WEALDEN GROUP.
The Wealden divisible into Weald Clay, Hastings Sand, and Purbeck
Beds--Intercalated between two marine formations--Weald clay and
Cypris-bearing strata--Iguanodon--Hastings sands--Fossil fish--Strata
formed in shallow water--Brackish water-beds--Upper, middle, and lower
Purbeck--Alternations of brackish water, freshwater, and
land--Dirt-bed, or ancient soil--Distinct species of fossils in each
subdivision of the Wealden--Lapse of time implied--Plants and insects
of Wealden--Geographical extent of Wealden--Its relation to the
cretaceous and oolitic periods--Movements in the earth's crust to
which it owed its origin and submergence.
Beneath the cretaceous rocks in the S.E. of England, a freshwater formation
is found, called the Wealden (see Nos. 5. and 6. Map, p. 242.), which,
although it occupies a small horizontal area in Europe, as compared to the
chalk, is nevertheless of great geological interest, not only from its
position, as being interpolated between two great marine formations (Nos.
7. and 9. Table, p. 103.), but also because the imbedded fossils indicate a
grand succession of changes in organic life, effected during its
accumulation. It is composed of three minor divisions, the Weald Clay, the
Hastings, and the Purbeck Beds, of which the aggregate thickness in some
districts may be 700 or 800 feet; but which would be much more considerable
(perhaps 2000 feet), were we to add together the extreme thickness acquired
by each of them in their fullest development.
The common name of Wealden was given to the whole, because it was first
studied in parts of Kent, Surrey, and Sussex, called the Weald, (see Map,
p. 242.), 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, Terebratulae, Echinites, Corals, and other marine fossils, so
characteristic of the cretaceous rocks above, and of the Oolitic strata
below, and to the presence of Paludinae, Melaniae, and various fluviatile
shells, as well as the bones of terrestrial reptiles and the trunks and
leaves of land plants.
[Illustration: Fig. 227. Position of the Wealden between two
marine formations.]
The evidence of so unexpected a fact as the infra-position of a dense mass
of purely freshwater origin to a deep-sea deposit (a phenomenon with which
we have since become familiar, in other chapters of the earth's
autobiography), 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 Greensand in various
parts of Surrey, Kent, and Sussex; and if we proceed from Sussex westward
to the Vale of Wardour, we there again observe the same formation, or, at
least, the lower division of it, the Purbeck, occupying the same relative
position, and resting on the Oolite (see fig. 228.). Or if we pass from the
base of the South Downs in Sussex, and cross to the Isle of Wight, we there
again meet with the Wealden series reappearing beneath the Greensand, and
cannot doubt that the beds are prolonged subterraneously, as indicated by
the dotted lines in fig. 229.
[Illustration: Fig. 228. Cross section.
O, Oolite.
G S, Greensand, or Lower Cretaceous.]
[Illustration: Fig. 229. Cross section.]
The minor groups into which the Wealden has been commonly divided in
England are, as before stated, three, and they succeed each other in the
following descending order[227-A]:--
Thickness.
1st. Weald Clay, sometimes including thin beds of
sand and shelly limestone 140 to 280 ft.
2d. Hastings Sand, in which occur some clays and
calcareous grits 400 to 500 ft.
3d. Purbeck Beds, consisting of various kinds of
limestones and marls 150 to 200 ft.
_Weald Clay._
The first division, or Weald Clay, is of purely freshwater origin. The
uppermost beds are not only conformable, as Dr. Fitton observes, to the
inferior strata of the Lower Greensand, 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 freshwater, 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 Lower Greensand. Hence we may infer that some of the
saurians which inhabited the country of the great river continued to live
when part of the country 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 freshwater 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 freshwater
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 freshwater strata.
The Iguanodon, first discovered by Dr. Mantell, has left more of its
remains in the Wealden strata of the south-eastern counties, and Isle of
Wight, than any other genus of associated saurians. It was an
herbivorous reptile, and regarded by Cuvier as more extraordinary than
any with which he was acquainted; for the teeth, though bearing a great
analogy to the modern Iguanas which now frequent the tropical woods of
America and the West Indies, exhibit many striking and important
differences (see fig. 230.). It appears that they have been worn by
mastication; whereas the existing herbivorous reptiles clip and gnaw off
the vegetable productions on which they feed, but do not chew them.
