded insects of many
orders, and the fruits of several plants; and lastly, they contain
“dirt-beds,” or old terrestrial surfaces and vegetable soils at
different levels, in some of which erect trunks and stumps of cycads
and conifers, with their roots still attached to them, are preserved.
Yet when the geologist inquires if any land-animals of a higher grade
than reptiles lived during any one of these three periods, the rocks
are all silent, save one thin layer a few inches in thickness; and this
single page of the earth’s history has suddenly revealed to us in a few
weeks the memorials of so many species of fossil mammalia, that they
already outnumber those of many a subdivision of the tertiary series,
and far surpass those of all the other secondary rocks put together!
_Lower Purbeck._—Beneath the thin marine band mentioned at p. 324 as
the base of the Middle Purbeck, some purely fresh-water marls occur,
containing species of _Cypris_ (Fig. 307 _a, c_), _Valvata,_ and
_Limnæa,_ different from those of the Middle Purbeck. This is the
beginning of the inferior division, which is about 80 feet thick. Below
the marls are seen, at Meup’s Bay, more than thirty feet of
brackish-water strata, abounding in a species of _Serpula,_ allied to,
if not identical with, _Serpula coacervites,_ found in beds of the same
age in Hanover. There are also shells of the genus _ Rissoa_ (of the
subgenus _Hydrobia_), and a little _ Cardium_ of the subgenus
_Protocardium,_ in these marine beds, together with _Cypris._ Some of
the cypris-bearing shales are strangely contorted and broken up, at the
west end of the Isle of Purbeck. The great dirt-bed or vegetable soil
containing the roots and stools of _Cycadeæ,_ which I shall presently
describe, underlies these marls, and rests upon the lowest fresh-water
limestone, a rock about eight feet thick, containing _Cyclas, Valvata,_
and _Limnæa,_ of the same species as those of the uppermost part of the
Lower Purbeck, or above the dirt-bed. The fresh-water limestone in its
turn rests upon the top beds of the Portland stone, which, although it
contains purely marine remains, often consists of a rock
undistinguishable in mineral character from the Lowest Purbeck
limestone.
Fig. 307: Cyprides from the Lower Purbeck.
Fig. 308: Mantellia nidiformis.
_Dirt-bed or ancient Surface-soil._—The most remarkable of all the
varied succession of beds enumerated in the above list is that called
by the quarrymen “the dirt,” or “black dirt,” which was evidently an
ancient vegetable soil. It is from 12 to 18 inches thick, is of a dark
brown or black colour, and contains a large proportion of earthy
lignite. Through it are dispersed rounded and sub-angular fragments of
stone, from 3 to 9 inches in diameter, in such numbers that it almost
deserves the name of gravel. I also saw in 1866, in Portland, a smaller
dirt-bed six feet below the principal one, six inches thick, consisting
of brown earth with upright _Cycads_ of the same species, _Mantellia
nidiformis,_ as those found in the upper bed, but no _Coniferæ._ The
weight of the incumbent strata squeezing down the compressible dirt-bed
has caused the _Cycads_ to assume that form which has led the quarrymen
to call them “petrified birds’ nests,” which suggested to Brongniart
the specific name of _nidiformis._ I am indebted to Mr. Carruthers for
Figure 308 of one of these Purbeck specimens, in which the original
cylindrical figure has been less distorted than usual by pressure.
Many silicified trunks of coniferous trees, and the remains of plants
allied to _Zamia_ and _Cycas,_ are buried in this dirt-bed, and must
have become fossil on the spots where they grew. The stumps of the
trees stand erect for a height of from one to three feet, and even in
one instance to six feet, with their roots attached to the soil at
about the same distances from one another as the trees in a modern
forest. The carbonaceous matter is most abundant immediately around the
stumps, and round the remains of fossil _Cycadeæ._
Besides the upright stumps above mentioned, the dirt-bed contains the
stems of silicified trees laid prostrate. These are partly sunk into
the black earth, and partly enveloped by a calcareous slate which
covers the dirt-bed. The fragments of the prostrate trees are rarely
more than three or four feet in length; but by joining many of them
together, trunks have been restored, having a length from the root to
the branches of from 20 to 23 feet, the stems being undivided for 17 or
20 feet, and then forked. The diameter of these near the root is about
one foot; but I measured one myself, in 1866, which was 3½ feet in
diameter, said by the quarrymen to be unusually large. Root-shaped
cavities were observed by Professor Henslow to descend from the bottom
of the dirt-bed into the subjacent fresh-water stone, which, though now
solid, must have been in a soft and penetrable state when the trees
grew. The thin layers of calcareous slate (Fig. 309) were evidently
deposited tranquilly, and would have been horizontal but for the
protrusion of the stumps of the trees, around the top of each of which
they form hemispherical concretions.
Fig. 309: Section in Isle of Portland, Dorset.
The dirt-bed is by no means confined to the island of Portland, where
it has been most carefully studied, but is seen in the same relative
position in the cliffs east of Lulworth Cove, in Dorsetshire, where, as
the strata have been disturbed, and are now inclined at an angle of
45°, the stumps of the trees are also inclined at the same angle in an
opposite direction—a beautiful illustration of a change in the position
of beds originally horizontal (see Fig. 310).
Fig. 310: Section of cliff east of Lulworth Cove.
From the facts above described we may infer, first, that those beds of
the Upper Oolite, called “the Portland,” which are full of marine
shells, were overspread with fluviatile mud, which became dry land, and
covered by a forest, throughout a portion of the space now occupied by
the south of England, the climate being such as to permit the growth of
the _Zamia_ and _Cycas._ Secondly. This land at length sank down and
was submerged with its forests beneath a body of fresh-water, from
which sediment was thrown down enveloping fluviatile shells. Thirdly.
The regular and uniform preservation of this thin bed of black earth
over a distance of many miles, shows that the change from dry land to
the state of a fresh-water lake or estuary, was not accompanied by any
violent denudation, or rush of water, since the loose black earth,
together with the trees which lay prostrate on its surface, must
inevitably have been swept away had any such violent catastrophe taken
place.
The forest of the dirt-bed, as before hinted, was not everywhere the
first vegetation which grew in this region. Besides the lower bed
containing upright _Cycadeæ,_ before mentioned, another has sometimes
been found above it, which implies oscillations in the level of the
same ground, and its alternate occupation by land and water more than
once.
