SECONDARY ROCKS.
_No. 5. The Chalk._
THE CRETACEOUS SYSTEM.
“His hands formed the dry land.”—DAVID.
At the conclusion of the Wealden period, the crust of our globe
underwent another great change. The fresh-water rocks we have just been
studying were probably submerged by some violent subterranean or
subaquean agency, and upon a great part of them a new deposit was
gradually formed. Foot by foot, and inch by inch, slowly accumulated
those cretaceous particles (Lat. _creta_, chalk) which now constitute
the chalk hills and cliffs of our own country, to say nothing of the
extent of territory in Europe occupied by this well-known mineral. To
those who like ourselves live in a chalk district, this formation
possesses special interest, simply because the “bounds of our
habitation” are fixed there; and though we miss here all traces of the
Ichthyosaurus and Iguanodon, although no Deinotherium nor Mastodon as
yet make their appearance on the stage of being, yet with a very little
knowledge of geology, coupled with a desire not to walk through the
world with “eyes and no eyes,” it is in our power to invest many an
otherwise motiveless walk with pleasurable interest. A cutting in the
hills may reveal some unthought-of geological curiosity; a heap of
flints may disclose some “rich and rare” fossils; and even a walk over
the Downs, those breezy “downs” or “heaths” of southern England, covered
with their short herbage, and dotted with their browsing sheep—those
downs that undulate in such well-known forms of beauty, may be fruitful
of suggestions concerning their origin, character and antiquity, that
may be of healthful power in the development of an intellectual life.
We live—that is, _we_ aforesaid—not on a fresh-water deposit, like the
people of Purbeck, Hastings, and so on, but upon an _old marine
deposit_. The huge cliffs of chalk that gave the name of _Albion_, the
“white isle,” to our country, with all these downs, which rest upon
chalk, having a depth or thickness of often more than two thousand feet,
were once formed at the bottom of pre-Adamite oceans; and in process of
time, as the secondary period drew near to its destined close, and other
and higher types of life were to make their appearance, were upheaved by
Him, “whose hands thus formed the dry land,” to serve the future
purposes and contribute to the comforts of the coming lord of all.
Of this Albion we may well be proud, as the home of religion, the
birth-place of true freedom, and the sanctuary of the oppressed. With
Coleridge we exclaim:—
“Not yet enslaved, nor wholly vile,
O Albion, O my mother isle!
Thy valleys fair, as Eden’s bowers,
Glitter green with sunny showers;
Thy grassy upland’s gentle swells
Echo to the bleat of flocks;
Those grassy hills, those glittering dells,
Proudly ramparted with rocks;
And Ocean, ’mid his uproar wild,
Speaks safety to his island-child.
Hence for many a fearless age
Has social quiet loved thy shore;
Nor ever proud invader’s rage
Or sack’d thy towers or stain’d thy fields with gore.”
If we were to tell the story of the Chalk formation, it would run
something after this fashion:—Once upon a time—but what time we don’t
know, for man’s records only go back a few thousand years; but long,
long ages before there was a man to till the earth—all this chalk lay
out of sight at the deep bottom of the old, old sea. The tall cliffs at
Dover, which Shakspeare would hardly recognise now with their tunnels
and railroads, the wavy downs of Sussex and Surrey, the chalk hills of
Hampshire, all that piece of Wiltshire called Salisbury Plain, and this
line of country stretching away north to bluff Flamborough Head, and
running down south to Weymouth and the isle of Purbeck, taking up in its
course a good part of Buckinghamshire, Bedfordshire, Cambridgeshire, and
Norfolk,—all this immense district, averaging probably a thousand feet
in thickness, lay quietly at the bottom of the sea. Yes! it wasn’t
created just pat in a moment, and set up in these hills and valleys, any
more than sand was created sand, but gradually, grain by grain, it
accumulated, ages rolling by while thus the earth was a-preparing. These
undulating downs knew then no boys pursuing on them their healthful and
gymnastic games; no fair riders then crossed these pleasant turfy
swards, inhaling life and vigour from the air of heaven; no evening
promenades for men of business, recreating the overworked physical
nature,—then diversified the scene: but instead thereof crumbling
particles of older rocks covered the floor of the ocean with their dust,
so making green sand, while the waste and débris of coral reefs, ground
up and pulverized, poured in their lavish contributions, so making
chalk; (?) while above this deposit thus forming, the ocean waves rolled
to and fro, and in their rise and fall, their daily ebb and flow,
sounded their eternal bass in the ear of old father Time, who certainly
then had no human children. Had we been there we should have seen as
tenants of the “vasty deep,” pectens, plagiostamas, hamites, belemnites,
and other odd-looking crustaceans, roaming about at their own sweet
will; some ganoid fishes that have now such hard names, that to write
them would interfere with the strain of this description; while here and
there we might have spied a veteran remnant of the oolite come down by
strange chance into the chalk, a solitary pterodactyle, snapped at and
pounced upon by an ugly mososaurus, a sea crocodile standing four feet
high. These, and such as these, were the tenants of that ocean, which
then rolled over our heaths and downs; and when at length their period
of life and purpose of creation were answered, these too all perished,
and the secondary formation ended its wondrous career. The old ocean
still rolls on, unchanged, unaltered still; but what changes has it seen
in these long distant epochs of which geology tries to tell the story!