Their teeth, when worn, present an appearance of having been chipped
off, and never, like the fossil teeth of the Iguanodon, have a flat
ground surface (see fig. 231.), resembling the grinders of herbivorous
mammalia. Dr. Mantell computes that the teeth and bones of this animal
which have passed under his examination during the last 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 24 inches in circumference. Yet
notwithstanding that the teeth were more numerous than any other bones,
it is remarkable that it was not till the relics of all these
individuals had been found, that a solitary example of part of a
jaw-bone was obtained. More recently remains both of the upper and lower
jaw have been met with in the Hastings Beds in Tilgate Forest. Their
size was somewhat greater than had been anticipated, and even allowing
that the tail was short, which Professor Owen infers from the short
bodies of the caudal vertebrae, Dr. Mantell estimates the probable length
of some of these saurians at between 30 and 40 feet. The largest femur
yet found measures 4 feet 8 inches in length, the circumference of the
shaft being 25 inches, and round the condyles 42 inches.
[2 Illustrations: Teeth of Iguanodon.
Fig. 230. Partially worn tooth of a young animal. (Mantell.)
Fig. 231. Crown of tooth in adult, worn down. (Mantell.)]
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.
[Illustration: Fig. 232. _Cypris spinigera_, Fitton.]
[Illustration: Fig. 233. _Cypris Valdensis_, Fitton. (_C. faba_,
Min. Con. 485.)]
[Illustration: Fig. 234. _Cypris tuberculata_, Fitton.]
[Illustration: Fig. 235. Sample with lamination.]
Shells of the _Cypris_, an animal belonging to the Crustacea, and before
mentioned (p. 31.) as abounding in lakes and ponds, are also plentifully
scattered through the clays of the Wealden, sometimes producing, like
the plates of mica, a thin lamination (see fig. 235.). Similar
cypriferous marls are found in the lacustrine tertiary beds of
Auvergne (see above, p. 183.).
_Hastings Sands._
This middle division of the Wealden consists of sand, calciferous grit,
clay, and shale; the argillaceous strata, notwithstanding the name,
being nearly in the same proportion as the arenaceous. The calcareous
sandstone and grit of Tilgate Forest, near Cuckfield, in which the
remains of the Iguanodon and Hyleosaurus were first found, constitute an
upper member of this formation. The white "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 it, consist partly of saurians,
already 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 Testudinata of the genera _Trionyx_ and
_Emys_, now confined to tropical regions.
[Illustration: Fig. 236. _Lepidotus Mantelli_, Agass. Wealden.
_a._ palate and teeth.
_b._ side view of teeth.
_c._ scale.]
The fishes of the Wealden belong partly to the genera _Pycnodus_ and
_Hybodus_ (see figure of genus in Chap. XXI.), forms common to the Wealden
and Oolite; but the teeth and scales of a species of _Lepidotus_ are most
widely diffused (see fig. 236.). The general form of these fish was that of
the carp tribe, although perfectly distinct in anatomical character, and
more allied to the pike. The whole body was covered with large rhomboidal
scales, very thick, and having the exposed part covered with enamel. Most
of the species of this genus are supposed to have been either river fish,
or inhabitants of the coasts, having not sufficient powers of swimming to
advance into the deep sea.
[Illustration: Fig. 237. _Corbula alata_, Fitton. Magnified.]
The shells of the Hastings beds belong to the genera _Melanopsis_,
_Melania_, _Paludina_, _Cyrena_, _Cyclas_, _Unio_, and others, which
inhabit rivers or lakes; but one band has been found in Dorsetshire
indicating a brackish state of the water, and, in some places, even a
saltness, like that of the sea, where the genera _Corbula_ (see fig. 237.),
_Mytilus_, and _Ostrea_ occur. At different heights in the Hastings Sand,
in the middle of the Wealden, 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, near Horsham, 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. 238.).
[Illustration: Fig. 238. Underside of slab of sandstone about one yard in
diameter. Stammerham, Sussex.]
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.[230-A] 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.
[Illustration: Fig. 239. _Sphenopteris gracilis_ (Fitton), from
near Tunbridge Wells.
_a._ portion of the same magnified.]
From such facts we may infer that, notwithstanding the great thickness of
this division of the Wealden (and the same observation applies to the Weald
Clay and Purbeck Beds), 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, such as the Portland Oolite, 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.