_Subdivisions of the Purbeck._—It will be observed that the division of
the Purbecks into upper, middle, and lower, was made by Professor
Forbes strictly on the principle of the entire distinctness of the
species of organic remains which they include. The lines of demarkation
are not lines of disturbance, nor indicated by any striking physical
characters or mineral changes. The features which attract the eye in
the Purbecks, such as the dirt-beds, the dislocated strata at Lulworth,
and the Cinder-bed, do not indicate any breaks in the distribution of
organised beings. “The causes which led to a complete change of life
three times during the deposition of the fresh-water and brackish
strata must,” says this naturalist, “be sought for, not simply in
either a rapid or a sudden change of their area into land or sea, but
in the great lapse of time which intervened between the epochs of
deposition at certain periods during their formation.”
Each dirt-bed may, no doubt, be the memorial of many thousand years or
centuries, because we find that two or three feet of vegetable soil is
the only monument which many a tropical forest has left of its
existence ever since the ground on which it now stands was first
covered with its shade. Yet, even if we imagine the fossil soils of the
Lower Purbeck to represent as many ages, we need not be surprised to
find that they do not constitute lines of separation between strata
characterised by different zoological types. The preservation of a
layer of vegetable soil, when in the act of being submerged, must be
regarded as a rare exception to a general rule. It is of so perishable
a nature, that it must usually be carried away by the denuding waves or
currents of the sea, or by a river; and many Purbeck dirt-beds were
probably formed in succession and annihilated, besides those few which
now remain.
The plants of the Purbeck beds, so far as our knowledge extends at
present, consist chiefly of Ferns, Coniferæ, and Cycadeæ (Fig. 308),
without any angiosperms; the whole more allied to the Oolitic than to
the Cretaceous vegetation. The same affinity is indicated by the
vertebrate and invertebrate animals. Mr. Brodie has found the remains
of beetles and several insects of the homopterous and trichopterous
orders, some of which now live on plants, while others are of such
forms as hover over the surface of our present rivers.
Portland Oolite and Sand (_b,_ Table p. 321).—The Portland Oolite has
already been mentioned as forming in Dorsetshire the foundation on
which the fresh-water limestone of the Lower Purbeck reposes (see p.
331). It supplies the well-known building-stone of which St. Paul’s and
so many of the principal edifices of London are constructed. About
fifty species of mollusca occur in this formation, among which are some
ammonites of large size. The cast of a spiral univalve called by the
quarrymen the “Portland screw” (_a,_ Figure 311), is common; the shell
of the same (_b_) being rarely met with. Also _Trigonia gibbosa_ (Fig.
313) and _Cardium dissimile_ (Fig. 314). This upper member rests on a
dense bed of sand, called the Portland Sand, containing similar marine
fossils, below which is the Kimmeridge Clay. In England these Upper
Oolite formations are almost wholly confined to the southern counties.
But some fragments of them occur beneath the Neocomian or Speeton Clay
on the coast of Yorkshire, containing many more fossils common to the
Portlandian of the Continent than does the same formation in
Dorsetshire. Corals are rare in this formation, although one species is
found plentifully at Tisbury, Wiltshire, in the Portland Sand,
converted into flint and chert, the original calcareous matter being
replaced by silex (Fig. 312).
Fig. 311: Cerithium Portlandicum.
Fig. 312: Isastræa oblonga. Fig. 313: Trigonia gibbosa.
Kimmeridge Clay.—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, with skeletons of fish and saurians, the bitumen may perhaps be
of animal origin. Some of the saurians (Pliosaurus) in Dorsetshire are
among the most gigantic of their kind.
Fig. 314: Cardium dissimile. Fig. 315: Ostrea expansa. Fig. 316:
Cardium striatulum. Fig. 317: Ostrea deltoidea. Fig. 318: Gryphæa
(Exogyra) virgula.
Among the fossils, amounting to nearly 100 species, may be mentioned
_Cardium striatulum_ (Fig. 316) and _Ostrea deltoidea_ (Fig. 317), the
latter found in the Kimmeridge Clay throughout England and the north of
France, and also in Scotland, near Brora. The _Gryphæa virgula_ (Fig.
318), also met with in the Kimmeridge Clay near Oxford, is so abundant
in the Upper Oolite of parts of France as to have caused the deposit to
be termed “marnes à gryphées 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.
Fig. 319: Trigonellites latus.
The _Trigonellites latus_ (_Aptychus_ of some authors) (Fig. 319) is
also widely dispersed through this clay. The real nature of the shell,
of which there are many species in oolitic rocks, is still a matter of
conjecture. Some are of opinion that the two plates have been the
gizzard of a cephalopod; others, that it may have formed a bivalve
operculum of the same.
Solenhofen Stone.—The celebrated lithographic stone of Solenhofen in
Bavaria, appears to be of intermediate age between the Kimmeridge clay
and the Coral Rag, presently to be described. It 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 reptiles or pterodactyls (see Fig. 320), 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 pterodactyls, and other contemporaneous
air-breathers, resorted.
Fig. 320: Skeleton of Pterodactylus crassirostris.
In the same slate of Solenhofen a fine example was met with in 1862 of
the skeleton of a bird almost entire, and retaining even its feathers
so perfect that the vanes as well as the shaft are preserved. The head
was at first supposed to be wanting, but Mr. Evans detected on the slab
what seems to be the impression of the cranium and beak, much
resembling in size and shape that of the jay or woodcock. This valuable
specimen is now in the British Museum, and has been called by Professor
Owen _Archæopteryx macrura._ Although anatomists agree that it is a
true bird, yet they also find that in the length of the bones of the
tail, and some other minor points of its anatomy, it approaches more
nearly to reptiles than any known living bird. In the living
representatives of the class Aves, the tail-feathers are attached to a
coccygian bone, consisting of several vertebræ united together, whereas
in the Archæopteryx the tail is composed of twenty vertebræ, each of
which supports a pair of quill-feathers. The first five only of the
vertebræ, as seen in A, have transverse processes, the fifteen
remaining ones become gradually longer and more tapering. The feathers
diverge outward from them at an angle of 45°.
Fig. 321: Tail and feather of Archæopteryx, from Solenhofen, and tail
of living bird for comparison.
