NOTES ON THE ARTIFICIAL UNMAKING OF FLINTS.

BY T. HAY WILSON.

(Read June 7th, 1889.)

Reading Professor Judd's interesting paper on "The Unmaking of Flints," which I was not fortunate enough to hear, it occurred to me that as this same "unmaking" has often been a source of great trouble and anxiety to me, a few notes of my special experience of an artificial unmaking may be of interest.

This experience lies in the behaviour of Flint Shingle when exposed to the action of hot gases, and the following destructive agents percussion, attrition, heat, with pressure, moisture, and acid.

The gases are the products of combustion discharged from the cylinder of a gas-engine, where, in a few cases, it has been essential, from surrounding circumstances, to have the slight noise from the exhaust completely silenced. The working parts of a gas-engine are very similar to those of the ordinary steamengine, the reciprocating motion of a piston being caused by impulses from the explosion and expansion of a combustible mixture of gas and air, in a cylinder, instead of by the expansion of steam. After the explosion of gas and air, the piston is driven forward, and the products are expelled during the return stroke of the piston, and (in these special cases) driven through a chamber filled with shingle, to reduce the pressure and quiet the noise. These chambers are sometimes made in the ground near the engine, and are about 6ft. deep and 3ft. square, built with fire-brick, and nearly filled with shingle. The hot gases, in some of these cases amounting to about 7,000 cubic feet per hour, are passed through the shingle, entering at the bottom with a pressure of about 2 atmospheres and a temperature of nearly 1,000° Fahrenheit, and escaping quietly through a ventilating pipe at the top, an iron plate being firmly fastened down to the brickwork, with an asbestos joint to prevent the gases from leaking into the air except through the ventilating pipe.

The constituents of the gases are-air, nitrogen, carbonic acid, *Proc. Geol. Assoc.,' x, 217.

and aqueous vapour; the acid being about 8 to 10 per cent. of the weight and the H2O about the same. Taking the specific heat of silica at say 02, enough heat is furnished by the combustion of 100 cubic feet of the live gas to raise the temperature of 5,000 lbs. of shingle about 15° F. Of course some of this heat is absorbed by and conducted through the walls of the chamber into the adjacent ground, a little is lost by radiation from the plate, and more goes into the atmosphere through the ventilating pipe.

The hot gases being continually poured through the chamber, in some cases for 10 to 12 hours per diem, the temperature would be very great, were it not for the aforesaid cooling agencies; as it is, the temperature of glowing cast-iron is sometimes reached, and probably the centre of the mass is for some time daily not less than 1,400° F., or possibly more.

A recent test by the Society of Arts gave the temperature of the products before leaving the cylinder at over 2,000° F.

Percussion is caused not only by the entrance of the gaseous products at a pressure of about 35 lbs. per square inch, but by their hammering the stones against each other. Occasionally explosions occur in the inlet pipe, which for the moment enormously increase the pressure and also the temperature.

Attrition is one of the chief sources of destruction to the shingle, the mass being moved by each influx of gas, these influxes being sometimes 80 or more per minute.

Moisture. While the gas is passing, the temperature is, of course, too great to allow of condensation; but for 12 or 14 hours of the 24, and for about 40 hours at the end of a week, the cooling influences alone are at work, and condensation often takes place. Moreover, a leakage often occurs through the walls of the pit, which becomes open and is destroyed by the expansion and contraction.

Acid.-Flint is the only shingle I can find to stand this action for any length of time, and I always test a sample with dilute sulphuric acid before filling a chamber. I have known cases where limestone has got amongst other shingle, and it is acted upon as if in a mill. In one case in particular, at Oxford, three days after a chamber was started, I found over two tons had almost disappeared, as a fine powder, and had been deposited over the adjacent roofs. The chamber was refilled with

good flint shingle, and has not been touched for about three years, the shingle having stood the daily influence of about 40,000 to 50,000 feet of gaseous products, without serious disintegration. This is the most successful case I know with this kind of chamber.

Pressure is considerable at all times when the gases are passing, and may be as much as 200 lbs. per square inch, when explosions occur. The shingle which appears to stand best is that composed of well-worn beach or river-gravel flint pebbles (Shown in sample 3). I am informed by the contractor that all those samples which I have recently found to last fairly well, are dredged between Gravesend and Sheerness.

The samples exhibited are of this sort. I do not know where the Oxford shingle was procured, though probably it was from the gravel on which the city stands. In the Report of the Excursion of the Association to Oxford in 1874 ( Proc. Geol. Assoc.,' Vol. iv, p. 91), Prof. Phillips thus describes the gravel overlying Oxford Clay, on which the city stands :

"In composition it is mostly Oolitic, but with some admixture of materials from the high-level gravel, which is often found capping the plateaux of the district. These materials are chiefly quartzites, felstones, and grits; flints are also found."

