CRETACEOUS GROUP.
Divisions of the cretaceous series in North-Western Europe--Upper
cretaceous strata--Maestricht beds--Chalk of Faxoe--White
chalk--Characteristic fossils--Extinct cephalopoda--Sponges and corals
of the chalk--Signs of open and deep sea--Wide area of white
chalk--Its origin from corals and shells--Single pebbles in
chalk--Siliceous sandstone in Germany contemporaneous with white
chalk--Upper greensand and gault--Lower cretaceous strata--Atherfield
section, Isle of Wight--Chalk of South of Europe--Hippurite
limestone--Cretaceous Flora--Chalk of United States.
Having treated in the preceding chapters of the tertiary strata, we have
next to speak of the uppermost of the secondary groups, called the Chalk
or Cretaceous (No. 6. Table, p. 103.), because in those parts of Europe
where it was first studied its upper members are formed of that
remarkable white earthy limestone, termed chalk (_creta_). The inferior
division consists, for the most part, of clays and sands, called
Greensand, because some of the sands derive a bright green colour from
intermixed grains of chloritic matter. The cretaceous strata in the
north-west of Europe may be thus divided[209-A]:
_Upper Cretaceous._
1. Maestricht beds and Faxoe limestone.
2. Upper white chalk, with flints.
3. Lower white chalk, without flints, passing downwards into chalk marl,
which is slightly argillaceous.
4. Upper greensand.
5. Gault.
_Lower Cretaceous._
6. Lower greensand--Ironsand, clay, and occasional beds of limestone
(Kentish rag).
_Maestricht Beds._--On the banks of the Meuse, at Maestricht, reposing on
ordinary white chalk with flints, we find an upper calcareous formation
about 100 feet thick, the fossils of which are, on the whole, very
peculiar, and all distinct from tertiary species. Some few are of species
common to the inferior white chalk, among which may be mentioned
_Belemnites mucronatus_ (see fig. 197.) and _Pecten quadricostatus_.
Besides the Belemnite there are other _genera_, such as Ammonite, Baculite,
and Hamite, never found in strata newer than the cretaceous, but frequently
met with in these Maestricht beds. On the other hand, Volutes and other
genera of univalve shells, usually met with only in tertiary strata, occur.
The upper part of the rock, about 20 feet thick, as seen in St. Peter's
Mount, in the suburbs of Maestricht, abounds in corals, often detachable
from the matrix; and these beds are succeeded by a soft yellowish
limestone 50 feet thick, extensively quarried from time immemorial for
building. The stone below is whiter, and contains occasional nodules of
grey chert or chalcedony.
M. Bosquet, with whom I lately examined this formation (August, 1850),
pointed out to me a layer of chalk from 2 to 4 inches thick, containing
green earth and numerous encrinital stems, which forms the line of
demarcation between the strata containing the fossils peculiar to
Maestricht and the white chalk below. The latter is distinguished by
regular layers of black flint in nodules, and by several shells, such as
_Terebratula carnea_ (see fig. 201.), wholly wanting in beds higher than
the green band. Some of the organic remains, however, for which St. Peter's
Mount is celebrated, occur both above and below that parting layer, and,
among others, the great marine reptile, called _Mosasaurus_, a saurian
supposed to have been 24 feet in length, of which the entire skull and a
great part of the skeleton have been found. Such remains are chiefly met
with in the soft freestone, the principal member of the Maestricht beds.
_Chalk of Faxoe._--In the island of Seeland, in Denmark, the newest member
of the chalk series, seen in the sea-cliffs at Stevens Klint resting on
white chalk with flints, is a yellow limestone, a portion of which, at
Faxoe, where it is used as a building-stone, is composed of corals, even
more conspicuously than is usually observed in recent coral reefs. It has
been quarried to the depth of more than 40 feet, but its thickness is
unknown. The imbedded shells are chiefly casts, many of them of univalve
mollusca, which, as they strictly belong to the Cretaceous era, are worthy
of notice, since such forms, whether spiral or patelliform, are wanting in
the white chalk of Europe generally. Thus, there are two species of
_Cypraea_, one of _Oliva_, two of _Mitra_, four of the genus _Cerithium_,
six of _Fusus_, two of _Trochus_, one _Patella_, one _Emarginula_, &c., on
the whole, more than thirty univalves, spiral or patelliform, not one of
which is common to the white chalk. At the same time, a large proportion of
the accompanying bivalve shells, echinoderms, and zoophytes, are
specifically identical with fossils of older parts of the Cretaceous
series. Among the cephalopoda of Faxoe, may be mentioned _Baculites
Faujasii_ and _Belemnites mucronatus_, shells of the white chalk.