_Purbeck Beds._
Immediately below the Hastings Sands we find a series of calcareous
slates, marls, and limestones, called the Purbeck Beds, because well
exposed to view in the sea-cliffs of the Peninsula of Purbeck,
especially in Durlestone Bay, near Swanage. They may also be
advantageously studied at Lulworth Cove and the neighbouring bays
between Weymouth and Dorchester. At Meup's Bay in particular, Prof. E.
Forbes has recently examined minutely the organic remains of the three
members of the Purbeck group, displayed there in a vertical section 155
feet thick. To the information previously supplied in the works of
Messrs. Webster, Fitton, De la Beche, Buckland, and Mantell, he has made
most ample and important additions, so that it will be desirable to give
them at some length, it appearing 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. This
result cannot fail to excite much wonder, and it leads us to suspect
that the Wealden period, which many geologists have scarcely deigned
to notice in their classification, may comprehend the history of a
lapse of time as great as that of the Oolitic or Cretaceous
eras respectively.[231-A]
_Upper Purbeck._--The highest of the three divisions is purely freshwater,
the strata, about 50 feet in thickness, containing shells of the genera
_Paludina_, _Physa_, _Lymnea_, _Planorbis_, _Valvata_, _Cyclas_, and
_Unio_, with cyprides, and fish.
_Middle Purbeck._--To these succeed the Middle Purbeck, about 30 feet
thick, the uppermost part of which consists of freshwater 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 _Corvulae_ and _Melaniae_.
These are based on a purely marine deposit, with _Pecten_, _Modiola_,
_Avicula_, and _Thracia_, all undescribed shells. Below this, again, come
limestones and shales, partly of brackish and partly of freshwater origin,
in which many fish, especially species of _Lepidotus_ and _Microdon
radiatus_, are found, and a reptile named _Macrorhyncus_. Among the
mollusks, a remarkable ribbed _Melania_, of the section _Chilira_, occurs.
Immediately below is the great and conspicuous stratum, 12 feet thick, long
familiar to geologists under the local name of "Cinder-bed," formed of a
vast accumulation of shells of _Ostrea distorta_ (fig. 240.). In the
uppermost part of this bed Mr. Forbes discovered the first echinoderm as
yet known in the Purbeck series, a species of _Hemicidaris_, a genus
characteristic of the Oolitic period. It was accompanied by a species of
_Perna_. Below the Cinder-bed freshwater strata are again seen, filled in
many places with species of _Cypris_, _Valvata_, _Paludina_, _Planorbis_,
_Lymnea_, _Physa_, and _Cyclas_, all different from any we had previously
seen above. Thick siliceous beds of chert, filled with these fossils, occur
in a beautiful state of preservation, often converted into chalcedony.
Among these Mr. Forbes met with gyrogonites (the spore vesicles of
_Charae_), plants never before discovered in rocks older than the Eocene.
Again, beneath these freshwater strata, a very thin band of greenish
shales, with marine shells and impressions of leaves, like those of a large
_Zostera_, succeeds, forming the base of the Middle Purbeck.
[Illustration: Fig. 240. Ostrea distorta. Cinder-bed.]
_Lower Purbeck._--Beneath the thin marine band last mentioned, purely
freshwater marls occur, containing species of _Cypris_, _Valvata_, and
_Lymnea_, 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 more than 30 feet of brackish-water beds, at Meup's Bay, abounding in
a species of _Serpula_, allied to, if not identical with, _Serpula
coacervites_, found in the Wealden of Hanover. There are also shells of the
genus _Rissoa_ (of the subgenus _Hydrobia_), and a little _Cardium_ of the
subgenus _Protocardium_, in the same 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 _Cycadeae_, which I shall presently
describe, underlies these marls, resting upon the lowest freshwater
limestone, a rock about 8 feet thick, containing _Cyclades_, _Valvata_, and
_Lymnea_, of the same species as those of the uppermost part of the Lower
Purbeck. This rock rests upon the top beds of the Portland stone, which is
purely marine, and between which and the Purbecks there is no passage.
The most remarkable of all the varied successions 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 fragments of stone,
from 3 to 9 inches in diameter, in such numbers that it almost deserves the
name of gravel. Many silicified trunks of coniferous trees, and the remains
of plants allied to _Zamia_ and _Cycas_, are buried in this dirt-bed (see
figure of living _Zamia_, fig. 241.).
