CHAPTER
XIV.
CRETACEOUS
SERIES.
WHEN the
continent described in last chapter had endured for a long period of
time, submergence
of the area began to take place, accompanied by the deposition of the
purely marine CRETACEOUS
SERIES, which in England is as follows, the oldest beds being placed at
the
bottom:
I may here
mention that in parts of the Continent of Europe, there are certain marine
formations intermediate in position and date between the Oolitic and
Cretaceous rocks, which
are known as the Neocomian beds, so called from Neocomium, the ancient
name of Neuchâtel, in
Switzerland, where they are well developed. The assumption that the
Hastings Sands and Weald
Clay are the fresh-water equivalents in time of the lower and middle
parts of these continental
beds, is undoubtedly correct, the Lower Greensand of English geologists
being the British
representative of the Upper Neocomian strata.
Mr. Judd has shown that at the south end of Filey
[Atherfield Clay. 213]
Bay, in Yorkshire, we have the actual marine representatives of the
continental Neocomian
strata. These Yorkshire beds were formerly called Speeton Clay, and lie
between the uppermost
Oolitic strata of the district, called by Mr. Judd, Portlandian, and
the Red Chalk or Hunstanton
Limestone, which, according to that author, cannot be of later age than
the Upper Greensand, and
may he as early as the Gault.1
The area occupied by the Purbeck and Wealden strata underwent a long
period of slow
depression, during which these fresh-water strata with occasional
marine interstratifications
were deposited; and by sinking still further, the purely marine beds of
the Atherfield Clay
began to be formed. In fact, but for the presence in it of marine
fossils, it is hard to draw any
line between the Wealden and the Atherfield Clays, and no doubt the mud
that formed the latter
was at first carried seaward by the same great river, in the manner,
for example, that muddy
sediments are now deposited at and near the mouth of the Amazons on the
east coast of South
America.
The Atherfield Clay takes its name from Atherfield, on the
south-west coast of the Isle of Wight,
where it is well seen overlying the Weald Clay, and is overlaid by the Lower
Greensand. Its
lowest beds form a kind of passage from the fresh-water strata of the
Weald into the overlying
marine beds of the Lower Greensand, both in the Isle of Wight and in
the Wealden district, round
which it circles at the edge of the Lower Greensand; for at Atherfield
there seems to have been a
depression of the fresh-water area and an influx of the sea,
accompanied by the appearance of
Cerithium carbonarium, accompanied by Pinna and Panopœa
standing vertically in the
position in which they lived. Many other shells
1
'Journal of the Geological Society,' 1868, vol. xxiv., p. 218.
[214 Lower Greensand.]
are scattered through the clay, including the well-known Perna Mulleti, Trigonia
caudata, Gervillia aviculoides, Areas, Pectens, Oysters,
Rostellaria Parkinsoni, and Hemicardium Austeni, &c.
&c.
THE LOWER GREENSAND, of which the Atherfield Clay is a subdivision,
comes next in
succession, in the Isle of Wight, beginning with a bed of sandstone
containing Gryphœa sinuata
and many other shells, succeeded by 29 feet of clay, vulgarly called
the 'lobster bed,' from the
presence of Meyeria magna, formerly called Astacus,
together with Ammonites Deshayesii, &c.,
overlaid by nodular bands with Gervillia aviculoides, &c..,
above which, clay is repeated, with
the same Meyeria. Above this, sands and clays alternate to the top of
the series, with many
fossils, among which may be mentioned as characteristic, Terebratula
sella, T. Gibbsii, T.
biplicata, Limas, Gryphœas, Gervillia solenoides,
Ammonites, Nautili, and other remarkable
Cephalopoda of the genera Crioceras, Ancyloceras, and Hamites.
The whole of these strata
overlying the Wealden beds occur in magnificent sections along the
southern cliffs of the Isle of
Wight, dipping northerly under the Gault, Upper Greensand, and Chalk,
which in a high ridge
stretches across the island from Culver Cliff to Alum Bay. Overlaid by
the Gault, and reposing
on the Weald Clay, the Lower Greensand also sweeps round the whole
Wealden area from
Sandgate to Guildford and Haslemere, and from thence to the coast north
of Beachy Head. Between
Guildford and Haslemere it forms high scarped terraces. The sands are
sometimes quite soft,
with intercalated hard bands, and they are frequently ferruginous. A
good building stone, very
fossiliferous, being sometimes an impure limestone, called the Kentish
rag, lies in the lower
part
FIG.