“Time writes no wrinkle on thine azure brow;
Such as creation’s dawn beheld, thou rollest now!”
No one can write the history of the Chalk formation, without remembering
his indebtedness to the patient and laborious toil of Dr. Mantell in
this department, from whose “Geological Excursions round the Isle of
Wight,”[104] we give the following extract. “The features of a chalk
district are so well known, that a brief notice will render local
details unnecessary. The rounded summits of the hills, covered with
short verdant turf—the smooth undulated outline of the downs unbroken,
save by the sepulchral mounds of the early inhabitants of the
country—the coombes and furrows ramifying and extending into the deep
valleys, which abruptly terminate at the base of the hills, and appear
like dried-up channels of rivulets and streams, though free from all
traces of alluvial débris, thus bearing the impress of physical
operations, of which the agents that produced them have long since
passed away—are phenomena familiar to every one who has travelled over
the downs of the south-east of England, and are displayed in the chalk
districts of the ‘beautiful isle.’ These features are restricted to the
hilly districts of the _white chalk_, and have resulted from the
peculiar nature of the sedimentary detritus,[105] of which the strata
comprised in the upper division of the Cretaceous system are composed:
for in the lower groups, clays, marls, sands, and sandstones prevail;
and where these deposits approach the surface, and form the subsoil, the
country is broken and diversified, and the landscape presents a striking
contrast with the down scenery, as may be observed in the picturesque
district which flanks the escarpment of the chalk hills. It may,
perhaps, be necessary to remind the unscientific reader, that these
strata are but an insulated portion of an ancient sea-bottom, or in
other words, a mass of consolidated sediments formed in the profound
depths of the ocean, in a very remote period of the earth’s physical
history. This detritus is made up of inorganic and organic materials.
The former consist of the débris of the cliffs and shores which
encompassed the ancient sea, of the spoils of islands and continents
brought down into the ocean by fresh-water currents, and of chemical
deposits thrown down from mineral solutions. The organic substances are
the durable remains of animals and plants which lived and died in the
ocean, and of fluviatile and terrestrial species that were transported
from the land by rivers and their tributaries; the whole forming such an
assemblage of sedimentary deposits as would probably be presented to
observation, if a _mass of the bed of the Atlantic ocean_, 2,000 feet in
thickness, were elevated above the waters, and became dry land.”
We have alluded to the undulating character of the downs, so “well
known,” as Mantell says, that “local details are unnecessary.” How
correct this is may be seen in the following drawing, which represents a
portion of Royston Heath.
[Illustration: Fossil]
All over this heath is found the “Royston crow,” during the winter
months. This fine bird migrates hither from Norway, to avoid its severe
winters, and is scientifically known as the “Hooded-crow, _corvus
cornix_.” On its first arrival, when it is in its best plumage, it is
comparatively tame, allowing the sportsman to approach very near; but as
the season advances, acquaintance with the gun makes it very knowing and
shy. It associates freely with the other crows, but its nest has never
yet been found in England. About March the hooded-crow wholly
disappears. The head, throat, and wings are black; the back and breast a
“clear smoke-grey.” Norman, the bird-stuffer of this town, has always
several fine specimens on hand.