The claws and entire shell of a small crab, _Brachyurus rugosus_
(Schlotheim), are scattered through the Faxoe stone, reminding us of
similar crustaceans enclosed in the rocks of many modern coral
reefs.[211-A] Some small portions of this coralline formation consist of
white earthy chalk; it is, therefore, clear that this substance must
have been produced simultaneously, a fact of some importance, as bearing
on the theory of the origin of white chalk; for the decomposition of
such corals as we see at Faxoe is capable, we know, of forming white
mud, undistinguishable from chalk, and which we may suppose to have
been dispersed far and wide through the ocean, in which such reefs as
that of Faxoe grew.
[Illustration: Fig. 193. Section from Hertfordshire, in England, to
Sena, in France.]
_White Chalk_ (2. and 3. Tab. p. 209.).--The highest beds of chalk in
England and France consist of a pure, white, calcareous mass, usually
too soft for a building stone, but sometimes passing into a more solid
state. It consists, almost purely, of carbonate of lime; the
stratification is often obscure, except where rendered distinct by
interstratified layers of flint, a few inches thick, occasionally in
continuous beds, but oftener in nodules, and recurring at intervals from
2 to 4 feet distant from each other.
This upper chalk is usually succeeded, in the descending order, by a great
mass of white chalk without flints, below which comes the chalk marl, in
which there is a slight admixture of argillaceous matter. The united
thickness of the three divisions in the south of England equals, in some
places, 1000 feet.[211-B]
The annexed section, fig. 193., will show the manner in which the white
chalk extends from England into France, covered by the tertiary strata
described in former chapters, and reposing on lower cretaceous beds.
Among the conspicuous forms of mollusca wholly foreign to the tertiary and
recent periods, and which we meet with in the white chalk, are the
Belemnite, Ammonite, Baculite, and Turrilite, all genera of _Cephalopoda_,
a family to which the living cuttle-fish and nautilus belong.
[Illustration: Fig. 194. Portion of _Baculites Faujasii_. Maestricht and
Faxoe beds and white chalk.]
[Illustration: Fig. 195. Portion of _Baculites anceps_. Maestricht and
Faxoe beds and white chalk.]
[Illustration: Fig. 196. Turrilites.
_a._ _Turrilites costatus._ Chalk marl.
_b._ Same, showing the indented border of the partition of the chambers.]
[Illustration: Fig. 197. Belemnites.
_a._ _Belemnites mucronatus._
_b._ Same, showing internal structure.
Maestricht, Faxoe, and white chalk.]
Among the brachiopoda in the white chalk, the _Terebratulae_ are very
abundant. These shells are known to live at the bottom of the sea, where
the water is tranquil and of some depth (see figs. 198, 199, 200, 201.).
With these are associated some forms of oyster (see figs. 202. and 204.),
and other bivalves (figs. 203, 205, 206, 207, 208.).
[Illustration: Fig. 198. _Terebratula plicatilis_, dorsal view.
Upper white chalk.]
[Illustration: Fig. 199. _Terebratula plicatilis_, side view.]
[Illustration: Fig. 200. _Terebratula pumilus._ (_Magas pumilus_, Sow.)
Upper white chalk.]
[Illustration: Fig. 201. _Terebratula carnea._ Upper white chalk.]
[Illustration: Fig. 202. _Ostrea vesicularis._ _Gryphaea globosa_, Min. Con.
Upper chalk and upper greensand.]
[Illustration: Fig. 203. _Pecten 5-costatus._ White chalk, upper
and lower greensands.]
[Illustration: Fig. 204. _Ostrea carinata._ Chalk marl, upper
and lower greensands.]
[Illustration: Fig. 205. _Crania Parisiensis_, inferior or attached valve.
Upper white chalk.]
[Illustration: Fig. 206. _Plagiostoma Hoperi_, Sow. Syn. _Lima Hoperi_.
White chalk and upper greensand.]
[Illustration: Fig. 207. _Plagiostoma spinosum_, Sow. Syn. _Spondylus
spinosus_. Upper white chalk.]