Professor Huxley in his late memoirs on the order of reptiles called
Dinosaurians, which are largely represented in all the formations, from
the Neocomian to the Trias inclusive, has shown that they present in
their structure many remarkable affinities to birds. But a reptile
about two feet long, called Compsognathus, lately found in the
Stonesfield slate, makes a much greater approximation to the class Aves
than any Dinosaur, and therefore forms a closer link between the
classes Aves and Reptilia than does the Archæopteryx.
It appears doubtful whether any species of British fossil, whether of
the vertebrate or invertebrate class, is common to the Oolite and
Chalk. But there is no similar break or discordance as we proceed
downward, and pass from one to another of the several leading members
of the Jurassic group, the Upper, Middle, and Lower Oolite, and the
Lias, there being often a considerable proportion of the mollusca,
sometimes as much as a fourth, common to such divisions as the Upper
and Middle Oolite.
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, most of them retaining the position in which they
grew at the bottom of the sea. In their forms they more frequently
resemble the reef-building polyparia of the Pacific than do the corals
of any other member of the Oolite. They belong chiefly to the genera
_Thecosmilia_ (Fig. 322), _Protoseris,_ and _Thamnastræa,_ and
sometimes form masses of coral fifteen feet thick.
Fig. 322: Thecosmilia annularis. Fig. 323: Thamnastræa.
Fig. 324: Ostrea gregaria. Fig. 325: Nerinæa Goodhallii.
In Fig. 323 of a _Thamnastræa_ 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, until those on the left side are
nearly filled up. The last-mentioned stars are supposed to represent a
perfected condition, and the others an immature state. These coralline
strata extend through the calcareous hills of the north-west of
Berkshire, and north of Wilts, and again recur in Yorkshire, near
Scarborough. The _ Ostrea gregarea_ (Fig. 324) is very characteristic
of the formation in England and on the Continent.
One of the limestones of the Jura, referred to the age of the English
Coral Rag, has been called “Nerinæan limestone” (Calcaire à Nérinées)
by M. Thirria; _Nerinæa_ being an extinct genus of univalve shells
(Fig. 325) much resembling the _Cerithium_ in external form. The
section shows the curious and continuous ridges on the columnella and
whorls.
Oxford Clay.—The coralline limestone, or “Coral Rag,” above described,
and the accompanying sandy beds, called “calcareous grits,” of the
Middle Oolite, rest on a thick bed of clay, called the “Oxford Clay,”
sometimes not less than 600 feet thick. In this there are no corals,
but great abundance of cephalopoda, of the genera Ammonite and
Belemnite (Figs. 326 and 327). In some of the finely laminated clays
ammonites are very perfect, although somewhat compressed, and are
frequently found with the lateral lobe extended on each side of the
opening of the mouth into a horn-like projection (Figure 327). These
were discovered in the cuttings of the Great Western Railway, near
Chippenham, in 1841, and have been described by Mr. Pratt (_An. Nat.
Hist.,_ Nov., 1841).
Fig. 326: Belemnites hastatus. Fig. 327: Ammonites Jason.
Similar elongated processes have been also observed to extend from the
shells of some Belemnites discovered by Dr. Mantell in the same clay
(see Figure 328), who, by the aid of this and other specimens, has been
able to throw much light on the structure of singular extinct forms of
cuttle-fish.[3]
Kelloway Rock.—The arenaceous limestone which passes under this name is
generally grouped as a member of the Oxford clay, in which it forms, in
the south-west of England, lenticular masses, 8 or 10 feet thick,
containing at Kelloway, in Wiltshire, numerous casts of ammonites and
other shells. But in Yorkshire this calcareo-arenaceous formation
thickens to about 30 feet, and constitutes the lower part of the Middle
Oolite, extending inland from Scarborough in a southerly direction. The
number of mollusca which it contains is, according to Mr. Etheridge,
143, of which only 34, or 23½ per cent, are common to the Oxford clay
proper. Of the 52 Cephalopoda, 15 (namely 13 species of ammonite, the
_Ancyloceras Calloviense_ and one Belemnite) are common to the Oxford
Clay, giving a proportion of nearly 30 per cent.
Fig. 328: Belemnites Puzosianus.
LOWER OOLITE.
Cornbrash and Forest Marble.—The upper division of this series, which
is more extensive than the preceding or Middle Oolite, is called in
England the Cornbrash, as being a brashy, easily broken rock, good for
corn land. It consists of clays and calcareous sandstones, which pass
downward 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 crustaceans, 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.[4]
Great (or Bath) Oolite.—Although the name of Coral Rag has been
appropriated, as we have seen, to a member of the Middle Oolite before
described, some portions of the Lower Oolite are equally entitled in
many places to be called coralline limestones. Thus the Great Oolite
near Bath contains various corals, among which the _Eunomia radiata_
(Fig. 329) 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.
Fig. 329: Eunomia radiata.
Different species of crinoids, or stone-lilies, are also common in the
same rocks with corals; and, like them, must have enjoyed a firm
bottom, where their base of attachment remained undisturbed for years
(_c,_ Fig. 330). Such fossils, therefore, are almost confined to the
limestones; but an exception occurs at Bradford, near Bath, where they
are enveloped in clay sometimes 60 feet thick. 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 encrinite, were scattered at
random through the argillaceous deposit in which some now lie
prostrate. These appearances are represented in the section _b,_ Fig.
330, where the darker strata represent the Bradford clay, which is
however a formation of such local development that in many places it
cannot easily be separated from the clays of the overlying
“forest-marble” and underlying “fuller’s earth.” 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
_serpulæ._ Now these _serpulæ_ 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
_serpulæ_ were full grown, they had become incrusted over with a
bryozoan, called _Diastopora diluviana_ (see _b,_ Fig. 331); and many
generations of these molluscoids had succeeded each other in the pure
water before they became fossil.
Fig. 330: Apiocrinites rotundus, or Pear Eucrinite.
Fig. 331: a. Aingle plate of body of Apiocrinus, overgrown with serpulæ
and bryozoa; b. Portion of same magnified, showing the bryozoan
Diastopora diluviana covering one of the serpulæ.
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.
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 law of
variation above alluded to. But we recognise 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, although there is scarcely any lithological
resemblance, 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.
