During and after the deposition of the Portland and Purbeck beds, the
entire Oolite Series, in the south and centre of England and other
regions, was raised above the sea-level and became dry land. Above these
Purbeck beds, as Professor Ramsay tells us [in the district known as the
Weald], “we have a series of beds of clays, sandstones, and shelly
limestones, indicating by their fossils that they were deposited in an
estuary where fresh water and occasionally brackish water and marine
conditions prevailed. The Wealden and Purbeck beds indeed represent the
delta of an immense river which in size may have rivalled the Ganges,
Mississippi, Amazon, &c., and whose waters carried down to its mouth the
remains of land-plants, small Mammals, and great terrestrial Reptiles,
and mingled them with the remains of Fishes, Molluscs, and other forms
native to its waters. I do not say that this immense river was formed
or supplied by the drainage of what we now call Great Britain--I do not
indeed know where this continent lay, but I do know that England formed
a part of it, and that in size it must have been larger than Europe, and
was probably as large as Asia, or the great continent of America.”
Speaking of the geographical extent of the Wealden, Sir Charles Lyell
says: “It cannot be accurately laid down, because so much of it is
concealed beneath the newer marine formations. It has been traced about
200 miles from west to east; from the coast of Dorsetshire to near
Boulogne, in France; and nearly 200 miles from north-west to
south-east, from Surrey and Hampshire to Beauvais, in France;”[75] but
he expresses doubt, supposing the formation to have been continuous, if
the two areas were contemporaneous, the region having undergone frequent
changes, the great estuary having altered its form, and even shifted its
place. Speaking of a hypothetical continent, Sir Charles Lyell says: “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.”[76]
[75] “Elements of Geology,” p. 349.
[76] Ibid, p. 350.
The proof that the Wealden series were accumulated under fresh-water
conditions and as a river deposit[77] lies partly in the nature of the
strata, but chiefly in the nature of the organic remains. The fish give
no positive proof, but a number of Crocodilian reptiles give more
conclusive evidence, together with the shells, most of them being of
fresh-water origin, such as Paludina, Planorbis, Lymnæa, Physa, and such
like, which are found living in many ponds and rivers of the present
day. Now and then we find bands of marine remains, not mixed with
fresh-water deposits, but interstratified with them; showing that at
times the mouth and delta of the river had sunk a little, and that it
had been invaded by the sea; then by gradual change it was lifted up,
and became an extensive fresh-water area. This episode at last comes to
an end by the complete submergence of the Wealden area; and upon these
fresh-water strata a set of marine sands and clays, and upon these again
thick beds of pure white earthy limestone of the Cretaceous period were
deposited. The lowest of these formations is known as the Lower
Greensand; then followed the clays of the Gault, which were succeeded by
the Upper Greensand. Then, resting upon the Upper Greensand, comes the
vast mass of Chalk which in England consists of soft white earthy
limestone, containing, in the upper part, numerous bands of
interstratified flints, which were mostly sponges originally, that have
since become silicified and converted into flint. The strata of chalk
where thickest are from 1,000 to 1,200 feet in thickness. Their upheaval
into dry land brought this epoch to an end; the conditions which had
contributed to its formation ceased in our area, and as the uppermost
member of the Secondary rocks, it closes the record of Mesozoic times in
England.
[77] “The Physical Geology and Geography of Great Britain,” by A. C.
Ramsay, F.R.S., p. 64.
Let us add, to remove any remaining doubts, that in the basin of a
modern European sea--the Baltic--a curious assemblage of phenomena,
bearing on the question, is now in operation. The bed and coast-line of
the Baltic continue slowly but unceasingly to rise, and have done so for
several centuries, in consequence of the constant deposit which takes
place of calcareous shells, added to the natural accumulations of sand
and mud. The Baltic Sea will certainly be filled up in time by these
deposits, and this modern phenomenon, which we find in progress, so to
speak, brings directly under our observation an explanation of the
manner in which the cretaceous rocks were produced in the ancient world,
more especially when taken in connection with another branch of the same
subject to which Sir Charles Lyell called attention, in an address to
the Geological Society. It appears that just as the northern part of the
Scandinavian continent is now rising, and while the middle part south of
Stockholm remains unmoved, the southern extremity in Scania is sinking,
or at least has sunk, within the historic period; from which he argues
that there may have been a slow upheaval in one region, while the
adjoining one was stationary, or in course of submergence.
