Thomas Beddoes to Thomas Henry, 1 November 1793
Some Observations on the Flints of Chalk-beds, in a Letter from Thomas Beddoes, M. D. Physician, at Bristol Hot Wells, to Mr. Thomas Henry, F.R.S &c
READ, NOVEMBER 29. 1793
Dear Sir,
During my residence at Oxford, I examined with great attention some chalk quarries, or pits, in that part of England, especially near Henley upon Thames. I hoped, or rather I wished to meet with appearances, indicating the manner in which the flints these beds contain have been formed and distributed. But I was totally unable to frame any hypothesis, such as would comprehend all the phenomena; nor could I acquiesce in any of the various explanations which mineralogical writers have offered. I inclose the few observations I made upon these curious bodies, and request that you will lay them before your Society.
I. It is well known that flints occur, for the most part, stratified. They seldom touch one another in the bed, but lie insulated, like the specimens in the drawer of a cabinet. In almost every stratum, I believe, nodules alternate with tables, or flat masses, varying in thickness and extent. I did, however, observe flints irregularly dispersed among the chalk. Suppose the floor of a room to be a flinty stratum, and the cieling a sieve, through which chalk is falling; suppose, at the fame time, that a by-stander tosses pebbles occasionally among the descending sand, and you will have an idea of the manner, in which these solitary masses are dispersed through the chalk.
II. The rough white crust, which surrounds each flint, and is at first sight so naturally mistaken for adhering chalk, is formed in consequence of the decay of the external coat of the flint itself. It perhaps is the immediate effect of the decomposition of the water that soaks through beds. That it is owing to the decay of the flint, evidently appears from the change that takes place on the fresh face of fragments, broken for the repair of the roads. These fragments, after a short exposure to the weather, become tarnished; by degrees they turn milky or opalescent, then white, opake and rough superficially. We often find, that the surface of smooth bodies grows rough, when it is undergoing any chemical change, as in the familiar instance of polished iron rusting. The acquisition, or the loss of some constituent principle, produces a disarrangement of the superficial particles. By filling a phial with fragments of flint and water; and then inverting it in water, the cause of the change which the surface undergoes might be ascertained: Its progress at least would, in all probability, be observed.
III. Many nodules are hollow. These contain either a white powder, or a cellular spungy substance, which latter is more usually the case. A few are spherical, or nearly so; most are of an irregular roundish or flatted shape, with processes perforated by a hole, within which the contained porous matter appears, pointing outwards, and generally protruding as far as the orifice. A specimen in my possession might be thus exactly imitated. Take one of those oval phials, into which bent tubes are commonly inserted, for the purpose of obtaining elastic fluids by solution. Into this phial, put just acid and chalk enough to raise a foam that shall fill it; then conceive the foam to become concrete. In some specimens, I have observed the spungy mass to protrude beyond the orifice. And it seems to me obvious, from inspection, that the rarefied cellular substance, the powder, the perforated processes, or mamillae, and the holes through them, must have been really produced by the extrication of some elastic fluid. The few imperforated hollow nodules I have seen, are much more nearly globular than the others. In these, what is now the compact semi-transparent coat, must have yielded so much during the effervescence, as to afford space enough for the whole of the extricated elastic fluid. When the effervescence was rapid, or when the air was produced in large quantity, it burst its way out, producing an elongated mammillary process; and carrying along with it the effervescing substance within, as far as the orifice or beyond it. In the specimens containing powder, the effervescing matter must have become concrete, while its parts were disunited by the issuing air. Something of the same kind frequently happens to bars of cast iron, used as a grate for reverberatory furnaces. I have several times seen such bars, after having lain for weeks or months in the furnace, converted superficially into malleable iron, and within containing a grey powder. In two papers, printed in the Philosophical Transactions, I have shewn, that air is extricated during the conversion of cast into malleable iron. Now, in the bars which are found to contain powder, the application of heat occasions throughout the whole substance of the bar, an effort towards the extrication of air. But from some curious circumstances, described at length in the latter of the two papers above-mentioned, it appears, that the air issues from the iron with very little force, even when the heat is considerable. Hence it is extricated from the surface only of the bar; and this alone is converted into malleable iron. During this conversion, the surface is heaved and separated from the internal parts; and some space within is afforded for the extrication of air: And if the bar should be cooled while the particles are disunited, in consequence of this extrication, it will be found to contain a powder. The dust and ashes, ejected in such abundance by volcanoes, must be produced by very nearly the same mechanism. Let us suppose a substance in fusion, from which, or from below which, air or steam is rapidly and copiously evolved—a very common occurrence at the time of an eruption. These elastic fluids issue with such prodigious violence as to dissipate the matter in fusion, and bear it forward, as dust is elevated by a strong wind. On its arrival in the atmosphere, or before, it is cooled, becomes concrete, and descends like snow upon the ground.
IV. The glassy texture and fracture of flints, in the first place, leads me to believe that they have been fused. I have sometimes, within the hollow specimens, seen filaments falling across from side to side—another analogy, as I apprehend the fact, with glass; these filaments having been drawn out in the manner of spun glass, as the flint bubble was blown up by elastic fluids formed within. In the same light I confider the white opake spots, so commonly appearing in the substance of flints.
Mr. Dolomieu (Journal de Physique, 1792) has related experiments, from which he concludes, that siliceous matter in a strong heat yields hydrogen air. His experiments are not indeed perfectly decisive; because the air might have come from the alkali, with which he fused his siliceous earth. Should it however come from the flint, the fact would furnish an explanation of the appearances I have described. Streaks or weals may sometimes be observed upon the internal surface of hollow flints. I have seen these streaks round, and dilated at their termination. They must, I imagine, have been formed by viscid matter running upon the inside of the nodule. Exactly the same appearance might be produced, by letting flow down the inside of a tea-cup a liquid, too tenacious to run off the side entirely, but thin enough to reach the bottom, and form a small button there. If we could suppose, with Dr. Hutton, that flints were spouted into the body of the chalk from subterraneous fires, we might imagine the surface of each clot to have been cooled by coming into contact with the chalk, or during its passage, while the internal parts continued to boil and bubble, and work themselves into foam or powder. I cannot however conceive, how the nodules and tables could have been arranged, in the manner they are, by injection
V. Balls of ochre are sometimes found in chalk-pits. These balls, on examination, clearly appear to have existed in some other state since their deposition in the chalk-beds. They contain lumps of matter, much resembling ferrugineous vitrifications: whence I conjecture, that the whole ball was once in the same state. Long since this conjecture was formed, I observed in the quarries about Clifton, appearances which strongly confirm it. I have before me brecciated ferrugineous masses, and masses of friable ochre, each having precisely the same structure. I have even a series of specimens, from the hardest, reddish, brown ferrugineous breccia, to the most friable ochre, in which the angular fragments are seen perfectly; and, what seems to remove all doubt, you can perceive specks of ochre, beginning to form here and there in the hardest of these masses.—All the compact ferrugineous masses, which I have seen about Clifton, are varieties of hematites. I add, as a circumstance which seems to point out the origin of these masses, that a Gentleman one day last summer picked up in my presence, in a quarry between Gloucester Row and the Mall, a specimen of hematites, which has exactly the shape of a drop, falling from one contiguous surface to another. To that, which may be supposed the upper surface, it is attached by a narrow neck; while the body, a little flattened by reason of its size and specific gravity, rests upon the lower surface.I am, dear Sir,
With great regard, your’s,
THOMAS BEDDOES.
BRISTOL HOT WELLS
NOV. 1. 1793
Published: Memoirs of the Manchester Literary and Philosophical Society, 4 (1793), 303–10