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.” In such
shallow-water beds shells of the genera _Patella, Nerita, Rimula,
Cylindrites_ are common (see Figs. 334 to 337); while cephalopods are
rare, and instead of ammonites and belemnites, numerous genera of
carnivorous trachelipods appear. Out of 224 species of univalves
obtained from the Minchinhampton beds, Mr. Lycett found no less than 50
to be carnivorous. They belong principally to the genera _Buccinum,
Pleurotoma, Rostellaria, Murex, Purpuroidea_ (Fig. 333), and Fusus, and
exhibit a proportion of zoophagous species not very different from that
which obtains in seas of the Recent period. These zoological 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,
and a great number of other representatives of the same class of
chambered shells, had become extinct.
Fig. 332: Terebratula digona. Fig. 333: Purpuroidea nodulata. Fig. 334:
Cylindrites acutus. Fig. 335: Patella rugosa. Fig. 336: Nerita
costulata. Fig. 337: Rimula (Emarginula) clathrata.
Stonesfield Slate: Mammalia.—The slate of Stonesfield has been shown by
Mr. Lonsdale to lie at the base of the Great Oolite.[5] It is a
slightly oolitic shelly limestone, forming large lenticular masses
imbedded in sand only six 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, trigoniæ,
and other marine shells, with fragments of wood, are common, and
impressions of ferns, cycadeæ, and other plants. Several insects, also,
and, among the rest, the elytra or wing-covers of beetles, are
perfectly preserved (see Fig. 338), some of them approaching nearly to
the genus Buprestis. The remains, also, of many genera of reptiles,
such as _Plesiosaur, Crocodile,_ and _ Pterodactyl,_ have been
discovered in the same limestone.
Fig. 338: Elytron of Buprestis?
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, had been discovered until the _Spalacotherium_ of the
Purbeck beds came to light in 1854. Yet we have seen that terrestrial
plants were not wanting in the Upper Cretaceous formation (see p. 302),
and that in the Wealden there was evidence of fresh-water sediment on a
large scale, containing various plants, and even ancient vegetable
soils. We had also in the same Wealden many land-reptiles and winged
insects, which render 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.
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 ten specimens of lower jaws of
mammiferous quadrupeds, belonging to four different species and to
three distinct genera, for which the names of _ Amphitherium,
Phascolotherium,_ and _Stereognathus_ have been adopted.
Fig. 339: Tupaia Tana. Right ramus of lower jaw.
It is now generally admitted that these or really the remains of
mammalia (although it was at first suggested that they might be
reptiles), and the only question open to controversy is 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. 342 and 343)
of the lower jaw, as a character of the genus _Didelphys_; and
Professor Owen has since confirmed the doctrine of its generality in
the entire marsupial series. In all these pouched quadrupeds this
process is turned inward, as at _c, d,_ Fig. 342, in the Brazilian
opossum, whereas in the placental series, as at _c,_ Figs. 340 and 341,
there is an almost entire absence of such inflection. The _Tupaia Tana_
of Sumatra has been selected by Mr. Waterhouse for this illustration,
because the jaws of that small insectivorous quadruped bear a great
resemblance to those of the Stonesfield _Amphitherium._ By clearing
away the matrix from the specimen of _Amphitherium Prevostii_ here
represented (Fig. 344), Professor Owen ascertained that the angular
process (_c_) bent inward 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 made him doubt whether the _Amphitherium_ might
not be an insectivorous placental, although it offered 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.[6] Another species of _Amphitherium_ has been found at
Stonesfield (Fig. 345), which differs from the former (Fig. 344)
principally in being larger.
Fig. 340: Part of lower jaw of Tupaia Tana. Fig. 341: Side view of
same. Fig. 342: Part of lower jaw of Didelphys Azaræ. Fig. 343: Side
view of same. Fig. 344: Amphitherium Prevostii.
Fig. 344: Amphitherium Prevostii.
Fig. 345: Amphitherium Broderipii. Fig. 346: Phascolotherium
Bucklandii.
The second mammiferous genus discovered in the same slates was named
originally by Mr. Broderip _Didelphys Bucklandi_ (see Fig. 346), 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 pre-molar and
molar teeth is complete.[7]
In 1854 the remains of another mammifer, small in size, but larger than
any of those previously known, was brought to light. The generic name
of _Stereognathus_ was given to it, and, as is usually the case in
these old rocks (see p. 328), it consisted of part of a lower jaw, in
which were implanted three double-fanged teeth, differing in structure
from those of all other known recent or extinct mammals.
Plants of the Oolite.—The Araucarian pines, which are now abundant in
Australia and its islands, together with marsupial quadrupeds, are
found in like manner to have accompanied the marsupials in Europe
during the Oolitic period (see Fig. 348). In the same rock endogens of
the most perfect structure are met with, as, for example, fruits allied
to the Pandanus, such as the _Kaidacarpum ooliticum_ of Carruthers in
the Great Oolite, and the _Podocarya_ of Buckland (see Fig. 347) in the
Inferior Oolite.
Fuller’s Earth.—Between the Great and Inferior Oolite near Bath, an
argillaceous deposit, called “the fuller’s earth,” occurs; but it is
wanting in the north of England. It abounds in the small oyster
represented in Fig. 349. The number of mollusca known in this deposit
is about seventy; namely, fifty Lamellibranchiate Bivalves, ten
Brachiopods, three Gasteropods, and seven or eight Cephalopods.
Fig. 347: Portion of a fossil fruit of Podocarya Bucklandii. Fig. 348:
Cone of fossil Araucaria sphærocarpa.
Fig. 349: Ostrea acuminata.
Inferior Oolite.—This formation consists of a calcareous freestone,
usually of small thickness, but attaining in some places, as in the
typical area of Cheltenham and the Western Cotswolds, a thickness of
250 feet. It sometimes rests upon yellow sands, formerly classed as the
sands of the Inferior Oolite, but now regarded as a member of the Upper
Lias. These sands repose upon the Upper Lias clays in the south and
west of England. The Collyweston slate, formerly classed with the Great
Oolite, and supposed to represent in Northamptonshire the Stonesfield
slate, is now found to belong to the Inferior Oolite, both by community
of species and position in the series. The Collyweston beds, on the
whole, assume a much more marine character than the Stonesfield slate.
Nevertheless, one of the fossil plants _Aroides Stutterdi,_ Carruthers,
remarkable, like the Pandanaceous species before mentioned (Fig. 347)
as a representative of the monocotyledonous class, is common to the
Stonesfield beds in Oxfordshire.