After these explanations as to the manner in which the cretaceous rocks
were formed, let us examine into the state of animal and vegetable life
during this important period in the earth’s history.
The vegetable kingdom of this period forms an introduction to the
vegetation of the present time. Placed at the close of the Secondary
epoch, this vegetation prepares us for transition, as it were, to the
vegetation of the Tertiary epoch, which, as we shall see, has a great
affinity with that of our own times.
The landscapes of the ancient world have hitherto shown us some species
of plants of forms strange and little known, which are now extinct. But
during the period whose history we are tracing, the vegetable kingdom
begins to fashion itself in a less mysterious manner; Palms appear, and
among the regular species we recognise some which differ little from
those of the tropics of our days. The dicotyledons increase slightly in
number amid Ferns and Cycads, which have lost much of their importance
in numbers and size; we observe an obvious increase in the dicotyledons
of our own temperate climate, such as the alder, the wych-elm, the
maple, and the walnut, &c.
“As we retire from the times of the primitive creation,” says Lecoq,
“and slowly approach those of our own epoch, the sediments seem to
withdraw themselves from the polar regions and restrict themselves to
the temperate or equatorial zones. The great beds of sand and
limestone, which constitute the Cretaceous formation, announce a state
of things very different from that of the preceding ages. The seasons
are no longer marked by indications of central heat; zones of latitude
already show signs of their existence.
“Hitherto two classes of vegetation predominated: the cellular
_Cryptogams_ at first, the dicotyledonous _Gymnosperms_ afterwards; and
in the epoch which we have reached--the transition epoch of
vegetation--the two classes which have reigned heretofore become
enfeebled, and a third, the dicotyledonous _Angiosperms_, timidly take
possession of the earth--they consist at first of a small number of
species, and occupy only a small part of the soil, of which they
afterwards take their full share; and in the succeeding periods, as in
our own times, we shall see that their reign is firmly established;
during the Cretaceous period, in short, we witness the appearance of the
first dicotyledonous _Angiosperms_. Some arborescent Ferns still
maintain their position, and the elegant _Protopteris Singeri_,
Preissl., and P. _Buvigneri_, Brongn., still unfold their light fronds
to the winds of this period. Some _Pecopteri_, differing from the
Wealden species, live along with them. Some _Zamites_, _Cycads_, and
_Zamiostrobi_ announce that in the Cretaceous period the temperature was
still high. New Palms show themselves, and, among others, _Flabellaria
chamæropifolia_ is especially remarkable for the majestic crown at its
summit.
“The _Conifers_ have endured better than the _Cycadeæ_; they formed
then, as now, great forests, where _Damarites_, _Cunninghamias_,
_Araucarias_, _Eleoxylons_, _Abietites_, and _Pinites_ remind us of
numerous forms still existing, but dispersed all over the earth.
“From this epoch date the _Comptonias_, attributed to the Myricaceæ;
_Almites Friesii_, Nils., which we consider as one of the Betulaceæ;
_Carpinites arenaceus_, Gœp., which is one of the Cupuliferæ; the
_Salicites_, which are represented to us by the arborescent willows; the
Acerinæ would have their _Acerites cretaceæ_, Nils., and the Juglanditæ,
the _Juglandites elegans_, Gœp. But the most interesting botanical event
of this period is the appearance of the _Credneria_, with its
triple-veined leaves, of which no less than eight species have been
found and described, but whose place in the systems of classification
still remains uncertain. The _Crednerias_, like the _Salicites_, were
certainly trees, as were most of the species of this remote epoch.”