42.
Group of Atherfield Clay and Lower Greensand Fossils.
[216
Lower Greensand.]
of the formation, on the north side of the Weald at Maidstone. It rests
on the Atherfield Clay. The
general grouping of the fossils in all this area correspon is with that
of the Isle of Wight. In
Dorsetshire and part of Somersetshire, at the south end of the western
escarpments of the
Cretaceous rocks, the Lower Greensand is absent, and the Upper
Greensand rests directly on the
Lias and New Bed series. Further north, the Lower Green sand reappears
in Wiltshire, near
Chapmanslade, about three miles east of Frome, and in a long narrow
band follows the direction
of the escarpment of Chalk, as far as the neighbourhood of Devizes,
where it widens for a space,
and runs north in a projecting tongue as far as Farringdon, where it is
known as the Sponge
gravel.
Beyond the Farringdon area, it is for a space of twelve miles
overlapped unconformably by the
Gault, to reappear a little south of Abingdon in a broad patch, that
extends eastward about six
miles to Chiselhampton, where it is again overlapped by the Gault, to
reappear in a narrow
strip between Great Hazeley and the neighbourhood of Thame. Several
small outliers of Lower
Greensand lie on the Purbeck strata, south, west, and north of
Aylesbury. At Leighton Buzzard
it appears in great force, covering all the country for miles round
Woburn, from whence it
trends away to the north-east, and disappears under the alluvium of the
Fens of
Cambridgeshire, and runs along the east side of the Wash, where,
crossing under sea, it
reappears in Lincolnshire, and following the line of the chalk
escarpment runs in a NNW. line
to the Humber. As a whole this formation may be described as consisting
of yellow, grey, white,
and green sands.
In the Weald country and on the north-west
[Lower Greensand. 217]
side of the great Chalk escarpment, between Devizes and the Wash, the
Lower Greensand is
often ferruginous, and has been worked for iron ore both in ancient and
modern times. Fossil
wood is of frequent occurrence, perhaps of Coniferous trees, and all
the evidence tends to show
that, in the English area, the strata were deposited in comparatively
shallow seas not far from
shore.
The general characters of the fossils of the series are
as follows:—Echinoderms of the genera Salenia, Cardiaster, Diadema, Discoidea,
Echinobrissus, together with Pentacrinites, are found in
it.
Terebratulœ and Rhynchonellœ are of frequent occurrence,
with a few other Brachiopoda.
Among the Lamellibranchiate molluscs are numerous Limas,
Gervillias, Perna, Oysters,
Pectens, and Pinnas, together with shells of the genera Cardium,
Venus, Trigonia, Myacites,
and Nucula. Gasteropoda are not generally numerous. Cephalopoda
of remarkable forms are
characteristic; for, in addition to several species of Ammonites,
Nautili, and Belemnites, there
are Crioceras, and Ancyloceras, like Ammonites half
unrolled, Crioceras Bowerbankii,
Ancyloceras gigas, A. grande, and A. Hillsii. Fishes are
scarce, and only three reptiles have
hitherto been described, one Chelonian, Protemys serrata, a Plesiosaurus,
and a crocodilian
saurian Polyptychodon continuus, said also to occur in the
Lower Chalk.
Out of about 300 Lower Greensand species, 18 or 20 per cent. pass into
the Upper Cretaceous
series. Partly for palæontological considerations, and also
because the Gault seems sometimes to
lie, as it were, unconformably on the eroded surface of the sand, the
dissimilarity in the
grouping of fossils is so great, that
[218 Gault.]
it has been considered advisable to draw a marked line between the two
groups; the Atherfield
Clay and the Lower Greensand, when the term Neocomian is not applied to
them, meaning Lower
Cretaceous, and all above them to the topmost beds of the Chalk
being considered as Upper
Cretaceous strata.
The GAULT forms the base of the Upper Cretaceous
series—or of the Cretaceous series, for those who choose to call the
Lower Greensand Neocomian.