The Inferior Oolite of Yorkshire consists largely of shales and
sandstones, which assume much the aspect of a true coal-field, thin
seams of coal having actually been worked in them for more than a
century. A rich harvest of fossil ferns has been obtained from them, as
at Gristhorpe, near Scarborough (Fig. 350). They contain also Cycadeæ,
of which family a magnificent specimen has been described by Mr.
Williamson under the name Zamia gigas, and a fossil called _Equisetum
Columnare_ (see Fig. 397), which maintains an upright position in
sandstone strata over a wide area. Shells of _Estheria_ and _Unio,_
collected by Mr. Bean from these Yorkshire coal-bearing beds, point to
the estuary or fluviatile origin of the deposit.
Fig. 350: Hemitelites Brownii.
At Brora, in Sutherlandshire, a coal formation, probably coeval with
the above, or at least 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 three and a half feet thick, and there are several feet
more of pyritous coal resting upon it.
Fig. 351: Terebratula fimbria. Fig. 352: Rhynchonella spinosa. Fig.
353: Pholadomya fidicula.
Among the characteristic shells of the Inferior Oolite, I may instance
_Terebratula fimbria_ (Fig. 351), _Rhynchonella spinosa_ (Fig. 352),
and _Pholadomya fidicula_ (Fig. 353). 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 deep cleft (_a,_ Figs. 354, 355) on one side of the mouth. The
_Collyrites (Dysaster) ringens_ (Fig. 356) is an Echinoderm common to
the Inferior Oolite of England and France, as are the two Ammonites
(Figs. 357, 358).
Fig. 354: Pleurotomaria granulata. Fig. 355: Pleurotomaria ornata. Fig.
356: Collyrites (Dysaster) ringens. Fig. 357: Ammonites Humphresianus.
Fig. 358: Ammonites Braikenridgii. Fig. 359: Ostrea Marshii.
Palæontological Relations of the Oolitic Strata.—Observations have
already been made on the distinctness of the organic remains of the
Oolitic and Cretaceous strata, and the proportion of species common to
the different members of the Oolite. Between the Lower Oolite and the
Lias there is a somewhat greater break, for out of 256 mollusca of the
Upper Lias, thirty-seven species only pass up into the Inferior Oolite.
Fig. 360: Ammonites macrocephalus.
In illustration of shells having a great vertical range, it may be
stated that in England some few species pass up from the Lower to the
Upper Oolite, as, for example, _Rhynchonella obsoleta, Lithodomus
inclusus, Pholadomya ovalis,_ and _Trigonia costata._
Of all the Jurassic Ammonites of Great Britain, _A. macrocephalus_
(Fig. 360), which is common to the Great Oolite and Oxford Clay, has
the widest range.
We have every reason to conclude that the gaps which occur, both
between the larger and smaller sections of the English Oolites, imply
intervals of time, elsewhere represented by fossiliferous strata,
although no deposit may have taken place in the British area. This
conclusion is warranted by the partial extent of many of the minor and
some of the larger divisions even in England.
[1] Elements of Geology, 4th edition.
[2] I allude to several Zeuglodons found in Alabama, and referred by
some zoologists to three species.
[3] See Phil. Trans. 1850, p. 363; also Huxley, Memoirs of Geol.
Survey, 1864; Phillips, Palæont. Soc.
[4] P. Scrope, Proc. Geol. Soc., March, 1831.
[5] Proceedings Geol. Soc., vol. i, p. 414.
[6] A figure of this recent _Myrmecobius_ will be found in my
Principles of Geology, chap. ix.
[7] Owen’s British Fossil Mammals, p. 62.
CHAPTER XX.
JURASSIC GROUP—_continued_—LIAS.
Mineral Character of Lias. — Numerous successive Zones in the Lias,
marked by distinct Fossils, without Unconformity in the Stratification,
or Change in the Mineral Character of the Deposits. — Gryphite
Limestone. — Shells of the Lias. — Fish of the Lias. — Reptiles of the
Lias. — Ichthyosaur and Plesiosaur. — Marine Reptile of the Galapagos
Islands. — Sudden Destruction and Burial of Fossil Animals in Lias. —
Fluvio-marine Beds in Gloucestershire, and Insect Limestone. — Fossil
Plants. — The origin of the Oolite and Lias, and of alternating
Calcareous and Argillaceous Formations.
Lias.—The English provincial name of Lias has been very generally
adopted for a formation of argillaceous limestone, marl, and clay,
which forms the base of the Oolite, and is classed by many geologists
as part of that group. The peculiar aspect which is most characteristic
of the Lias in England, France, and Germany, is an alternation of thin
beds of blue or grey limestone, having a surface which becomes
light-brown when weathered, these beds being separated by
dark-coloured, narrow argillaceous partings, so that the quarries of
this rock, at a distance, assume a striped and ribbon-like appearance.
The Lias has been divided in England into three groups, the Upper,
Middle, and Lower. The Upper Lias consists first of sands, which were
formerly regarded as the base of the Oolite, but which, according to
Dr. Wright, are by their fossils more properly referable to the Lias;
secondly, of clay shale and thin beds of limestone. The Middle Lias, or
marl-stone series, has been divided into three zones; and the Lower
Lias, according to the labours of Quenstedt, Oppel, Strickland, Wright,
and others, into seven zones, each marked by its own group of fossils.
This Lower Lias averages from 600 to 900 feet in thickness.
From Devon and Dorsetshire to Yorkshire all these divisions, observes
Professor Ramsay, are constant; and from top to bottom we cannot assert
that anywhere there is actual unconformity between any two
subdivisions, whether of the larger or smaller kind.
In the whole of the English Lias there are at present known about 937
species of mollusca, and of these 267 are Cephalopods, of which class
more than two-thirds are Ammonites, the Nautilus and Belemnite also
abounding. The whole series has been divided by zones characterised by
particular Ammonites; for while other families of shells pass from one
division to another in numbers varying from about 20 to 50 per cent,
these cephalopods are almost always limited to single zones, as
Quenstedt and Oppel have shown for Germany, and Dr. Wright and others
for England.
As no actual unconformity is known from the top of the Upper to the
bottom of the Lower Lias, and as there is a marked uniformity in the
mineral character of almost all the strata, it is somewhat difficult to
account even for such partial breaks as have been alluded to in the
succession of species, if we reject the hypothesis that the old species
were in each case destroyed at the close of the deposition of the rocks
containing them, and replaced by the creation of new forms when the
succeeding formation began. I agree with Professor Ramsay in not
accepting this hypothesis. No doubt some of the old species
occasionally died out, and left no representatives in Europe or
elsewhere; others were locally exterminated in the struggle for life by
species which invaded their ancient domain, or by varieties better
fitted for a new state of things. Pauses also of vast duration may have
occurred in the deposition of strata, allowing time for the
modification of organic life throughout the globe, slowly brought about
by variation accompanied by extinction of the original forms.