SAMPLES.

No. 1. This shows the original and disintegrated flint also; the test having been very severe, as from the nature of the ground (near Wallbrook) a brick pit could not be made, and an old boiler-end about 6ft. long formed the receptacle. The internal pipes show signs of heat-action, due to very high temperatures only. The disintegrated portion is from the bottom where the gases enter. This has been down twelve months, and about 30,000 cubic feet of gas passed through daily on week-days. No. 2. From same chamber, previously opened, had been in for about six months: the bulk was badly destroyed.

No. 3. From a brick chamber with frequently up to 80,000 feet per diem of products passed through, and subjected to severe explosions; most of the bulk quite disintegrated. The undamaged portion will stand anything as far as I can test it.

No. 4. Small shingle (I believe from St. Leonards). This was closely packed in a small iron vertical chamber, and gases

passed through from above: after about six years it has not wasted appreciably. This chamber has cool air occasionally passed through, say 10 or 15 times per minute.

In some instances, where space is available, it is not necessary to cover the top, and the shingle is merely heaped up, the vapour flying about; in such cases the temperature is much lower, as radiation is considerable, and gases escape freely. In many cases the small cylindrical iron chamber just named (No. 4) is used instead of the pit, and closely packed with fine shingle, being placed vertically in the exhaust pipe, and the gases entering at the top, the grinding action does not occur, as the mass is not lifted, and shingle in these lasts for years with scarcely any waste. Iron chambers like this are known as "Justice's" patent, and have been largely used; the heat passes through them much more quickly, and there is considerably more radiation in proportion.

It seems to me that percussion and attrition have been, in all these cases but the Oxford one, the most troublesome agents of destruction, as the action of the acid appears to be successfully resisted by the specimens I have shown you.

It seems further clear that flint ultimately becomes reduced to mud, not sand.

DESCRIPTION OF SPECIMENS NAMED.

Nos. 1 and 2 showed pieces of Flint-shingle of the normal size used, varying from an equivalent of, say, 14 inch cube to the size of a walnut, more or less water-worn and rounded; also a disintegrated portion reduced to about the size of hazel-nuts, partly flaked and split, varying in size down to powder.

No. 3 showed some completely rounded pieces of same shingle showing no reduction, and some ordinary less-worn Chalkflints, burnt red and brittle, flaked, and disintegrated, as in Nos. 1 and 2.

No. 4.-Small shingle, about inch cube, showing no reduction from normal bulk; gases having entered at top of shingle column the mass was not shaken up.

Other similar specimens not named in these notes were also shown.

THE GEOLOGY OF UPTON AND CHILTON, IN BERKS.

By A. J. JUKES-BROWNE, B.A., F.G.S.

(Read June 7th, 1889.)

PART I. CRETACEOUS ROCKS.

This paper is intended as explanatory of a section through the Chalk and Upper Greensand escarpments from Chilton Downs to Didcot, and as a guide to the cuttings and exposures of the Chalk near the villages of Chilton and Upton, a district which will probably be visited by the Members of the Geologists' Association during the summer.

I propose to give a brief description of each of the divisions of the Upper Cretaceous series which will be shown on the new edition of the Geological Survey Map, and to mention the exposures which occur in each division within easy reach from the stations on the Didcot and Newbury line.*

Gault. The marly clays of the Upper Gault are exposed in the railway-cutting north-west of Didcot. The lowest beds seen are grey argillaceous marls, with Avicula gryphoides. These pass up into more shaly micaceous marls with the same fossil and small scattered phosphatic nodules. These beds were included in the Upper Greensand on the old edition of the map, but the base of this is now taken where the first layers of Malmstone occur.

The Gault at Didcot is more than 200 feet thick, for a well at one of the hotels near the station was bored to a depth of 208 feet, passing through clay into rock and sand, and the site of this must be 30 or 40 feet below the line which is taken as the base of the Malmstone.

Malmstone. The Malmstone of Berks closely resembles that of Surrey and Hants. It is a light grey calcareo-siliceous stone, which lies in layers, separated by bands and seams of soft micaceous marl. In the lower part the stone beds come in as courses in the marl, but in the higher part the beds are thick, with only thin partings of marl. Much of the rock is very siliceous, the silica consisting partly of the spicules_of

The information was obtained during the official survey of the district, and is published by permission of the Director-General of the Geological Survey.