It is a stiff blue clay, about 300 feet thick in its thickest
development, but sometimes it is hard
to separate it lithologically from the Upper Greensand. It appears in
the Isle of Wight, overlying
the Lower Greensand all across the Island; and ranges round the
escarpment of the Weald in the
same position, with occasional signs of a kind of unconformable erosion
between them; and in
the centre of England, from the neighbourhood of Devizes to the Wash in
Norfolk, the Gault
occasionally completely overlaps the Lower Green sand in an
unconformable manner. In proof of
this unconformity, occasional outlying patches of the Lower Greensand
occur north of the Chalk
escarpment, without any visible signs of it immediately at the base of
the neighbouring Upper
Cretaceous strata, which there ought to be, if these formations lay
everywhere conformably on
the Lower Greensand.
Many Foraminifera have been found in the Gault,
and a few Corals, Cyclocyathus Fittoni, Trochosmilia
sulcata, and Caryophyllia Bowerbankii. Its sea-urchins
are of the genera Cidarius (C. Gaultina), Hemiaster
(H. Asterias, H. Baileyi), and Diadema tumida. It
contains many Crustaceans, such as Astacus, Etyus Martini, Diaulax
Carteriana,
Palœocorystes Stokesii, &c. Among the Brachiopoda and
Lamellibranchiate
[Gault. 219]
molluscs the following are characteristic :—Terebratula
biplicata, Rhynchonella sulcata, Oysters, Pectens,
Plicatula pectinoides, Pinna tetragona, &c. ; Gervillia
solenoides, Inoceramus sulcatus, &c.;
Lima parallela, Cucullœa, Arca, Nucula pectinata, &c. it also
yields Gasteropoda of the genera
Dentalium, Solarium, Scalaria, Natica Gaultina, Pleurotomnaria
Gibbsii, Rostellaria
carinata, &c., and many Cephalopoda, such
FIG.
43.
Group of Gault Fossils.
as Belemnites minimus, &C. ; Nautilus inequalis,
&c. ; Ammonites splendens, A. dentatus, A.
interruptus, A. lautus, &c. &c. ; Ancyloceras
spinigerum, Hamites attenuatus; H. rotundus,
&.c. Traces of the Gault may probably be found along the lower
outskirts of the Chalk all the way
to Filey Bay in Yorkshire, where the Red Chalk has by some been
considered to be its equivalent,
or, at all events, to be of a
[220
Upper Greensand.]
geological date, not later than its successor the Upper Greensand.
It would be a great comfort to a proportion of the geological
population, if the different
formations were as clearly distinguishable on the ground, as they are
on a map, by different
colours, aided by numbers or letters for the use of the colour-blind.
If to this, in the economy
of Nature, it had so happened that no species had been permitted to
stray from its own formation
into the next in succession, the benefit would have been much enhanced,
for to those with keen
eyes for form, the finding of any single fossil would be sufficient to
mark the place in the
geological scale of any given formation. Then we should have a perfect
and orderly symmetrical
accuracy of detail, so that he who runs may read. But it so happens
that this is not the case, for
Nature loves variety, and performs her functions in various ways, and
thus it happens that in
certain cases the dividing lines between two formations, if we follow
them far enough, are
sometimes difficult to determine. Of these unruly formations in England
the Upper Greensand
forms one, in its occasional physical relations to its neighbours, the
Gault below and the Chalk
above.
THE UPPER GREENSAND in the West of England appears in great force,
forming part of the
strata that extend from the coast between Lyme Regis and Sidmouth,
northward to the Black
Down Hills, south of Taunton. West of the estuary of the Exe, it forms,
in two outlying patches,
the broad-topped hills of Great Hal-don and Little Hal-don, and south
of the angle of the Teign,
near Newton Bushell, there is another outlier on Milber Down. These lie
so near the main mass,
and approximately are so much on the same level, that they
[Upper Greensand. 221]
are obviously the work of ordinary denuding agencies on a broader area
of Greensand. It is,
however, at first sight somewhat surprising, to meet with a small
outlier of Upper Greensand,
only a few acres in extent, nearly fifty miles to the west of Black
Down, at Orleigh Court, three
miles south-east of Bideford Bay. This patch is mentioned by De la
Beche, in his Report on the
Geology of Cornwall, Devon, and West Somerset, and of late its
existence as a solid outlier has
been doubted. There is, however, so much angular chert on the spot,
that sufficient material
remains to show that the Greensand once spread westward so far, and in
my opinion probably
much farther,
Throughout these areas, the Upper Greensand may be briefly described as
consisting of
yellowish brown sand, partly compact, partly soft, with layers and
detached pieces of chert, and
towards the base it is partly green with specks of silicate of iron.