Fig. 361: Plagiostoma (Lima) giganteum. Fig. 362: Gryphæa incurva.
Fossils of the Lias.—The name of Gryphite limestone has sometimes been
applied to the Lias, in consequence of the great number of shells which
it contains of a species of oyster, or _ Gryphæa_ (Fig. 362). A large
heavy shell called _Hippopodium_ (Fig. 365), allied to _Cypricardia,_
is also characteristic of the upper part of the Lower Lias. In this
formation occur also the Aviculas, Figs. 363 and 364. The Lias
formation is also remarkable for being the newest of the secondary
rocks in which brachiopoda of the genera _Spirifer_ and _ Leptæna_
(Figs. 366, 367) occur, although the former is slightly modified in
structure so as to constitute the subgenus Spiriferina, Davidson, and
the Leptæna has dwindled to a shell smaller in size than a pea. No less
than eight or nine species of Spiriferina are enumerated by Mr.
Davidson as belonging to the Lias. Palliobranchiate mollusca
predominate greatly in strata older than the Trias; but, so far as we
yet know, they did not survive the Liassic epoch.
Fig. 363: Avicula inæquivalvis. Fig. 364: Avicula cygnipes. Fig. 365:
Hippopodium ponderosum. Fig. 366: Spiriferina (Spirifera). Fig. 367:
Leptæna Moorei.
Fig. 368: Ammonites Bucklandi. Fig. 369: Ammonites planorbis. Fig. 370:
Nautilus truncatus. Fig. 371: Ammonites bifrons.
Allusion has already been made, p. 354, to numerous zones in the Lias
having each their peculiar Ammonites. Two of these occur near the base
of the Lower Lias, having a united thickness, varying from 40 to 80
feet. The upper of these is characterised by _Ammonites Bucklandi,_ and
the lower by _Ammonites planorbis_ (see Figs. 368, 369).[1] Sometimes,
however, there is a third intermediate zone, that of _Ammonites
angulatus,_ which is the equivalent of the zone called the infra-lias
on the Continent, the species of which are for the most part common to
the superior group marked by _Ammonites Bucklandi._
Fig. 372: Ammonites margaritatus.
Among the Crinoids or Stone-lilies of the Lias, the Pentacrinites are
conspicuous. (See Fig. 373.) Of _ Palæocoma (Ophioderma) Egertoni_
(Fig. 374), referable to the _Ophiuridæ_ of Muller, perfect specimens
have been met with in the Middle Lias beds of Dorset and Yorkshire.
Fig. 373: Extracrinus (Pentacrinus) Briareus. Fig. 374: Palæocoma
(Ophioderma) tenuibrachiata.
The _Extracrinus Briareus_ (removed by Major Austin from Pentacrinus on
account of generic differences) occurs in tangled masses, forming thin
beds of considerable extent, in the Lower Lias of Dorset,
Gloucestershire, and Yorkshire. The remains are often highly charged
with pyrites. This Crinoid, with its innumerable tentacular arms,
appears to have been frequently attached to the driftwood of the
liassic sea, in the same manner as Barnacles float about on wood at the
present day. There is another species of _ Extracrinus_ and several of
_Pentacrinus_ in the Lias; and the latter genus is found in nearly all
the formations from the Lias to the London Clay inclusive. It is
represented in the present seas by the delicate and rare _Pentacrinus
caput-medusæ_ of the Antilles, which, with Comatula, is one of the few
surviving members of the ancient family of the Crinoids, represented by
so many extinct genera in the older formations.
Fig. 375: Scales of Lepidotus gigas. Fig. 376: a. Scales of Æchmodus
Leachii, b. Æchmodus (restored outline), c. Scales of Dapedius
monilifer.
Fishes of the Lias.—The fossil fish, of which there are no less than
117 species known as British, resemble generically those of the Oolite,
but differ, according to M. Agassiz, from those of the Cretaceous
period. Among them is a species of _ Lepidotus_ (_L. gigas,_ Agassiz),
Fig. 375, which is found in the Lias of England, France, and
Germany.[2] This genus was before mentioned (p. 316) as occurring in
the Wealden, and is supposed to have frequented both rivers and
sea-coasts. Another genus of Ganoids (or fish with hard, shining, and
enamelled scales), called _Æchmodus_ (Fig. 376), is almost exclusively
Liassic. The teeth of a species of _ Acrodus,_ also, are very abundant
in the Lias (Fig. 377).
Fig. 377: Acrodus nobilis. Fig. 378: Hybodus reticulatus, a. Part of
fin, commonly called Ichthyodorylite, b. Tooth.
But the remains of fish which have excited more attention than any
others are those large bony spines called ichthyodorulites (a, Figure
378), which were once supposed by some naturalists to be jaws, and by
others weapons, resembling those of the living Balistes and Silurus;
but which M. Agassiz has shown to be neither the one nor the other. The
spines, in the genera last mentioned, articulate with the backbone,
whereas there are no signs of any such articulation in the
ichthyodorulites.
Fig. 379: Chimæra monstrosa. Fig. 379: Chimæra monstrosa.[3]
These last appear to have been bony spines which formed the anterior
part of the dorsal fin, like that of the living genera _ Cestracion_
and _Chimæra_ (see _a,_ Figure 379). In both of these genera, the
posterior concave face is armed with small spines, as in that of the
fossil _Hybodus_ (Fig. 378), a placoid fish of the shark family found
fossil at Lyme Regis. Such spines are simply imbedded in the flesh, and
attached to strong muscles. “They serve,” says Dr. Buckland, “as in the
_Chimæra_ (Fig. 379), to raise and depress the fin, their action
resembling that of a movable mast, raising and lowering backward the
sail of a barge.”[4]
Reptiles of the Lias.—It is not, however, the fossil fish which form
the most striking feature in the organic remains of the Lias; but the
_Enaliosaurian_ reptiles, which are extraordinary for their number,
size, and structure. Among the most singular of these are several
species of _Ichthyosaurus_ and _Plesiosaurus_ (Figs. 380, 381). The
genus _ Ichthyosaurus,_ or fish-lizard, is not confined to this
formation, but has been found in strata as high as the White Chalk of
England, and as low as the Trias of Germany, a formation which
immediately succeeds the Lias in the descending order. It is evident
from their fish-like vertebræ, their paddles, resembling those of a
porpoise or whale, the length of their tail, and other parts of their
structure, that the Ichthyosaurs were aquatic. Their jaws and teeth
show that they were carnivorous; and the half-digested remains of
fishes and reptiles, found within their skeletons, indicate the precise
nature of their food.