Among the rest, no form marks the cretaceous era in Europe, America, and
India, in a more striking manner than the extinct genus _Inoceramus_
(_Catillus_ of Lamk.), the shells of which are distinguished by a
fibrous texture, and are often met with in fragments, having, probably,
been extremely friable.
[Illustration: Fig. 208. _Inoceramus Lamarckii._
Syn. _Catillus Lamarckii_.
White Chalk (Dixon's Geol. Sussex, Tab. 28. fig. 29.)]
[Illustration: Fig. 209. _Eschara disticha._
_a._ Natural size.
_b._ Portion magnified.
White chalk.]
[2 Illustrations: Fig. 210. Fig. 211. A branching sponge in a flint, from
the white chalk. From the collection of Mr. Bowerbank.]
With these mollusca are many corals (figs. 209, 210, 211.) and sea urchins
(fig. 212.), which are alike marine, and, for the most part, indicative of
a deep sea. They are dispersed indifferently through the soft chalk, and
hard flint, and some of the flinty nodules owe their irregular forms to
inclosed zoophytes, as in the specimen represented in fig. 211., where the
hollows in the exterior are caused by the branches of a sponge seen on
breaking open the flint, fig. 210.
[Illustration: Fig. 212. _Ananchytes ovata_. White chalk, upper and lower.
_a_. Side view.
_b_. Bottom of the shell on which both the oral and anal apertures are
placed; the anal being more round, and at the smaller end.]
Of the singular family called _Rudistes_, by Lamarck, hereafter to be
mentioned, as extremely characteristic of the chalk of Southern Europe,
a single representative only (fig. 213.) has been discovered in the
white chalk of England.
[4 Illustrations: _Hippurites Mortoni_, Mantell. Houghton, Sussex. White
chalk. Diameter one seventh of nat. size.
Fig. 213. Two individuals deprived of their opercula, adhering together.
Fig. 214. Same seen from above.
Fig. 215. Transverse section of part of the wall of the shell, magnified
to show the structure.
Fig. 216. Vertical section of the same.
On the side where the shell is thinnest, there is one external furrow and
corresponding internal ridge, a, b. figs. 213, 214.; but they are usually
less prominent than in these figures. This species has been referred to
_Hippurites_, but does not, I believe, fully agree in character with that
genus. I have never seen the opercular piece, or _valve_, as it is called
by those conchologists who regard the _Rudistes_ as bivalve mollusca. The
specimen above figured was discovered by the late Mr. Dixon.]
The remains of fishes of the Upper Cretaceous formations consist chiefly
of teeth of the shark family of genera, in part common to the tertiary,
and partly distinct. But we meet with no bones of land animals, nor any
terrestrial or fluviatile shells, nor any plants, except sea weeds, and
here and there a piece of drift wood. All the appearances concur in
leading us to conclude that the white chalk was the product of an open
sea of considerable depth.
The existence of turtles and oviparous saurians, and of a Pterodactyl or
winged-lizard, found in the white chalk of Maidstone, implies, no doubt,
some neighbouring land; but a few small islets in mid-ocean, like
Ascension, so much frequented by migratory droves of turtles, might perhaps
have afforded the required retreat where these creatures might lay their
eggs in the sand, or from which the flying species may have been blown out
to sea. Of the vegetation of such islands we have scarcely any indication,
but it consisted partly of cycadeous plants; for a fragment of one of these
was found by Capt. Ibbetson in the chalk marl of the Isle of Wight, and is
referred by A. Brongniart to _Clathraria Lyellii_, Mantell, a species
common to the antecedent Wealden period.
_Geographical extent and origin of the While Chalk._--The area over
which the white chalk preserves a nearly homogeneous aspect is so vast,
that the earlier geologists despaired of discovering any analogous
deposits of recent date. Pure chalk, of nearly uniform aspect and
composition, is met with in a north-west and south-east direction, from
the north of Ireland to the Crimea, a distance of about 1140
geographical miles; and in an opposite direction it extends from the
south of Sweden to the south of Bordeaux, a distance of about 840
geographical miles. In Southern Russia, according to Sir R. Murchison,
it is sometimes 600 feet thick, and retains the same mineral character
as in France and England, with the same fossils, including _Inoceramus
Cuvieri_, _Belemnites mucronatus_, and _Ostrea vesicularis_.
