The Geology of the Isle of Purbeck, 1898
type: Beyond Scotland - Dorset
The Geology of the Isle of Purbeck
MEMOIRS OF GEOLOGICAL SURVEY OF ENGLAND AND WALES, 1898, xi 278.
The Geology of the Isle of Purbeck and Weymouth. By A. Strachan.
The so-called 'coal' is a highly bituminous layer of shaley stone about 2 feet 10 inches thick with its partings, and of a dark brown colour, whence its local name of' Blackstone. It breaks with a conchoidal fracture and readily ignites, burning with a bright flame and an offensive smell, and leaving a copious grey ash.
When exposed to the weather, it is apt to develop a fissile structure, and the laminae curl up so as to closely so as to imitate layers of brown paper or leather. It is free from pyrites, but contains lenticular masses of calcareous; matter. The Kimmeridge 'Coal' has been in requisition from time immemorial. The earliest traces of its use consist of the so-called 'coal-money,' and of various ornaments and vessels which have been found in barrows and among Roman remains in the neighbourhood of Weymouth, and recently at Silchester. The supposed coins which occur in the soil in various places in the Isle of Purbeck, are circular discs of two or three inches diameter, and have obviously been turned in a lathe. They are doubtless waste pieces formed in the process of turning cups or vases, and were thrown aside in heaps, as they are now found. See J. C. Mansel-Pleydell, Proc. Dorset Field Club, vol. xiii, p. 178, and vol. xv, p. 172.
But as a fuel the coal has been more extensively used, principally in the neighbourhood of Kimmeridge, for which purpose, however, its abominable odour renders it unsuitable.
Of late years the mineral property at Kimmeridge has been leased to the Kimmeridge Oil and Carbon Company, by whom the coal has been used for fuel for improving the illuminating power of coal-gas and for the manufacture of paraffin. It is stated by them that the residual coke or carbon after the distillation of the oil possesses the properties of animal charcoal, and can be employed as a deodoriser, disinfectant, and decoloriser, and that it serves also as a manure. From the oil an insecticide and a preparation for the prevention of mildew, oidium, &c., on vines and other plants have been made.
The Blackstone of Kimmeridge Bay, by the same account, is the richest seam yet discovered, and in the laboratory has been known to yield 120 gallons of oil to the ton, or, when distilled on a large scale, 66 gallons to the ton, while the common shale gives about 33 gallons to the ton. For this information I am indebted to Mr Charles Beaumont.
An Analysis of Kimmeridge Coal
by Mr J. W. Keates, F.C.S,, gave the following result :-
One ton yielded 9,000 cubic feet of gas, which, burning at the rate of 9·2 cubic feet per hour in a fifteen-holed argand burner, equalled ~6 sperm F .candles consuming 120 grs. per hour. The coal was composed as follows.-
- Volatile matter = 61%
- Carbon or Coke (Carbon 13·15) (Ash 25·85) = 39%
The ash contained;
- Insoluble residue = 29 ·01 %
- Peroxide or Iron = 7·10 %
- Silica = 21·75%
- Alumina = 10.60%
Notwithstanding, however, the variety of uses to which the coal has been put, it has never paid the expense of extraction, and at the time the re-survey of the district was in progress, the works were almost at a standstill.
The Blackstone of Kimmeridge Bay, by the same account, is the richest seam yet discovered, and in the laboratory has been known to yield 120 gallons to one ton, or, when distilled on a large scale, 66 gallons to the ton while the common shale gives about 33 gallons to the ton......Not withstanding, however, the variety of uses to which the coal has been put, it has never paid the cost of extraction, and at the time the resurvey of the district was in progress, the works were almost at a standstill. Just to the east of headland of Clavells' Hard the "coal" is thrown down about 6 feet westwards along a line of crush. It then runs along a terrace in the cliff at the east end of Hen Cliff. There it strikes inland, the outcrop being marked for about 800 yards northwards by a line of old surface workings. Several faults of 5 to 12 feet can be seen in its outcrop in the cliff, all with one exception, with downthrows westwards. The coal has been worked also by means of a shaft 200 yards from the edge of the cliff.
At Portisham an energetic attempt has been made of late years to turn the "coal" to account. About 1856 it had been discovered in the Potisham Dairy (100 yards west of the station) at 14 ft deep, and a few tons were got out. A pit was sunk on the east side of the building and intersected the bed at 12ft deep, but in both cases the works were drowned out. In 1877 the railway cutting east of the Station, having disclosed the outcrop, the "coal" was mixed with clay and burnt for ballast. Various other shafts were sunk, but all were overpowered by water until the Manfield Shaft was made in 1883. This was dry until coal was reached, when a feeder, which yielded at first 11,000 gallons per hour was struck. A very rich oil shale was met in the shaft at 46 ft 7 inches depth, but the principal bed at 137 ft, a hard rock band occurring 33 feet below it..... The shaft was sunk to a total depth of 189 ft with a boring to a further depth of 126 feet. It is situated 170 yards north of the railway, and an incline was driven to it in the "coal from the outcrop at the side of the line, the inclination being 1 in 4 (15 degrees). One hundred tons of both parts of the principal bed were sent to Scotland for distillation, but in consequence of the death of Mr. Manfield the trial seems to have been abandoned.
The Kimmeridge coal, though still used as fuel in the village, has not been profitably worked for export of late years. The older workings are confined to the neighbourhood of Hen Cliff, but the same or a similar seam has been proved in a shaft and several bore holes at Portisham by the late Mr. Manfield.