The sands are often coarse,
and contain layers of shells, frequently broken and fragmentary. The
whole is little more than
200 feet in thickness.
East of Lyme Regis, this Greensand appears near Abbotsbury, on the
southern and western
flanks of the Chalk Downs at Whitehill. West of Chideock outlying
patches lie on the maristone of
the Middle Lias, between Chideock and Bridport on the sand that
underlies the Inferior Oolite, at
Abbotsbury Castle on the Forest Marble, and from thence stretching
north and west, at Shipton
Beacon they lie on the Fuller's Earth. Beyond this the Cretaceous
strata make a great. curve east
of Poorstock and Beaminster, lying generally on the Fuller's Earth.
East of Cheddingtori, for
some miles the Greensand lies on the Oxford Clay, then for a short
space on the sand of the
Calcareous
[222 Upper Greensand.]
Grit, then from Buckland Newton, in Dorsetshire, to the neighbourhood
of Shaftesbury, on
Kimeridge Clay.
Near Shaftesbury the Gault comes on in force, and separates the Upper
Greensand from the
Oolitic rocks as far as the neighbourhood of East Knoyle, on the north
side of the mouth of the
Vale of Wardour. Between East Knoyle and Chapmanslade in Wiltshire, the
Greensand lies
chiefly on the Coral Rag, but partly on the underlying Oxford Clay, and
the Gault, if present at
all, is so thin that it cannot be mapped. It is probable that when the
Gault was deposited
elsewhere, this part of the Oolitic area was above the sea-level. From
Chapmanslade, the
Greensand, underlaid by Gault, runs along the lower part of the Chalk
escarpment in an ENE.
direction, by Westbury to Urchfont and Devizes, and from thence, lying
nearly flat, the strata
form the surface of a broad tract of country, eighteen miles in length
from west to east, bounded
on both sides by Chalk, the whole forming a low anticlinal north and
south curve. Still further
east, at Shalbourne and Sidmonton, near .Kingsclere in Hampshire, two
other tracts of Upper
Greensand rise through the Chalk to the surface in anticlinal curves of
an oval form.
Between Devizes, the Fens of Cambridgeshire, and the east coast of the
Wash, the Upper
Greensand runs to the north-east, in a long somewhat sinuous line, and
nearly all along the
strike it forms the lower part of the bold escarpment of the Chalk,
which overlooks the great
plain or table-land of Oolitic strata that runs across England from the
coast of Dorsetshire to
the Humber. North of the Humber, it is marked in ordinary maps as
skirting the Chalk Wolds,
first to the north and then to the east, as far as Filey Bay, but if
such be the case, its sandy
character is not always
[Upper Greensand. 223]
easily recognisable, which, indeed, is also the case much further south.
In the south-eastern part of Dorsetshire, the Upper Greensand crosses
the Isle of Purbeck from
west to east in a narrow line, overlying the well-known Punfield beds,
and overlaid by the
Chalk of the long and imposing ridge of Purbeck Hill, Knowl Hill, Nine
Barrow Down, and
Ballard Down. The Greensand itself makes no feature in the landscape.
Fig. 75, p. 347.
Striking
east under the sea,
the Upper Greensand barely escapes forming part cf the great seacliff
of
chalk, that runs from Sun Corner near the Needles, to Compton Bay below
Afton
Down, from whence, overlying the Gault, it crosses the Island to the
sea
close under Bembridge Down. In this course, wherever the Chalk Downs
are
narrow, owing to the high angle of northern dip, there the line of
Upper
Greensand is also narrow, but where the angle of inclination is
comparatively
low, there both Chalk and Greensand spread over a broad space, between
Mollestone
Down and Carisbrook. A large outlier of Upper Greensand, capped by two
outliers
of Chalk, overlooks the sea on the south side of the Island between
Chale
Bay and Chine Head, the strata being nearly flat.