But it would be an error to imagine, that the chalk was ever spread out
continuously over the whole of the space comprised within these limits,
although it prevailed in greater or less thickness over large portions of
that area. On turning to those regions of the Pacific where coral reefs
abound, we find some archipelagoes of lagoon islands, such as that of the
Dangerous Archipelago, for instance, and that of Radack, with several
adjoining groups, which are from 1100 to 1200 miles in length, and 300 or
400 miles broad; and the space to which Flinders proposed to give the name
of the Corralline Sea is still larger; for it is bounded on the east by the
Australian barrier--all formed of coral rock,--on the west by New
Caledonia, and on the north by the reefs of Louisiade. Although the islands
in these areas may be thinly sown, the mud of the decomposing zoophytes may
be scattered far and wide by oceanic currents. That this mud would resemble
chalk I have already hinted when speaking of the Faxoe limestone, p. 211.;
and it was also remarked in an early part of this volume, that some even of
that chalk which appears to an ordinary observer quite destitute of organic
remains, is nevertheless, when seen under the microscope, full of fragments
of corals and sponges; together with the valves of entomostraca, the shells
of foraminifera, and still more minute infusoria.[215-A] (See p. 26.)
Now it had been often suspected, before these discoveries, that white
chalk might be of animal origin, even where every trace of organic
structure has vanished. This bold idea was partly founded on the fact,
that the chalk consisted of pure carbonate of lime, such as would result
from the decomposition of testacea, echini, and corals; and partly on
the passage observable between these fossils when half decomposed and
chalk. But this conjecture seemed to many naturalists quite vague and
visionary, until its probability was strengthened by new evidence
brought to light by modern geologists.
We learn from Lieutenant Nelson, that, in the Bermuda Islands, there are
several basins or lagoons almost surrounded and enclosed by reefs of
coral. At the bottom of these lagoons a soft white calcareous mud is
formed by the decomposition of _Eschara_, _Flustra_, _Cellepora_, and
other corallines. This mud, when dried, is undistinguishable from common
white earthy chalk; and some portions of it, presented to the Museum of
the Geological Society of London, might, after full examination, be
mistaken for ancient chalk, but for the labels attached to them. About
the same time Mr. C. Darwin observed similar facts in the coral islands
of the Pacific; and came also to the opinion, that much of the soft
white mud found at the bottom of the sea near coral reefs has passed
through the bodies of worms, by which the stony masses of coral are
everywhere bored; and other portions through the intestines of fishes;
for certain gregarious fishes of the genus _Sparus_ are visible through
the clear water, browsing quietly, in great numbers, on living corals,
like grazing herds of graminivorous quadrupeds. On opening their bodies,
Mr. Darwin found their intestines filled with impure chalk. This
circumstance is the more in point, when we recollect how the fossilist
was formerly puzzled by meeting, in chalk, with certain bodies, called
cones of the larch, which were afterwards recognized by Dr. Buckland to
be the excrement of fish.[216-A] These spiral coprolites (see figures),
like the scales and bones of fossil fish in the chalk, are composed
chiefly of phosphate of lime.
[2 Illustrations: Fig. 217. Fig. 218. Coprolites of fish called
_Iulo-eido-copri_, from the chalk.]
Mr. Dana, when describing the elevated coral reef of Oahu, in the Sandwich
Islands, says, that some varieties of the rock consist of aggregated
shells, imbedded in a compact calcareous base as firm in texture as any
secondary limestone; while others are like chalk, having its colour, its
earthy fracture, its soft homogeneous texture, and being an equally good
writing material. The same author describes, in many growing coral reefs, a
similar formation of modern chalk, undistinguishable from the
ancient.[216-B] The extension over a wide submarine area of the calcareous
matrix of the chalk, as well as of the imbedded fossils, would take place
the more readily, in consequence of the low specific gravity of the shells
of mollusca and zoophytes, when compared with ordinary sand and mineral
matter. The mud also derived from their decomposition would be much lighter
than argillaceous and other inorganic mud, and very easily transported by
currents, especially in salt water.
_Single pebbles in chalk._--The general absence of sand and pebbles in
the white chalk has been already mentioned; but the occurrence here and
there, in the south-east of England, of a few isolated pebbles of quartz
and green schist, some of them 2 or 3 inches in diameter, has justly
excited much wonder. If these had been carried to the spots where we now
find them by waves or currents from the lands once bordering the
cretaceous sea, how happened it that no sand or mud were transported
thither at the same time? We cannot conceive such rounded stones to have
been drifted like erratic blocks by ice[217-A], for that would imply a
cold climate in the Cretaceous period; a supposition inconsistent with
the luxuriant growth of large chambered univalves, numerous corals, and
many fish, and other fossils of tropical forms.