Mr. Conybeare was enabled, in 1824, after examining many skeletons
nearly perfect, to give an ideal restoration of the osteology of this
genus, and of that of the _ Plesiosaurus._[5] (See Figs. 380, 381.) The
latter animal had an extremely long neck and small head, with teeth
like those of the crocodile, and paddles analogous to those of the _
Ichthyosaurus,_ but larger. It is supposed to have lived in shallow
seas and estuaries, and to have breathed air like the Ichthyosaur and
our modern cetacea.[6] Some of the reptiles above mentioned were of
formidable dimensions. One specimen of _ Ichthyosaurus platydon,_ from
the Lias at Lyme, now in the British Museum, must have belonged to an
animal more than 24 feet in length; and there are species of
_Plesiosaurus_ which measure from 18 to 20 feet in length. The form of
the _ Ichthyosaurus_ may have fitted it to cut through the waves like
the porpoise; as it was furnished besides its paddles with a tail-fin
so constructed as to be a powerful organ of motion; but it is supposed
that the _Plesiosaurus,_ at least the long-necked species (Fig. 381),
was better suited to fish in shallow creeks and bays defended from
heavy breakers.
It is now very generally agreed that these extinct saurians must have
inhabited the sea; and it was urged that as there are now chelonians,
like the tortoise, living in fresh water, and others, as the turtle,
frequenting the ocean, so there may have been formerly some saurians
proper to salt, others to fresh water. The common crocodile of the
Ganges is well-known to frequent equally that river and the brackish
and salt water near its mouth; and crocodiles are said in like manner
to be abundant both in the rivers of the Isla de Pinos (Isle of Pines),
south of Cuba, and in the open sea round the coast. In 1835 a curious
lizard (_Amblyrhynchus cristatus_) was discovered by Mr. Darwin in the
Galapagos Islands.[7] It was found to be exclusively marine, swimming
easily by means of its flattened tail, and subsisting chiefly on
seaweed. One of them was sunk from the ship by a heavy weight, and on
being drawn up after an hour was quite unharmed.
Fig. 380: Skeleton of Ichthyosaurus communis, restored by Conybeare and
Cuvier. Fig. 381: Skeleton of Plesiosaurus dolichodeirus, restored by
Rev. W. D. Conybeare.
The families of Dinosauria, crocodiles, and Pterosauria or winged
reptiles, are also represented in the Lias.
Sudden Destruction of Saurians.—It has been remarked, and truly, that
many of the fish and saurians, found fossil in the Lias, must have met
with sudden death and immediate burial; and that the destructive
operation, whatever may have been its nature, was often repeated.
“Sometimes,” says Dr. Buckland, “scarcely a single bone or scale has
been removed from the place it occupied during life; which could not
have happened had the uncovered bodies of these saurians been left,
even for a few hours, exposed to putrefaction, and to the attacks of
fishes and other smaller animals at the bottom of the sea.”[8] Not only
are the skeletons of the Ichthyosaurs entire, but sometimes the
contents of their stomachs still remain between their ribs, as before
remarked, so that we can discover the particular species of fish on
which they lived, and the form of their excrements. Not unfrequently
there are layers of these coprolites, at different depths in the Lias,
at a distance from any entire skeletons of the marine lizards from
which they were derived; “as if,” says Sir H. De la Beche, “the muddy
bottom of the sea received small sudden accessions of matter from time
to time, covering up the coprolites and other exuviæ which had
accumulated during the intervals.”[9] It is further stated that, at
Lyme Regis, those surfaces only of the coprolites which lay uppermost
at the bottom of the sea have suffered partial decay, from the action
of water before they were covered and protected by the muddy sediment
that has afterwards permanently enveloped them.
Numerous specimens of the Calamary or pen-and-ink fish, (_Geoteuthis
bollensis_) have also been met with in the Lias at Lyme, with the
ink-bags still distended, containing the ink in a dried state, chiefly
composed of carbon, and but slightly impregnated with carbonate of
lime. These Cephalopoda, therefore, must, like the saurians, have been
soon buried in sediment; for, if long exposed after death, the membrane
containing the ink would have decayed.[10]
As we know that river-fish are sometimes stifled, even in their own
element, by muddy water during floods, it cannot be doubted that the
periodical discharge of large bodies of turbid fresh water in the sea
may be still more fatal to marine tribes. In the “Principles of
Geology” I have shown that large quantities of mud and drowned animals
have been swept down into the sea by rivers during earthquakes, as in
Java in 1699; and that indescribable multitudes of dead fishes have
been seen floating on the sea after a discharge of noxious vapours
during similar convulsions. But in the intervals between such
catastrophes, strata may have accumulated slowly in the sea of the
Lias, some being formed chiefly of one description of shell, such as
ammonites, others of gryphites.
Fig. 382: Wing of a neuropterous insect.
Fresh-water Deposits.—Insect-beds.—From the above remarks the reader
will infer that the Lias is for the most part a marine deposit. Some
members, however, of the series have an estuarine character, and must
have been formed within the influence of rivers. At the base of the
Upper and Lower Lias respectively, insect-beds appear to be almost
everywhere present throughout the Midland and South-western districts
of England. These beds are crowded with the remains of insects, small
fish, and crustaceans, with occasional marine shells. One band in
Gloucestershire, rarely exceeding a foot in thickness, has been named
the “insect limestone.” It passes upward, says the Reverend P. B.