In the area of the Weald of Kent and Sussex (fig. 72), the Upper
Greensand at the base of the
escarpment of the Chalk, sweeps round the vast oval, from East Wear
Bay, near Folkestone, to
East Meon, near Petersfield, and from thence to the sea at Eastbourne,
near Beechy Head, but not
with absolute certainty all the way, for only here and there can the
Greensand be faintly
discovered, between the sea and Chevening, along a line of about fifty
miles in length. Beyond
this point it begins to get more distinct, and the malm-rock,
fire-stone,
[224
Upper Greensand.]
and other
lithological varieties, can be traced all along by Westerham, Merstham,
Guildford, the Hog's Back, Farnham, and the extreme west of the area in
the country round
Binstead, Selbourne, and the ground about two miles west and south of
Petersfield, where, as far
as colour goes, it might often be taken for chalk.
On the south side of the Wealden area, the Upper Greensand maintains
the same general
character by Cocking and Barlavington as far as Steyning, where its
lithological character
begins to change, and the beds pass into 'sandy marl and marly sand,'
and near Eastbourne the
strata are decidedly sandy.
Important deductions are to be drawn from the consideration of the
lithological changes that take
place in the character of the Upper Greensand, which will afterwards
appear. A gradual change
may be traced all the way from Devonshire to Cambridgeshire and the
east end of the Wealden
area, which throws some light on the physical geography of the time,
especially when taken in
connection with the circumstance, that out of more than 200 species of
fossils in the Gault,
about 46 per cent. pass onward into the Upper Greensand.1
The Upper Greensand is often
fossiliferous, containing Cycads and Coniferous woods;
Sponges, Siphonia, pyriformis, &c.; a
few Foraminifera; Corals, Trochosmilia tuberosa, Micrabacia coronula;
many Echinoidea, the
chief of which belong to the genera Cidaris, Cardiaster, Echinus,
Pseudo-diadema, Salenia,
&c. Brachiopoda are common, Terebratulœ and Rhynchonellœ (T.
biplicata, Rh. latissima,
1 For much information
on the Upper Greensand of the Wealden area see 'Memoirs of the
Geological Survey, Geology of the Weald,' by W. Topley, 1875.
[Upper Greensand. 225]
&c.). In Lamellibranchiate molluscs it is even richer than the
Lower Greensand, abounding
especially in species of the genera Inoceramus, Gryphœa (1avigata),
Lima, Pecten asper,
Astarte, Trigonia, Cucullœa, Cyprina, and Cytherea. It is
also rich in Gasteropoda, such as
Turritella, Pleurotomaria, Natica (Y. Gentii),
&c., and yields many species of Ammonites, Nautili,
FIG.
44.
Group of Upper Greensand Fossils.
Hamites,
Baculites, Scaphites, and Belemnites. Crustacea, Hoploparia
longimana, Necrocarcinus Bechii, &c. Probably three species of
Reptilia belong to this formation,
Plesiosaurus pachycomus, a Crocodile,
and a Turtle.
THE CHALK, from its familiar characters and general uniformity of
structure, is the most
easily recognisable
[226 Chloritic and Chalk Marl.]
of all the British formations. From west to east it stretches from the
neighbourhood of
Beaminster in Dorsetshire, to Beachy Head and the North Foreland, and
passing beneath the
Eocene formations of the Hampshire and London basins it spreads
northward to Speeton, in
Yorkshire.
The Chloritic Marl indicates a passage from the Upper
Greensand into the Chalk. It consists of a
chalky base specked with green grains, and varies from a few inches to
a few feet in thickness.
It is highly fossififerous, abounding in Ammonites, Nautili (N.
lœvigata), and a small Scaphite
(S. œqualis), besides Oysters, Trigonias, Holaster,
&c., and many other Echinodermata.
The Chalk Marl, which lies above the Chloritic Marl when both
are present, is merely chalk
with a slight admixture of argillaceous matter, and with its
predecessor by no means deserves
to be considered as a separate formation. The whole, therefore, may be
massed as The Chalk. It
consists of a soft white limestone, generally much jointed where
exposed in quarries, and but
for lines of flints, the bedding would often be scarcely
distinguishable. On minute examination
with the microscope, much of the Chalk is found to consist of the
shells of Foraminifera,
Diatomaceæ, spiculæ and other remains of Sponges, Polyzoa,
and shells, highly comminuted.