Now in Keeling Island, one of those detached masses of coral which rise up
in the wide Pacific, Captain Ross found a single fragment of greenstone,
where every other particle of matter was calcareous; and Mr. Darwin
concludes that it must have come there entangled in the roots of a large
tree. He reminds us that Chamisso, the distinguished naturalist who
accompanied Kotzebue, affirms, that the inhabitants of the Radack
archipelago, a group of lagoon islands, in the midst of the Pacific,
obtained stones for sharpening their instruments by searching the roots of
trees which are cast up on the beach.[217-B]
It may perhaps be objected, that a similar mode of transport cannot have
happened in the cretaceous sea, because fossil wood is very rare in the
chalk. Nevertheless wood is sometimes met with, and in the same parts of
the chalk where the pebbles are found, both in soft stone and in a
silicified state in flints. In these cases it has often every appearance of
having been floated from a distance, being usually perforated by
boring-shells, such as the _Teredo_ and _Fistulana_.[217-C]
The only other mode of transport which suggests itself is sea-weed. Dr.
Beck informs me, that in the Lym-Fiord, in Jutland, the _Fucus
vesiculosus_, often called kelp, sometimes grows to the height of 10
feet, and the branches rising from a single root form a cluster several
feet in diameter. When the bladders are distended, the plant becomes so
buoyant as to float up loose stones several inches in diameter, and
these are often thrown by the waves high up on the beach. The _Fucus
giganteus_ of Solander, so common in Terra del Fuego, is said by Captain
Cook to attain the length of 360 feet, although the stem is not much
thicker than a man's thumb. It is often met with floating at sea, with
shells attached, several hundred miles from the spots where it grew.
Some of these plants, says Mr. Darwin, were found adhering to large
loose stones in the inland channels of Terra del Fuego, during the
voyage of the Beagle in 1834; and that so firmly, that the stones were
drawn up from the bottom into the boat, although so heavy that they
could scarcely be lifted in by one person. Some fossil sea-weeds have
been found in the Cretaceous formation, but none, as yet, of large size.
But we must not imagine that because pebbles are so rare in the white
chalk of England and France there are no proofs of sand, shingle, and
clay having been accumulated contemporaneously even in the European
seas. The siliceous sandstone, called "upper quader" by the Germans,
overlies white argillaceous chalk, or "plaener-kalk," a deposit
resembling in composition and organic remains the chalk marl of the
English series. This sandstone contains as many fossil shells common to
our white chalk as could be expected in a sea-bottom formed of such
different materials. It sometimes attains a thickness of 600 feet, and
by its jointed structure and vertical precipices, plays a conspicuous
part in the picturesque scenery of Saxon Switzerland, near Dresden.
_Upper greensand_ (4. Tab. p. 209.).--The lower chalk without flints passes
gradually downwards, in the south of England, into an argillaceous
limestone, "the chalk marl," already alluded to, in which ammonites and
other cephalopoda, so rare in the higher parts of the series, appear. This
marly deposit passes in its turn into beds containing green particles of a
chloritic mineral, called the upper greensand. In parts of Surrey
calcareous matter is largely intermixed, forming a stone called
_firestone_. In the cliffs of the southern coast of the Isle of Wight, this
upper greensand is 100 feet thick, and contains bands of siliceous
limestone and calcareous sandstone with nodules of chert.
[2 Illustrations: Fossils of the Upper Greensand.
Fig. 219.
_a._ _Terebratula lyra._ } Upper greensand.
_b._ Same, seen in profile. } France.
Fig. 220. _Ammonites Rhotomagensis._ Upper greensand.]
[Illustration: Fig. 221. _Hamites spiniger_ (Fitton);
near Folkstone. Gault.]
_Gault._--The lowest member of the upper Cretaceous group, usually about
100 feet thick in the S.E. of England, is provincially termed Gault. It
consists of a dark blue marl, sometimes intermixed with greensand. Many
peculiar forms of cephalopoda, such as the _Hamite_ (fig. 221.) and
_Scaphite_, with other fossils, characterize this formation, which, small
as is its thickness, can be traced by its organic remains to distant parts
of Europe, as, for example, to the Alps.