Brodie,[11] into a shale containing _Cypris_ and _ Estheria,_ and is
full of the wing-cases of several genera of Coleoptera, with some
nearly entire beetles, of which the eyes are preserved. The nervures of
the wings of neuropterous insects (Figure 382) are beautifully perfect
in this bed. Ferns, with Cycads and leaves of monocotyledonous plants,
and some apparently brackish and fresh-water shells, accompany the
insects in several places, while in others marine shells predominate,
the fossils varying apparently as we examine the bed nearer or farther
from the ancient land, or the source whence the fresh water was
derived. After studying 300 specimens of these insects from the Lias,
Mr. Westwood declares that they comprise both wood-eating and
herb-devouring beetles, of the Linnean genera _Elater, Carabus,_ etc.,
besides grasshoppers (_Gryllus_), and detached wings of dragon-flies
and may-flies, or insects referable to the Linnean genera _Libellula,
Ephemera, Hemerobius,_ and _Panorpa,_ in all belonging to no less than
twenty-four families. The size of the species is usually small, and
such as taken alone would imply a temperate climate; but many of the
associated organic remains of other classes must lead to a different
conclusion.
Fossil Plants.—Among the vegetable remains of the Lias, several species
of _Zamia_ have been found at Lyme Regis, and the remains of coniferous
plants at Whitby. M. Ad. Brongniart enumerates forty-seven liassic
acrogens, most of them ferns; and fifty gymnosperms, of which
thirty-nine are cycads, and eleven conifers. Among the cycads the
predominance of _ Zamites,_ and among the ferns the numerous genera
with leaves having reticulated veins (as in Fig. 349), are mentioned as
botanical characteristics of this era.[12] The absence as yet from the
Lias and Oolite of all signs of dicotyledonous angiosperms is worthy of
notice. The leaves of such plants are frequent in tertiary strata, and
occur in the Cretaceous, though less plentifully (see p. 303). The
angiosperms seem, therefore, to have been at the least comparatively
rare in these older secondary periods, when more space was occupied by
the Cycads and Conifers.
Origin of the Oolite and Lias.—The entire group of Oolite and Lias
consists of repeated alternations of clay, sandstone, and limestone,
following each other in the same order. Thus the clays of the Lias are
followed by the sands now considered (see p. 353) as belonging to the
same formation, though formerly referred to the Inferior Oolite, and
these sands again by the shelly and coralline limestone called the
Great or Bath Oolite. So, in the Middle Oolite, the Oxford Clay is
followed by calcareous grit and coral rag; lastly, in the Upper Oolite,
the Kimmeridge Clay is followed by the Portland Sand and limestone (see
Fig. 298).[13] The clay beds, however, as Sir H. de la Beche remarks,
can be followed over larger areas than the sand or sandstones.[14] It
should also be remembered that while the Oolite system becomes
arenaceous and resembles a coal-field in Yorkshire, it assumes in the
Alps an almost purely calcareous form, the sands and clays being
omitted; and even in the intervening tracts it is more complicated and
variable than appears in ordinary descriptions. Nevertheless, some of
the clays and intervening limestones do retain, in reality, a pretty
uniform character for distances of from 400 to 600 miles from east to
west and north to south.
In order to account for such a succession of events, we may imagine,
first, the bed of the ocean to be the receptacle for ages of fine
argillaceous sediment, brought by oceanic currents, which may have
communicated with rivers, or with part of the sea near a wasting coast.
This mud ceases, at length, to be conveyed to the same region, either
because the land which had previously suffered denudation is depressed
and submerged, or because the current is deflected in another direction
by the altered shape of the bed of the ocean and neighbouring dry land.
By such changes the water becomes once more clear and fit for the
growth of stony zoophytes. Calcareous sand is then formed from
comminuted shell and coral, or, in some cases, arenaceous matter
replaces the clay; because it commonly happens that the finer sediment,
being first drifted farthest from coasts, is subsequently overspread by
coarse sand, after the sea has grown shallower, or when the land,
increasing in extent, whether by upheaval or by sediment filling up
parts of the sea, has approached nearer to the spots first occupied by
fine mud.
The increased thickness of the limestones in those regions, as in the
Alps and Jura, where the clays are comparatively thin, arises from the
calcareous matter having been derived from species of corals and other
organic beings which live in clear water, far from land, to the growth
of which the influx of mud would be unfavourable. Portions therefore of
these clays and limestones have probably been formed contemporaneously
to a greater extent than we can generally prove, for the distinctness
of the species of organic beings would be caused by the difference of
conditions between the more littoral and the more pelagic areas and the
different depths and nature of the sea-bottom. Independently of those
ascending and descending movements which have given rise to the
superposition of the limestones and clays, and by which the position of
land and sea are made in the course of ages to vary, the geologist has
the difficult task of allowing for the contemporaneous thinning out in
one direction and thickening in another, of the successive organic and
inorganic deposits of the same era.
[1] Quart. Journ., vol. xvi, p. 376.
[2] Agassiz, Poissons Fossiles, vol. ii, tab. 28, 29.
[3] Agassiz, Poissons Fossiles, vol. iii, tab. C, Fig. 1.
[4] Bridgewater Treatise, p. 290.
[5] Geol. Soc. Transactions, Second Series, vol. i, p. 49.
[6] Conybeare and De la Beche, Geol. Trans., First Series, vol. v, p.
559; and Buckland, Bridgewater Treatise, p. 203.
[7] See Darwin, Naturalist’s Voyage, p. 385. Murray.
[8] Bridgewater Treatise, p. 115.
[9] Geological Researches, p. 334.
[10] Buckland, Bridgewater Treatise, p. 307.
[11] A History of Fossil Insects, etc., 1846. London.
[12] Tableau des Vég. Foss., 1849, p. 105.
[13] Conybeare and Philips’s Outlines, etc., p. 166.
[14] Geological Researches, p. 337.
CHAPTER XXI.
TRIAS, OR NEW RED SANDSTONE GROUP.
Beds of Passage between the Lias and Trias, Rhætic Beds. — Triassic
Mammifer. — Triple Division of the Trias. — Keuper, or Upper Trias of
England. — Reptiles of the Upper Trias. — Foot-prints in the Bunter
formation in England. — Dolomitic Conglomerate of Bristol. — Origin of
Red Sandstone and Rock-salt. — Precipitation of Salt from inland Lakes
and Lagoons. — Trias of Germany. — Keuper. — St. Cassian and Hallstadt
Beds. — Peculiarity of their Fauna. — Muschelkalk and its Fossils. —
Trias of the United States. — Fossil Foot-prints of Birds and Reptiles
in the Valley of the Connecticut. — Triassic Mammifer of North
Carolina. — Triassic Coal-field of Richmond, Virginia. — Low Grade of
early Mammals favourable to the Theory of Progressive Development.