Somewhat similar deposits are now forming in the open Atlantic at great
depths, chiefly of
Foraminifera of the genus Globigerina, Polycystina and
Diatomaceæ, and spiculæ of Sponges. In
the Pacific, also, from Java to the Low Archipelago, over an area of
about 4,000 miles in
length, all the deep-sea deposits are of fine, white, calcareous mud,
like unconsolidated chalk.
In its thickest development in England the Chalk is
[The
Chalk. 227]
about 1,200 feet thick in Dorsetsliire, Hampshire, &c. The Lower
Chalk usually contains no
flints and,as already stated, is somewhat many at the base, while the
Upper Chalk is
interstratified with many beds of interrupted flints. These are of
irregular form, and lie in
layers in the lines of bedding. A great proportion of them are stated
by Dr. Bowerbank to be
silicified Sponges, which often inclose other organic bodies, such as
shells, fragments of
Belemnites, &c.; others of large size, called Paramoudras, which
are rare, stand vertically
across the beds. These sometimes resemble, in general form, the large
cup-shaped sponges of
the Indian Ocean Alcyonium, or Neptune's cup.
As a whole, the Chalk dips gently from its western escarpment to the
east and south, and round
the Wealden area to the south and north, underlying the Tertiary strata
of the Hampshire and
London basins, and reappearing with precisely the same characters on
the coast of France. Its
area in Europe and Asia is immense. In the north of Ireland, between
Belfast and the Giant's
Causeway, there are patches of very hard Chalk on the coast, overlaid
by columnar basalt of
Miocene age. The great superincumbent pressure of these masses of
igneous rocks has hardened
the chalk, and therefore they have not, as is usually supposed, been
altered by the heat of
overflowing lavas, except possibly for an inch or two at the immediate
point of junction, but
this is somewhat foreign to our present subject. Traces of Chalk and
Upper Greensand occur at
Bogingarry, &c., in Aberdeenshire. These consist partly of chalk
flints, partly of sandstone,
possibly in place, and sufficient to show that Cretaceous rocks, which
have been removed by
denudation, probably once spread over that country. Cretaceous strata,
discovered by Mr.
[228 The Chalk]
Judd, also occur in the Island of Mull beneath the Miocene basalts.
About half the genera, and a considerable number of Chalk species, are
identical with those of
the Gault and Upper Greensand, but it contains a far greater number,
nearly 800, most of
which are peculiar to itself. Plants are few, as might he expected in a
wide deep-sea deposit. A
great many Sponges have been described, chiefly from flints. Among the
most numerous are
species belonging to the genera Ventriculites, Gephalites, Spongia,
and Siphonia. A large number
of genera and species of Foraminifera are also described, among which Globigerina
bulloides,
Dentalina gracilis, and Rotalina ornata, are common. Of
Corals about 15 species are known,
several of which belong to the genus Parasmilia (centralis,
&c.), Caryophillia lœvigata, &c.
Echinodermata are very numerous, among others including the genera Ananchytes,
Cardiaster, Cidaris, Cyphosoma, Diadema, Echinopsis, Galerites and
Echinobrissus, Holaster, Micraster,
and Solenia, &c. Among its starfish are comprised the
genera Arthraster, Goniaster, and
Oreaster. Of these Goniaster is exceedingly characteristic. In
addition it has yielded an Ophiura
and several Crinoids, Bourgueticrinus ellipticus, Marsupites Milleri,
&c. On shells, &c.,
found in the Chalk, are frequent Serpulæ. It also yields
Cirripeds and a few Crustaceans,
Enoploclytia Sussexensis &c. Polyzoa are numerous, of many
species. Like other members of
the Cretaceous rocks, its Brachiopoda generically resemble those of the
Oolites, including
Rhynconella, Terebratulina, and Terebratula. The
Lamellibranchiate molluscs of the Chalk are
in some cases specifically identical with those of the Gault and Upper
Greensand; and,
generically, they bear the
[229]
FIG.
45.
Group of Fossils from the Chalk.
[230
The chalk.]
strongest resemblance, consisting, among others, of
many species of Inoceramus, Lima, Pecten, Oyster,
Spondylus, Radiolites, Trigonia, &c. Being a deepsea deposit,
it is poor in Gasteropoda, but rich
in Cephalopoda, especially in Nautili (N. elegans, &c), Ammonites
(A. Rothomagensis, &c.),
and Turrilites (T. costatus), besides Baculites, Hamites
simplex, Scaphites (S. œqualis), and
Belemnites.