The phosphate of lime, found lately near Farnham, in Surrey, in such
abundance as to be used largely by the agriculturist for fertilizing soils,
occurs exclusively, according to Mr. R. A. C. Austen, in the upper
greensand and gault. It is doubtless of animal origin, and partly
coprolitic, probably derived from the excrement of fish.
LOWER CRETACEOUS DIVISION. (No. 6. Tab. p. 209.)
That part of the Cretaceous series which is older than the Gault has been
commonly called the Lower Greensand. The greater number of its fossils are
specifically distinct from those of the upper cretaceous system. Dr.
Fitton, to whom we are indebted for an excellent monograph on this
formation as developed in England, gives the following as the succession of
rocks seen in parts of Kent.
No. 1. Sand, white, yellowish, or ferruginous, with
concretions of limestone and chert 70 feet.
2. Sand with green matter 70 to 100 feet.
3. Calcareous stone, called Kentish rag 60 to 80 feet.
In his detailed description of the fine section displayed at Atherfield,
in the south of the Isle of Wight, we find the limestone wholly wanting;
in fact, the variations in the mineral composition of this group, even
in contiguous districts, is very great; and on comparing the Atherfield
beds with corresponding strata at Hythe in Kent, distant 95 miles,
the whole series has lost half its thickness, and presents a very
dissimilar aspect.[219-A]
On the other hand, Professor E. Forbes has shown that when the sixty-three
strata at Atherfield are severally examined, the total thickness of which
he gives as 843 feet, there are some fossils which range through the whole
series, others which are peculiar to particular divisions. As a proof that
all belong chronologically to one system, he states that whenever similar
conditions are repeated in overlying strata the same species reappear.
Changes of depth, or of the mineral nature of the 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 nothing to do with
the extinction, throughout a large area, of certain animals or plants. The
rule laid down by this eminent naturalist for enabling us to test the
arrival of a new state of things in the animate world, is the
representation by new and different species of corresponding genera of
mollusca or other beings. When the forms proper to loose sand or soft clay,
or a stony or calcareous bottom, or a moderate or a great depth of water,
recur with all the same species, 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. On passing from the lower greensand to the gault, we suddenly
reach one of these new epochs, scarcely any of the fossil species being
common to the lower and upper cretaceous systems, a break in the chain
implying no doubt many missing links in the series of geological monuments
which we may some day be able to supply.
One of the largest and most abundant shells in the lowest strata of the
lower greensand, as displayed in the Atherfield section, is the large
_Perna mulleti_ of which a reduced figure is here given (fig. 222.).
[Illustration: Fig. 222. _Perna mulleti._ Desh. in Leym.
_a._ Exterior.
_b._ Hinge of upper valve.]
In the south of England, during the accumulation of the lower greensand
above described, the bed of the sea appears to have been continually
sinking, from the commencement of the period, when the freshwater
Wealden beds were submerged, to the deposition of those strata on which
the gault immediately reposes.
Pebbles of quartzose sandstone, jasper, and flinty slate, together with
grains of chlorite and mica, speak plainly of the nature of the
pre-existing rocks, from the wearing down of which the greensand beds
were derived. The land, consisting of such rocks, was doubtless
submerged before the origin of the white chalk, as corals can only
multiply in the clear waters of the sea in spaces to which no mud or
sand are conveyed by currents.
HIPPURITE LIMESTONE.
_Difference between the chalk of the north and south of Europe._--By the
aid of the three tests of relative age, namely, 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.
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 the chalk and Greensand in the neighbourhood of London and Paris 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 the annexed map, fig. 223., in which the
shaded part represents chalk).
[Illustration: Fig. 223. Map of south-western France.]
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 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 a formation which we at once recognize by its
mineral character to be chalk, although there are some places where the
rock becomes oolitic. The fossils are, upon the whole, very similar;
especially certain species of the genera _Spatangus_, _Ananchytes_,
_Cidarites_, _Nucula_, _Ostrea_, _Gryphaea_ (_Exogyra_), _Pecten_,
_Plagiostoma_ (_Lima_), _Trigonia_, _Catillus_, (_Inoceramus_), and
_Terebratula_.[221-A] 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.
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_, _Sphaerulites_, 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.
[Illustration: Fig. 224.