As, in the case of the Carboniferous system, we ventured to say to the
reader that it was not all _coal_, so in the Cretaceous system, we would
remind him that it is not all _chalk_; but without going minutely into
the subdivisions which the chalk formation has received, because this
unpretending elementary treatise does not profess to _teach_ geology,
but simply aims, as we have ventured again and again to repeat, to
infuse into the mind a desire of acquaintance with the marvels and
truths of this science, we will just indicate the leading divisions and
nomenclature of this deposit. First, there is the green sand; that is,
_first_, beginning at the bottom or lower part of the formation: this
may be well seen and studied in the neighbourhood of Cambridge, where we
have procured many of its characteristic fossils, including several
vertebræ and teeth of the otodus, a fish allied to the shark family,
such as are figured in the opposite diagram.
[Illustration:
FOSSIL TEETH OF FISHES:
FROM UPPER GREEN SAND, CAMBRIDGE.
1. OTODUS.
2. CARCHARIAS.
3. CORAX.
4. OXYRHINA.
5. NOTIDANUS.
6. LAMNA.
7. PTYCHODUS.
]
[Illustration:
FOSSILS FROM THE GAULT, FOLKSTONE.
1. AMMONITE DENTATUS.
2. AM. LAUTUS.
3. AM. SPLENDENS.
4. AM. CRISTATUS.
5. AM. DENARIUS.
6. CATILLUS SULCATUS.
]
At Potton and Gamlingay in Bedfordshire, and in the neighbourhood, this
green sand is highly ferruginous, and the roads and fields present that
peculiarly dark-red colour which is first singular and then wearisome to
the eye. In the case of the Potton beds, the red colour is caused by
oxidization or rust of iron; in the neighbourhood of Cambridge, &c.,
where there is the _green_ sand, this is owing to the influence of
“chloritous silicate of iron.” Then we have the galt or gault, a local
term of which we cannot trace the etymology. The gault, however, is not
of great thickness, but is likely to be the most interesting department
of the Chalk to the beginner, on account of the abundance and peculiar
appearance of its fossils. A ramble under the cliffs at Folkstone,[106]
where the gault may be seen in perfection, will amply repay any one for
toil, dirt, and a few slips and bruises. He will there find evidences of
a prolific and prodigal bestowment of life in the innumerable fragments
of organic remains every where observable; and if he be patient,—if he
won’t go running on from spot to spot, saying, as some do, “Oh, there’s
nothing here;” if he will just persevere in a minute examination of
every spot where organic remains may be detected, he will not come away
without his reward in ammonites, hamites, and other cephalopodous
mollusks, and most of them with that peculiar nacreous or
mother-of-pearl lustre upon them which renders the fossils of this
period so beautiful and attractive. Only we caution the explorer not to
buy of the so-called guides. At Dover and Folkstone the rogues have a
knack of getting a lump of gault, and sticking into it one or two common
pyrites, which are very abundant in the cliff, bits of shell, ammonites,
&c.; they then offer this conglomerate for sale, all rounded and smooth,
assuring you upon their “sacred honour,” the honour of men who always
draw upon their imagination for their facts, that they would not ask so
much for it, only on account of its excessive rarity. As good economists
always avoid cheap houses, and go to the best shops, so let the young
geologist always go to the best shop: let him go to the cliff with his
hammer, and work for himself. We picture a few fossils from the gault,
only regretting that it is out of the power of our artist to convey
their lustrous colours, as well as their curious forms.
[Illustration:
1. NATICA CANICULATA.
2.} VENTICRULITES.
3.}
4.} ROSTELLARIA MARGINATA.
5.}
6.}
7.} CATILLUS CRISPI.
8.}
]
[Illustration: Fossil]
Then comes, lastly, the _Chalk_: that is, the _white_ chalk, divided
into lower and upper; the _lower_ being harder and mostly without
flints, and the upper characterised by layers and bands of flint,
sometimes nodular, as in Cambridgeshire, and sometimes flat almost as a
pancake, as in the neighbourhood of Woolwich.
Above are some of the most characteristic fossils of the Chalk. No. 1 is
a _pecten_, or oyster, called the “five-ribbed,” or _quinque costatus_;
No. 2 is the _plagiostoma spinosa_, so called on account of its spines,
a shell found frequently in our chalk or lime-pits; No. 3 is the
intermediate hamite (Lat. _hamus_, a hook), “_hamites intermedius_;” No.
4 is the _spatangus cor-anguinum_, a very common fossil _echinus_ in the
chalk; No. 5 is the _ananchytes ovata_, found frequently in the Brighton
and Ramsgate cliffs; No. 6 is a scaphite (Gr. _skaphē_, a skiff or
boat); and the last is our old friend the belemnite, who has survived so
many of this earth’s changes, and now finds himself a contemporary of
the cretaceous inhabitants of the globe.
