TRANSACTIONS OF THE HULL GEOLOGICAL SOCIETY
THE FORMATION OF FLINT.
Abstract of a paper by the late Canon H. E. Maddock, M.A.
(Read February 20th, 1890)
Flint is one of the commonest stones met with in east
Yorkshire. It occurs plentifully in the glacial deposits, and also in great
abundance in the form of nodules and layers, in the middle division of the
Yorkshire Chalk, a formation which in the form of a broad irregular crescent
stretches from Flambro' Head to the Humber.
Chemical analysis has shown that flint is almost entirely
composed of silica, a combination of oxygen and silicon, which probably forms
not less than two-thirds of the earth's crust.
There is great variety in the structure of flint. Some
specimens exhibit their original structure and appear to be simply silicified
chalk- mud, others show signs of crystallization and consequent more or less
complete obliteration of primary structure. In the former case the outlines of
foraminifera, sponge spicules and other organisms can often be clearly seen; in
the latter, and especially is this the case with black flint, traces of organic
forms can seldom be detected.
Flint occurs in the following forms :--
(1) Potstones or "Paramoudra.'' Flint in this form was
observed by Sir Chas. Lyell in the White Chalk near Norwich. He describes the
potstones, as pear-shaped masses of flint, 3 feet in height by foot in diameter,
occurring 20 feet to 30 feet apart, in rows, like a series of pillars. On
breaking them open the flinty mass was found to contain a cylindrical core, of
pure white chalk, of exceptional hardness. Mr. J. R. Mortimer records similar
potstones at Flambro' and Speeton. He describes them as stump-like columns of
flint, with an admixture of chalk, measuring from 2 feet to 5 feet in diameter,
having a dish-shaped hollow or cavity in the centre of the almost flat top,
filled with hard chalk. The term “paramoudra" appears to have come from Ireland.
(2)
Nodules. Flint occurs commonly in the form of nodules of various shapes and
sizes, from five inches in diameter downwards. The nodules though sometimes
scattered, generally run in lines, from two feet to eight feet apart, parallel
with the bedding. These lines can sometimes be traced many miles.
(3) Beds or seams. Flint also occurs in continuous seams
or beds varying from one inch to eight inches in thickness. These beds also,
like the lines of nodules, exist over large areas.
(4) Fissures. Flint is also found in oblique or vertical
fissures in the chalk. The fissures are generally titled with calcite, but
occasionally with flint from top to bottom.
It is also worthy of note that the Middle Chalk of
Yorkshire, leaving its characteristic flint bands and nodules out of
consideration, contains less silica than the Upper and Lower divisions of the
Chalk, which in Yorkshire contains no flints.
Formation.
Many geologists think that flint of every description was
formed under the waters of an extensive sea or ocean. Microscopic examination
reveals remains of marine organisms in flint, and there is no doubt that certain
siliceous rocks are almost entirely made up of sponge spicules still unchanged.
The "Challenger" reports prove conclusively that at the bottom of the Atlantic
and Pacific oceans arc vast quantities of sponges bearing siliceous spicules.
These extend to all depths, but attain their maximum development between 500
fathoms and 1000 fathoms. The forms represented by the Ventriculidae and allied
families in the Chalk and Greensand are found in plenty at that depth in all
parts of the ocean. Nearly all the deep sea sponges of all orders are stalked,
or are provided with beards of fringes of radiating spicules, or are otherwise
suppled with means of supporting themselves above the surface of the soft ooze
in which they grow. These sponges must have swarmed on the floor of the
Cretaceous seas. In all the beds of chalk their siliceous spicules are very
abundant, and entire individuals are not uncommon. There is a very interesting
point in connection with sponge spicules brought to light by Professor Sollas,
in his report on the Tetracinnelid sponges of the Challenger Expedition. He has
shown that certain species have the power of periodically shedding immense
quantities of spicules. In some specimens he found that the passages were nearly
always closely packed with a substance resembling cotton-wool. On examination
this was found to consist of spicules densely matted together, everyone similar
to those forming the sponge. Some of the Chalk sponges are closely allied to
this species, and in all probability possessed the same power of shedding
spicules during their growth. Hence we find that to account for the silica in
flint, we have to take into consideration not only the spicules contained in
dead sponges, but also a large number produced by the sponges during their
life-time. In addition to this Radiolaria and possibly Diatoms assisted.