_a._ _Radiolites radiosus_, D'Orb. (_Hippurites_, Lamk.)
_b._ Opercular valve of same.
White chalk of France.]
[Illustration: Fig. 225. _Radiolites foliaceus_, D'Orb. Syn. _Sphaerulites
agariciformis_, Blainv. White chalk of France.]
[Illustration: Fig. 226. _Hippurites organisans_, Desmoulins. Upper
chalk:--chalk marl of Pyrenees?[222-A]
_a._ Young individual; when full grown they occur in groups adhering
laterally to each other.
_b._ Upper side of the opercular valve, showing a reticulated structure
in those parts, _b_, where the external coating is worn off.
_c._ Upper side of the lower and cylindrical valve.
_d._ Cast of the interior of the lower conical valve.]
The species called _Hippurites organisans_ (fig. 226.) is more abundant
than any other in the south of Europe; and the geologist should make
himself well acquainted with the cast _d_, which is far more common in many
compact marbles of the upper cretaceous period than the shell itself, which
has often wholly disappeared. The flutings, or smooth, rounded,
longitudinal ribs, representing the form of the interior, are wholly
unlike the hippurite itself, and in some individuals, which attain a great
size and length, are very conspicuous.
Between the region of chalk last mentioned in which Perigueux is situated,
and the Pyrenees, the space B intervenes. (See Map, p. 221.) Here the
tertiary strata cover, and for the most part conceal, the cretaceous rocks,
except in some spots where they have been laid open by the denudation of
newer formations. In these places they are seen still preserving the form
of a white chalky rock, which is charged in part with grains of green sand.
Even as far south as Tercis, on the Adour, near Dax, where I examined them
in 1828, the cretaceous rocks retain this character. In that region M.
Grateloup has found in them _Ananchytes ovata_ (fig. 212.), and other
fossils of the English chalk, together with _Hippurites_.
FLORA OF THE CRETACEOUS PERIOD.
Although the fossil plants of the Cretaceous era at present known are
few in number, the rocks being principally marine, they suffice,
according to M. Ad. Brongniart, to show a transition character between
the vegetation of the secondary and that of the tertiary formations. The
tertiary strata, when compared to the older rocks, are marked by the
predominance of _Exogens_, which now constitute three-fourths of the
living plants of the globe.[223-A]
These exogens are wanting in the secondary strata generally, but in the
Cretaceous period they equal in number the _Gymnogens_ (_Coniferae_ and
_Cycadeae_) which abounded so much in the preceding Oolitic period, and
disappeared before the Eocene rocks were formed.[223-B] The discovery of a
tree-fern in the ferruginous sands of the Lower Cretaceous group of the
department of Ardennes in France is one of many signs of the contrast of
the flora, and doubtless of the climate, of this era with that of the
Pliocene and Modern periods.
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 our Upper Cretaceous
system; which we can, nevertheless, recognize as referable,
paleontologically, to the same division.
That they were about the same age generally as the European chalk and
greensand, was the conclusion to which Dr. Morton and Mr. Conrad came after
their investigation of the fossils in 1834. The strata consist chiefly of
greensand 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_, _Gryphaea costata_, _Pecten quinque-costatus_, _Belemnites
mucronatus_. 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 are different, the generic forms, such as
the Baculite and certain sections of Ammonites, as also the Inoceramus (see
above, fig. 208.) 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.[224-A]
Fish of the genera _Lamna_, _Galeus_, and _Carcharias_ are common to New
Jersey and the European cretaceous rocks. So also is the genus _Mosasaurus_
among reptiles, and _Pliosaurus_ (Owen), another saurian likewise obtained
from the English chalk. From New Jersey the cretaceous formation extends
southwards to North Carolina, Georgia, and Alabama, cropping out at
intervals from beneath the tertiary strata, between the Appalachian
Mountains and the Atlantic. They then sweep round the southern extremity of
that chain, and stretch northwards again to Tennessee and Kentucky. They
have also been traced far up the valley of the Missouri 275 English miles
above its mouth, to the neighbourhood of Fort Leavenworth; and southwards
to Texas, according to the observations of Ferdinand Roemer; so that already
the area which they are ascertained to occupy in North America may perhaps
equal their extent in Europe. So little do they resemble mineralogically
the European white chalk, that limestone in North America is, upon the
whole, an exception to the rule; and, even in Alabama, where I saw a
calcareous member of this group, the marlstones are much more like the
English and French Lias than any other secondary deposit of the Old World.