In many respects, the Chalk presents us with remarkable anomalies: we
have _sand_, the green sand, but unlike in colour and in texture the
sand of the old and new red sandstone, where we find it compressed and
hardened into solid and compact masses of stone; we have _clay_,
argillaceous beds such as the gault, but it is not clay hard and pressed
into slaty rocks, but soft and compressible; and we have _carbonate of
lime_, the chalk constituting the calcareous beds of this formation; but
where we have met with it before it has been hard and solid limestone,
and marble, not pliable and soft as in the Cretaceous system; and yet
apparently it is all the same material as we have found in the earlier
stages of the earth’s crust—the washings, degradations, and deludations
of older and harder rocks, along with the secretions and remains of
organized animals that once peopled this ancient earth; thus affording
us, on a large scale, another illustration of the economy observable in
all the works of God.
[Illustration:
FOSSIL FISH FROM LEWES.
]
Having spoken of the fossil fishes of the Chalk, we here give drawings
of two procured from the neighbourhood of Lewes, the famous fossil
fishing-ground of the late Dr. Mantell; and it is due to the name and
memory of the Chalk historian and geologist, to inform the reader that
Dr. M. was the first who succeeded, by skilful removal of the
surrounding chalk, in procuring a perfect ichthyolite from the
cretaceous formation of England. The British Museum is now enriched by
Dr. Mantell’s collection of fossil fishes, that once so much excited the
admiration of Agassiz, when he saw them at Brighton.
[Illustration:
FOSSIL FISH (ICHTHYOLITE) FROM LEWES.
]
Here let us again advert to the Deluge theory, not because our own minds
are not satisfied on the point, but because theology and science alike
demand a true statement of the facts of the case. We believe, as we said
in a previous chapter, in the plenary inspiration of nature, just as we
believe that the Scriptures were given by inspiration of God; and we are
quite sure that both books, if they are not misinterpreted, will declare
the glory of God in one common speech, and elevate the mind of man, to
whom they speak, up to a more adoring trust and a profounder reverence.
With Dr. Hitchcock we say, “It seems to me that the child can easily
understand the geological interpretation of the Bible and its reasons.
Why, then, should it not be taught to children, that they may not be
liable to distrust the whole Bible, when they come to the study of
geology? I rejoice, however, that the fears and prejudices of the pious
and the learned are so fast yielding to evidence; and I anticipate the
period, when on this subject the child will learn _the same thing in the
Sabbath school and in the literary institution_. Nay, I anticipate the
time as not distant when the high antiquity of the globe will be
regarded as no more opposed to the Bible, than the earth’s revolution
round the sun, and on its axis. Soon shall the horizon, where geology
and revelation meet, be cleared of every cloud, and present only an
unbroken and magnificent circle of truth.”[107] But to return; this
Deluge theory refers all existing fossils to “Noah’s flood,”—to that
violent diluvial action, the graphic account of which is in the book of
Genesis. Now, it is impossible to believe this if we look at a fossil:
look, for instance, at this terebratula, and observe how perfectly
uninjured it is, frail as is its shelly covering; or at this plagiostoma
spinosa, and mark how susceptible it is of injury, and yet that its
brittle spines are all unbroken; or at this inoceramus or catillus, and
observe its delicate flutings, still in exquisite preservation, without
fracture or distortion; or these specimens of echinites, the ananchytes
ovata, or the spatanguscor-anguinum, and see the markings on the shell,
the apertures of the mouth and stomach still perfect; who can see all
this and not come to the conclusion, that these creatures, and thousands
such as these, endured not only no violence in death, such as a deluge
would suppose, but that at death they subsided quietly to the bottom of
the sea, there to find a fitting sepulchre of soft cretaceous matter
prepared for them, which in process of time was lifted up, to exhibit in
a hard chalky bed their forms of pristine beauty?