Numerically as individually, though still very characteristic,
Cephalopoda are less numerous in
the Cretaceous than in the Oolitic and Liassic strata, though the
latter contain fewer genera. In
the Lias and Oolites there are nearly 300 species of Cephalopoda, most
of which are Ammonites.
In the Cretaceous rocks less than 200 species are known, about 70 of
which are Ammonites.
More than 80 species of fish are known in the Chalk, including all the
four orders of Agassiz,
Placoids, Ganoids, Cycloids, and Ctenoids. Many of the Placoids are
Cestraciont fish, numerous
species being of the genus Ptychodus. Ten genera of reptiles are known,
two of which are allied
to the Crocodilia, Acanthophilis horridus, and Leiodon anceps
; the great Mosasaurus, 3
species; Plesiosaurus, 2 species, Ichthyosaurus and Pterodactyle,
one of which is said to have
measured eighteen feet across the expanded wings. Several Turtles occur
in the Chalk, Chelone
Bertstedi, &c.
Having thus briefly described the Tipper Cretaceous strata of England,
I shall next endeavour to
show what inferences may be drawn with regard to the physical geography
of the British area,
during the time occupied by their deposition.
We have already seen that, during the deposition of the Purbeck and
Wealden strata, England
formed part
[Physical Geography. 231]
of a great continent, and that, during the formation of the Lower
Greensand, this land suffered
partial submergence, but by no means to such an extent that the Oolitic
strata, which then
extended far to the west, round Wales, were entirely sunk beneath the
sea in which our Lower
Greensand was deposited.
As a whole, the Lower Greensand, being a coarse and sandy formation,
was deposited in shallow
water, and great part of it was in the long run tranquilly heaved out
of the sea, to undergo
terrestrial waste and denudation before the deposition of the Gault
began.
The deposition of the Gault in our area, first took place on a surface
of country that was being
gradually submerged, and part of the sediment was laid on the Lower
Greensand, and part on
various members of the Oolitic strata, from which the Lower Greensand
had been removed by
denudation. This Gault Clay is, however, so difficult to separate from
the Upper Greensand in the
eastern part of England, and the Upper Greensand is so difficult to
separate by any clear line
from the Chalk, that it now becomes necessary to consider the question
of the mode of deposition
of all three, if, indeed, except as local developments of different
sedimentary character, they
ought not to be considered, on a broad scale, as only one formation.
Right or wrong, the origin of
this idea was first declared by Mr. Godwin-Austen, whose large grasp of
questions in physical
geology, (to be found only in scattered memoirs, and unfortunately
often only spoken in
accidental remarks,) is by no means so well known as it would have
been, had he printed all his
stores of geological knowledge in consecutive form. All that I know of
this subject with respect
to these Cretaceous formations, is in the first
[232 Physical Geography.]
instance derived from him, subsequently aided by personal observation
on the ground.
The story revealed by these various strata is this: When, after the
temporary upheaval of the
Lower Greensand, the land gradually sank in part beneath the sea, it
happened that the Upper
Greensand was being deposited far in the west on a sea-bottom that now
forms an eastern part of
Devonshire. Not far from its margin, a fragment of the old greater
land, in our day known in a
modified form as the granite hills of Dartmoor, stood high above the
level of the sea, and at the
same time, on the opposite side of what is now the Bristol Channel,
Wales also formed high land.
The pebbly shore of the lower land near Dartmoor, has long ago been
destroyed by denudation,
but the sediments laid down not far from the shore still exist in the
coarse sandy strata that
form the Upper G-reensand west and east of the river Exe. As we go
eastward from that area
towards Devizes, the Upper Greensand still continues to be
comparatively coarse, and by
degrees in Buckinghamshire, and Bedfordshire, and on into
Cambridgeshire, it gets finer and
finer, and at length becomes white, calcareous, and marly, and, as it
were, seems to mingle with
the Gault beneath and the Chalk above, and the Gault, indeed, in a
lithological point of view,
sometimes seems to disappear altogether.