These plants must have become fossil on the spots where they grew. The
stumps of the trees stand erect for a height of from 1 to 3 feet, and even
in one instance to 6 feet, with their roots attached to the soil at about
the same distances from one another as the trees in a modern
forest.[233-A] The carbonaceous matter is most abundant immediately around
the stumps, and round the remains of fossil _Cycadeae_.[233-B]
[Illustration: Fig. 241. Zamia spiralis; Southern Australia.[233-C]]
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 3 or 4
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 roots is about 1 foot.[233-D] Root-shaped
cavities were observed by Professor Henslow to descend from the bottom of
the dirt-bed into the subjacent freshwater stone, which, though now solid,
must have been in a soft and penetrable state when the trees grew.[233-E]
[Illustration: Fig. 242. Section in Isle of Portland, Dorset.
(Buckland and De la Beche.)]
The thin layers of calcareous slate (fig. 242.) 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.
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 deg., 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. 243.). Traces of the dirt-bed have also been observed
by Dr. Buckland, about two miles north of Thame, in Oxfordshire; and by Dr.
Fitton, in the cliffs of the Boulonnois, on the French coast; but, as might
be expected, this freshwater deposit is of limited extent when compared to
most marine formations.
[Illustration: Fig. 243. Section in cliff east of Lulworth Cove.
(Buckland and De la Beche.)]
From the facts above described, we may infer, first, that the superior
beds of the 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 admit the growth of
the _Zamia_ and _Cycas_. 2dly. 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. 3dly. 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
freshwater 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 then taken place.
The dirt-bed has been described above in its most simple form, but in some
sections the appearances are more complicated. The forest of the dirt-bed
was not everywhere the first vegetation which grew in this region. Two
other beds of carbonaceous clay, one of them containing _Cycadeae_, in an
upright position, have been found below it, and one above it[234-A], which
implies other oscillations in the level of the same ground, and its
alternate occupation by land and water more than once.
_Table showing the changes of medium in which the strata were formed,
from the Lower Greensand to the Portland Stone inclusive, in the
south-east of England._
1. Marine Lower greensand.
2. Freshwater Weald clay.
3. Freshwater }
Brackish } Hastings sand.
Freshwater }
4. Freshwater Upper Purbeck.
5. Freshwater }
Brackish }
Marine }
Brackish } Middle Purbeck.
Marine }
Freshwater }
Marine }
6. Freshwater }
Brackish }
Land }
Freshwater }
Land (dirt-bed) } Lower Purbeck.
Freshwater }
Land }
Freshwater }
Land }
Freshwater }
7. Marine Portland stone.
The annexed tabular view will enable the reader to take in at a glance the
successive changes from sea to river, and from river to sea, or from these
again to a state of land, which have occurred in this part of England
between the Cretaceous and Oolitic periods. That there have been at least
four changes in the species of testacea during the deposition of the
Wealden, seems to follow from the observations recently made by Professor
E. Forbes, so that, should we hereafter find the signs of many more
alternate occupations of the same area by different elements, it is no more
than we might expect. Even during a small part of a zoological period, not
sufficient to allow time for many species to die out, we find that the same
area has been laid dry, and then submerged, and then again laid dry, as in
the deltas of the Po and Ganges, the history of which has been brought to
light by Artesian borings.[235-A] We also know that similar revolutions
have occurred within the present century (1819) in the delta of the Indus
in Cutch[235-B], where land has been laid permanently under the waters both
of the river and sea, without its soil or shrubs having been swept away.
Even, independently of any vertical movements of the ground, we see in the
principal deltas, such as that of the Mississippi, that the sea extends its
salt waters annually for many months over considerable spaces, which, at
other seasons, are occupied by the river during its inundations.
It will be observed that the division of the Purbecks into upper, middle,
and lower, has been made by Professor E. Forbes, strictly on the principle
of the entire distinctness of the species of organic remains which they
include. The lines of demarcation 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 organized beings. "The causes which led to a
complete change of life three times during the deposition of the freshwater
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 2 or 3 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 imagined the fossil soils of the Lower Purbeck to represent as
many ages, we need not expect on that account to find them constituting the
lines of separation between successive strata characterized 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 dirt-beds were probably formed in succession, and annihilated in
the Wealden, besides those few which now remain.