It must be borne in mind that the colloid or gelatinous
form of silica can be dissolved with no great difficulty. Indeed it is by no
means uncommon to find sponge spicules, once siliceous, which have lost every
particle of silica. In many cases also we find that silica, has taken the place
particle by particle, of structures which were once calcareous. Hence we are
forced to the conclusion that at certain periods the floor of the Chalk-ocean
was covered by an ooze of colloid silica dissolved from the remains of sponge
spicules, Radiolaria etc. It is a well ascertained fact that where the
decomposition of any organic substance is going on in the proximity of colloid
silica there is a tendency for the particles of decaying matter to be replaced
by particles of silica, or else the hollows left by decomposition are filled in
with that material. Professor Sollas has shown that when such animal matters as
albumen and gelatine are treated with silicic acid a definite chemical compound
is formed. In process of time this complex organic substance would decompose,
the organic constituents would be evolved, and the silica would remain behind.
In other cases the silica would fill the cavities left by decomposition or the
natural cavities of the sponges, whilst in still others it would form an entire
covering, enveloping the animal or vegetable organisms. All these processes seem
to have taken place during the formation of flint. In some instances the colloid
silica seems certainly to have replaced the organism, molecule by molecule. In
others it seems to have filled the hollow left in the interior of a sponge,
shell, or urchin, whilst occasionally it has formed a deposit around the object.
Perhaps the best example of the first of these are the paramoudra or Potstones.
Sir Chas. Lyell pointed out how clearly the resembled in shape ad size the large
sponges called Neptune's Cups, which grow in the seas of Sumatra. In these cases
it is evident that the flint must have been deposited before that consolidation
of the Chalk took place by which the smaller fossils were mostly destroyed, and
must have taken the place of the part of the sponges, leaving unchanged the
cretaceous mud in the interior.
With regard to the parallel layers of nodules of flint,
there can be little doubt that each layer represents an ancient sea floor. From
the plentiful occurrence of perfect sponges in these nodules, it is obvious that
they owe their form in many cases to the growth of sponges in the exact place in
which they now occur. The alternate beds of chalk and flint, however, requires
some explanation. In my opinion we must look for the solution of this difficulty
in the phenomena observed in the manufacture of porcelain in our Pottery
districts. For the production of certain of the finer kinds of china, flints are
ground up and mixed in definite proportions with clay. If this mixture is well
stirred up and used at once, the particles of silica and alumina remain
intermingled, but if it is allowed to stand for any time the siliceous particles
separate from the clay and form into nodules. I believe some such process took
place in the formation of flint nodules, and that when a certain amount of ooze
had been deposited over the area where the layer now occurs, the law of
aggregation of similar particles took effect. Wherever there was a decaying
organism, there would be a nucleus for the aggregation of siliceous particles,
and thus a layer of flint nodules would be formed. This would also account for
the scarcity of silica in the Chalk with flints, as compared with the Chalk
without flints.
The third form of flint, viz., tabular layers, has
probably a similar origin to the beds of nodules, only representing a more
extensive deposit. The "Challenger reports" contain much interesting information
in this connection.
The fourth and last form of flint to which I have referred is of comparatively
recent origin. The fissures would no doubt be caused by the upheaval and
displacement of the Chalk strata. Water continually acting on the layers of
flint would in course of time dissolve some of the colloid silica. This would be
carried down and re-deposited on the sides of the crack, and eventually fill it
with a mass of flint.
In conclusion it may be not uninteresting to refer to the
evidence of the time which must have been necessary for the deposition of the
flint. Professor Sollas, in the report already referred to, has carefully
estimated the proportion of silica found in the sponges most resembling those of
the Cretaceous ocean. He finds that it would take eight full-sized sponges to
furnish sufficient silica for an ordinary flint nodule. It would therefore
appear that the time required to form an ordinary nodule, equals the period
necessary for the growth of eight successive generations of sponges. It has to
been found by experiment carried on in the Adriatic Sea and the Gulf of Florida,
that it takes from five to seven years for a sponge to grow to the size of an
average flint nodule. Hence it is argued that it would take about fifty years to
form a single nodule, or, (as the process would probably go on all over the
sea-bottom at once,) a layer of nodules. Is not a fact like this cause for
wonder and admiration, as we look up at our chalk cliffs, remembering that to
form one single layer of one or two inches, half a century has passed away, and
that to form a single fragment of flint, such as we see on the roadside, and are
apt to pass by quite unnoticed, there has been such a wealth of animal and
vegetable life?
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