In like manner, at the western end of the Wealden area, and along the
base of the South and North
Downs, the Upper Greensand for many miles consists of fine, white sand,
and in the Malm-rock
is somewhat chalk-like and calcareous, till going further east towards
Folkestone, it gradually
becomes untraceable as a special formation, and merges into the
underlying Gault and the
overlying Chalk.
[Physical Geography. 233]
The meaning of this is, that distinct coarse Upper Greensand strata
were deposited not far from
shore in the west, gradually getting finer towards the east, because
the finer and lighter
material was drifted further from shore. At the very same time, in the
farther east of what is
now England, the sediments were still finer, and depositions akin to
Chalk, and even the Chalk
itself, had begun to be formed in a deeper sea, far removed from land,
so that according to this
view, part of the lowest strata of the Chalk, in the eastern and
southeastern parts of England,
were deposited contemporaneously with the coarse Upper Greensand of
eastern Devonshire. On
no other hypothesis that I know than this of Godwin-Austen's, can the
phenomena connected with
the Gault, Upper Greensand, and the lower strata of the Chalk, be
rationally accounted for, and I
believe that hypothesis to be true.
The upper strata of the Chalk consist of nearly pure chalk with lines
of flint, and as it
accumulated, the sinking of the western and northern fragments of the
old continent steadily
continued, till at length they almost, if not entirely, sank beneath a
sea, broad and silent,
except when roused by storms, like the Atlantic of our own time, for
though the Echini and
shells found in our chalk, show that the sea of those days was not so
deep as the present Atlantic,
yet the prevalence of prodigious numbers of Globigerina and other
Foraminifera shows that the
old and the new seas are akin in the nature of their organic sediments.
If the whole of the older
land was not submerged, (making an allowance for the lowering of the
mountain lands by
subsequent subaerial waste,) even then we can only suppose that a few
insignificant islets rose
above a waste of waters, that spread not only over Britain, but also
[234 Physical Geography.]
over a very large part of the Europe of the present day, long before
the Alps and the Pyrenees
rose into mountain chains, and only a few islands formed of
Palæozoic rocks stood above the
waves. This surely was a striking phase of an older physical geography,
which affected areas far
wider than Europe alone, but which in the course of time came to an end
in a manner which we
shall presently see. To do so thoroughly we must consider the rocks of
the continent for a little.
A vast lapse of time took place between the close of the deposition of
the uppermost Cretaceous
strata of England, and the commencement of the deposition of the
succeeding Eocene formations,
for in England we have no deposits of intermediate age. What, however,
helps to prove this great
hiatus is, that on the Meuse, at Maestricht, there is a calcareous
formation about 100 feet
thick, which lies unconformably on the Chalk, the line of unconformity
being marked by a line
of water-worn flint pebbles. Some of the fossils are of the same
species with those found in the
Chalk, and Cephalopoda of the genera Baculites and Hamites,
not yet known in strata younger
than the Cretaceous rocks of Europe, are found in the Maestricht beds.
On the other hand,
Volutes, and other genera of Tertiary type, are found in the strata, so
that this marine fauna
may be said to be of a type intermediate to those of the Cretaceous and
Eocene epochs.
Extending for great distances round Paris, there are numerous small
patches of pisolitic
limestone, once united, but now separated by denudation. These contain
some Cretaceous species,
but many others are more Eocene than Cretaceous in their affinities.
At Faxoe also, in the Isle of Seeland, in Denmark, there is a yellow
limestone so full of corals
that it was
[Physical Geography. 235]
probably a coral reef. It contains among other shells many univalves
which are unknown or
rare in the Chalk, such as Cyprœa, Oliva, &c., and along
with these Baculites and Belemnitella,
both unknown in European Eocene strata, though the latest intelligence
from Australia tells of a
Belemnite in certain late Tertiary strata there. Overlying the common
white Chalk, this Faxoe
formation by its fauna also seems to be intermediate in date to the
ordinary Cretaceous and
Eocene strata.
But without such data as these it is evident to any reflective mind,
that a great gap in time,
unrepresented by any sedimentary formations in England, took place in
our area between the
deposition of the latest bed of English Chalk, and that of the earliest
Eocene stratum, for,
excepting a few Foraminifera, the species found in the Chalk seem all
to have been remodelled
before our Eocene epoch began, in so far that palæontologists
recognise none of the species as
identical, and before the days of Darwin they would generally have been
spoken of as new
creations.