In the following illustration are represented two of the Palms belonging
to the Cretaceous period, restored from the imprints and fragments of
the fossil remains left by the trunk and branches in the rocks of the
period (Fig. 130.)
[Illustration: Fig. 130.--Fossil Palms restored.]
But if the vegetation of the Cretaceous period exhibits sensible signs
of approximation to that of our present era, we cannot say the same of
the animal creation. The time has not yet come when Mammals analogous to
those of our epoch gave animation to the forests, plains, and shores of
the ancient world; even the Marsupial Mammals, which made their
appearance in the Liassic and Oolitic formations, no longer exist, so
far as is known, and no others of the class have taken their place. No
climbing Opossum, with its young ones, appears among the leaves of the
Zamites. The earth appears to be still tenanted by Reptiles, which alone
break the solitudes of the woods and the silence of the valleys. The
Reptiles, which seem to have swarmed in the seas of the Jurassic period,
partook of the crocodilian organisation, and those of this period seem
to bear more resemblance to the Lizards of our day. In this period the
remains of certain forms indicate that they stood on higher legs; they
no longer creep on the earth, and this is apparently the only
approximation which seems to connect them more closely with higher
forms.
It is not without surprise that we advert to the immense development,
the extraordinary dimensions which the Saurian family attained at this
epoch. These animals which, in our days, rarely exceed a yard or so in
length, attained in the Cretaceous period as much as twenty. The marine
lizard, which we notice under the name of _Mosasaurus_, was then the
scourge of the seas, playing the part of the Ichthyosauri of the
Jurassic period; for, from the age of the Lias to that of the Chalk, the
Ichthyosauri, the Plesiosauri, and the Teleosauri were, judging from
their organisation, the tyrants of the waters. They appear to have
become extinct at the close of the Cretaceous period, and to give place
to the _Mosasaurus_, to whom fell the formidable task of keeping within
proper limits the exuberant production of the various tribes of Fishes
and Crustaceans which inhabited the seas. This creature was first
discovered in the celebrated rocks of St. Peter’s Mount at Maestricht,
on the banks of the Meuse. The skull alone was about four feet in
length, while the entire skeleton of _Iguanodon Mantelli_, discovered by
Dr. Mantell in the Wealden strata, has since been met with in the
Hastings beds of Tilgate Forest, measuring, as Professor Owen estimates,
between fifty and sixty feet in length. These enormous Saurians
disappear in their turn, to be replaced in the seas of the Tertiary
epoch by the Cetaceans; and henceforth animal life begins to assume,
more and more, the appearance it presents in the actually existing
creation.
Seeing the great extent of the seas of the Cretaceous period, Fishes
were necessarily numerous. The pike, salmon, and dory tribes, analogous
to those of our days, lived in the seas of this period; they fled before
the sharks and voracious dog-fishes, which now appeared in great
numbers, after just showing themselves in the Oolitic period.
The sea was still full of Polyps, Sea-urchins, Crustaceans of various
kinds, and many genera of Mollusca different from those of the Jurassic
period; alongside of gigantic Lizards are whole piles of
animalculæ--those Foraminifera whose remains are scattered in infinite
profusion in the Chalk, over an enormous area and of immense thickness.
The calcareous remains of these little beings, incalculable in number,
have indeed covered, in all probability, a great part of the terrestrial
surface. It will give a sufficient idea of the importance of the
Cretaceous period in connection with these organisms to state that, in
the rocks of the period, 268 genera of animals, hitherto unknown, and
more than 5,000 species of special living beings have been found; the
thickness of the rocks formed during the period being enormous. Where is
the geologist who will venture to estimate the time occupied in creating
and destroying the animated masses of which this formation is at once
both the cemetery and the monument? For the purposes of description it
will be convenient to divide the Cretaceous series into lower and upper,
according to their relative ages and their peculiar fossils.