At the base of the system in Alabama I found dense masses of shingle,
perfectly loose and unconsolidated, derived from the waste of paleozoic
(or carboniferous) rocks, a mass in no way distinguishable, except
by its position, from ordinary alluvium, but covered with marls
abounding in Inocerami.
In Texas, according to F. Roemer, the chalk assumes a new lithological type,
a large portion of it consisting of hard siliceous limestone, but the
organic remains leaving no doubt in regard to its age.
In South America the cretaceous strata have been discovered in Columbia, as
at Bogota and elsewhere, containing Ammonites, Hamites, Inocerami, and
other characteristic shells.[225-A]
In the South of India, also, at Pondicherry, Verdachellum, and
Trinconopoly, Messrs. Kaye and Egerton have collected fossils belonging to
the cretaceous system. Taken in connection with those from the United
States they prove, says Prof. E. Forbes, that those powerful causes which
stamped a peculiar character on the forms of marine animal life at this
period, exerted their full intensity through the Indian, European, and
American seas.[225-B] Here, as in North and South America, the cretaceous
character can be recognized even where there is no specific identity in the
fossils; and the same may be said of the organic type of those rocks in
Europe and India which succeed next in the ascending and descending order,
the Eocene and the Oolitic.
FOOTNOTES:
[209-A] M. Alcide d'Orbigny, in his valuable work entitled Paleontologie
Francaise, has adopted new terms for the French subdivisions of the
Cretaceous Series, which, so far as they can be made to tally with English
equivalents, seem explicable thus:
Danien. Maestricht beds.
Senonien. Upper and lower white chalk, and chalk marl.
Turonien. Part of the chalk marl and the upper greensand, the latter
being in his last work (Cours Elementaire) termed
Cenomanien.
Albien. Gault.
Aptien. Upper part of lower greensand.
Neocomien. Lower part of same.
[211-A] See paper by the author, Trans. of Geol. Soc., vol. v.
p. 246., 1840.
[211-B] Fitton, Geol. Trans., 2d series, vol. iv. p. 319.
[215-A] Proceedings of Geol. Soc., vol. iii. pp. 7, 8., 1842.
[216-A] Geol. Trans. Second Series, vol. iii. p. 232. plate 31.
figs. 3. and 11.
[216-B] Geol. of U. S. Exploring Exped. p. 252. 1849.
[217-A] See Chapters X. and XI.
[217-B] Darwin, p. 549. Kotzebue's First Voyage, vol. iii. p. 155.
[217-C] Mantell, Geol. of S. E. of England, p. 96.
[219-A] Dr. Fitton, Quart. Geol. Journ., vol. i. p. 179., ii. p. 55.,
and iii. p. 289., where comparative sections and a valuable table
showing the vertical range of the various fossils of the lower greensand
at Atherfield is given.
[221-A] Archiac, sur la Form. Cretacee du S. O. de la France, Mem. de la
Soc. Geol. de France, tom. ii.
[222-A] D'Orbigny's Paleontologie Francaise, pl. 533.
[223-A] 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 palaeontology.
| Brongniart.
| | |Lindley.
| | | Examples.
Cryptogamic.
| |1. Cryptogamous amphigens, or cellular cryptogamic.
| | |Thallogens.
| | | |Lichens, sea-weeds, fungi.
| |2. Cryptogamous acrogens.
| | |Acrogens.
| | | |Mosses, equisetums, ferns,
| | | |lycopodiums--Lepidodendron.
| | | |
Phanerogamic.
| |3. Dicotyledonous gymnosperms.
| | |Gymnogens.
| | | |Conifers and Cycads.
| |4. Dicot. Angiosperms.
| | |Exogens.
| | | |Compositae, leguminosae, umbelliferae,
| | | |cruciferae, heaths, &c. All native
| | | |European trees except conifers.
| |5. Monocotyledons.
| | |Endogens.
| | | |Palms, lilies, aloes, rushes,
| | | |grasses, &c.
[223-B] A. Brongniart, Veget. Foss. Dict. Univ., p. 111., 1849.
[224-A] See a paper by the author, Quart. Journ. Geol. Soc., vol. i. p. 55.
[225-A] Proceed. Geol. Soc. iv. p. 391.
[225-B] See Forbes, Quart. Geol. Journ. vol. i. p. 79.