[Illustration: Fig. 244. Cone from the Isle of Purbeck, resembling the
_Dammara_ of the Moluccas. (Fitton.)]
The plants of the Wealden, so far as our knowledge extends at present,
consist chiefly of Ferns, Coniferae (see fig. 244.), and Cycadeae, without
any exogens; the whole more allied to the Oolitic than to the Cretaceous
vegetation, although some of the species seem to be common to the chalk.
But the vertebrate and invertebrate animals indicate, in like manner, a
relationship to both these periods, though a nearer affinity to the
Oolitic. Mr. Brodie has found the remains of beetles and several insects of
the homopterous and trichopterous orders, some of which now live on plants,
like those of the Wealden, while others hover over the surface of our
present rivers. But no bones of mammalia have been met with among those of
land-reptiles. Yet, as the reader will learn, in Chapter XX., that the
relics of marsupial quadrupeds have been detected in still older beds, and,
as it was so long before a single portion of the jaw of an iguanodon was
met with in the Tilgate quarries (see p. 228.), we need by no means despair
of discovering hereafter some evidence of the existence of warm-blooded
quadrupeds at this era. It is, at least, too soon to infer, on mere
negative evidence, that the mammalia were foreign to this fauna.
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 200 English miles from west to
east, from Lulworth Cove to near Boulogne, in France; and about 220 miles
from north-west to south-east, from Whitchurch, in Buckinghamshire, to
Beauvais, 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 change 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.[237-A] 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[237-B]; and
the possibility of such movements, and their effects, must not be lost
sight of when we speculate on the origin of the Wealden.
If it 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, we are half tempted to speculate on the former existence
of the Atlantis of Plato. The story of the submergence of an ancient
continent, however fabulous in history, must have been true again and
again as a geological event.
The real difficulty consists in the persistence of a large hydrographical
basin, from whence a great body of fresh water was poured into the sea,
precisely at a period when the neighbouring area of the Wealden was
gradually going downwards 1000 feet or more perpendicularly. If the
adjoining land participated in the movement, how could it escape being
submerged, or how could it retain its size and altitude so as to continue
to be the source of such an inexhaustible supply of fresh water and
sediment? In answer to this question, we are fairly entitled to suggest
that the neighbouring land may have been stationary, or may even have
undergone a contemporaneous slow upheaval. There may have been an ascending
movement in one region, and a descending one in a contiguous parallel zone
of country; just as the northern part of Scandinavia is now rising, while
the middle portion (that south of Stockholm) is unmoved, and the southern
extremity in Scania is sinking, or at least has sunk within the historical
period.[237-C] We must, nevertheless, conclude, if we adopt the above
hypothesis, that the depression of the land became general throughout a
large part of Europe at the close of the Wealden period, a subsidence which
brought in the cretaceous ocean.
FOOTNOTES:
[227-A] Dr. Fitton, Geol. Trans. vol. iv. p. 320. Second Series.
[230-A] Mantell, Geol. of S. E. of England, p. 244.
[231-A] "On the Dorsetshire Purbecks," by Prof. E. Forbes, Edinb. Brit.
Assoc., Aug. 1850.
[233-A] Mr. Webster first noticed the erect position of the trees and
described the Dirt-bed.
[233-B] Fitton, Geol. Trans., Second Series, vol. iv. pp. 220, 221.
[233-C] See Flinders' Voyage.
[233-D] Fitton, ibid.
[233-E] Buckland and De la Beche, Geol. Trans., Second Series, vol. iv.
p. 16. Mr. Forbes has ascertained that the subjacent rock is a
freshwater limestone, and not a portion of the Portland oolite, as
was previously imagined.
[234-A] E. Forbes, ibid.
[235-A] See Principles of Geol., 8th ed. pp. 260-268.
[235-B] Ibid. p. 443.
[237-A] Fitton, Geol. of Hastings, p. 58.; who cites Lander's Travels.
[237-B] See above, p. 85.; and Second Visit to the U. S. vol. ii.
chap. xxxiv.
[237-C] See the Author's Anniv. Address, Geol. Soc. 1850, Quart. Geol.
Journ. vol. vi. p. 52.