In the upper chalk every one has seen the layers of flint, and marked
their singular distribution, in layers; and here we would add, that the
existence of flint in chalk is one of those hard nuts which geology has
not yet cracked. The geologist, the chemist, and the zoologist have all
puzzled themselves in vain to find a truly satisfactory origin for these
nodules of siliceous matter. We have heard it suggested that they may be
_coprolites_; but no one who examines the texture of a flint, can hold
that theory, to say nothing of the idea that the coprolites have been
preserved, while the animal remains have perished. We may sum up all we
have to say about flints in the following words, from that useful little
book, Chambers’s “Rudiments of Geology:”
“_The formation of flint_, within a mass so different in composition as
chalk, is still in some respects an unsolved problem in geology. It
occurs in nodular masses of very irregular forms and variable magnitude;
some of these not exceeding an inch, others more than a yard in
circumference. Although thickly distributed in horizontal layers, they
are never in contact with each other, each nodule being completely
enveloped by the chalk. Externally, they are composed of a white cherty
crust; internally, they are of a grey or black silex, and often contain
cavities lined with chalcedony and crystallized quartz. When taken from
the quarry they are brittle and full of moisture, but soon dry, and
assume their well-known hard and refractory qualities. Flints, almost
without exception, enclose remains of sponges, alcyonia, echinida, and
other marine organisms, the structures of which are often preserved in
the most delicate and beautiful manner. In some specimens the organism
has undergone decomposition, and the space it occupied either left
hollow, or partially filled with some sparry incrustation. From these
facts, it would seem that flints are as much an aggregation of silex
around some organized nucleus, as septaria are aggregations of clay and
carbonate of iron. This is now the generally received opinion; and when
it is remembered that the organisms must have been deposited when the
chalk was in a pulpy state, there can be little difficulty in conceiving
how the silex dissolved through the mass would, by chemical affinity,
attach itself to the decaying organism. Chalk is composed of carbonate
of lime, with traces of clay, silex, and oxide of iron; flint, on the
other hand, consists of 98 per cent. of pure silex, with a trace of
alumine, oxide of iron, and lime. Silex is quite capable of solution: it
occurs in the hot-springs of Iceland and most thermal waters; has been
found in a pulpy state within basalt; forms the _tabasheer_ found in the
cavities of the bamboo, and the thin pellicle or outer covering of
canes, reeds, grasses, &c.; and siliceous concretions are common in the
fruits and trees of the tropics. All these facts point to a very general
diffusion of silex in a state of solution; and whatever may have caused
its abundance in the waters during the deposition of the upper chalk,
there can be little doubt respecting the mode in which it has been
collected around the organic remains of these early seas.”
At Scratchell’s Bay in the Isle of Wight, it will be seen that the
flints are in a vertical position; and to the most casual observer the
perpendicular arrangement of these flints will supply the strongest
evidence of disturbance by upheaval from below. The bay in front, called
Scratchell’s Bay, is a small but romantic indentation in the coast of
the south side of the island, in which are the famous Needles. In the
face of the cliff is a noble archway between 200 and 300 feet high,
which has been created by the constant action of water eating and
wearing away the lower beds; while the Needles themselves are only
isolated masses of chalk, separated or eroded from the main land by the
same erosive action. “To the late Sir Henry Inglefield belongs the merit
of having first observed and directed attention to the highly
interesting phenomena of vertical chalk strata, occasioned by the
disruption and elevation of the eocene and cretaceous formations, which
are so remarkably displayed in the Isle of Wight, where the vertical
position of the strata, and the shattered condition of the flint
nodules, thought still embedded in the solid chalk, may be conveniently
studied in the cliffs in the neighbourhood of Scratchell’s Bay.”[108]
With the study of the Chalk formation, we close what has been
appropriately termed the “secondary period, or middle epoch of the
ancient world;” of which it has been well said, “In reviewing the
characters of the Cretaceous group, we have evidence that these varied
strata are the mineralized bed of an extensive ocean, which abounded in
the usual forms of marine organic life, as algæ, sponges, corals,
shells, crustacea, fishes, and reptiles. These forms are specifically
distinct from those which are discovered in the tertiary strata; in many
instances, the genus, in all the species, became extinct with the close
of the Cretaceous period. It affords a striking illustration of creative
power, that of the hundreds of species which composed the Fauna and the
Flora of the Cretaceous group, not one species passed into the
succeeding epoch.”[109]
Of that old ocean with its countless tenants we have already spoken, and
conclude by applying to it the well-known lines of Montgomery, in his
celebration of the coral insect in his “Pelican Island:”—
Millions of millions here, from age to age,
With simplest skill, and toil unweariable,
No moment and no movement unimproved,
Laid line on line, on terrace, terrace spread,
To swell the heightening, brightening gradual mound,
By marvellous structure climbing towards the day.
Omnipotence wrought in them, with them, by them;
Hence what Omnipotence alone could do,
Worms did.
[Illustration:
WALTONIAN AND MANTELLIAN FISHERMEN.
]