Page images

In the district south of Frome the Great Oolite thins away entirely, and is represented partly by the Forest Marble, and perhaps also by the Fuller's Earth.

The Great Oolite generally forms a well-marked feature in Gloucestershire and the borders of Oxfordshire.

At Coln St. Denis the beds, consisting of white limestone resting on marls, have a thickness of about 145 feet. At Burford the beds have a thickness of about 100 feet.

The upper rock-bed, according to Mr. Hull, is frequently pierced by Lithodomi, and it also affords interesting evidence of having been consolidated contemporaneously with its deposition. Occasionally he has noticed beds of conglomerate formed by waterworn fragments of the underlying lime


On Minchinhampton Common, where the beds have yielded a rich collection of fossils to Mr. Lycett, there appears to be no positive line of separation between the Great Oolite and the Forest Marble.

Other fossiliferous localities are near the Seven Springs, North Leach, Burford, and Sherborne Park (Gloucestershire).

At Sapperton Tunnel Mr. Hull estimates the thickness of the limestones at 20 feet. He observes that in the country near Woodstock, at Wychwood Forest, &c., the beds are blue when reached at some depth, but they weather white, and are called white limestones. The limestones are not usually oolitic, but compact and sometimes marly. The beds contain plant-remains and jet, and have a thickness of from 20 to 30 feet.

1 Mr. Judd has pointed out that when dug at great depths or otherwise obtained at points where they have not been exposed to atmospheric influences, all the Oolitic rocks exhibit an almost uniform deep-blue tint, which is apparently communicated to them by a diffusion through their substance of small quantities of sulphide of iron.

Great Oolite Limestone.

The term Great Oolite Limestone is used in Lincolnshire and the northern part of Northamptonshire because the Great Oolite series is divided into :

[merged small][merged small][ocr errors][ocr errors]

1. Upper Estuarine series (Stonesfield Slate).

The Great Oolite Limestone, as described by Mr. Judd, consists of alternate beds of white limestone and marly clay, with seams made up of the shells of small oysters. Oolitic structure is seldom developed.

Among the fossils are Ostrea subrugulosa, O. Sowerbyi, Homomya gibbosa, Terebratula maxillata, Rhynchonella concinna, Clypeus Mülleri, Isastrea, also bones of Cetiosaurus, and some fish-remains.

Near Banbury, according to Mr. Beesley, it is a white compact earthy limestone, often blue within, and seldom oolitic in structure; moreover, it never assumes the character of a freestone. It is chiefly used as limestone and road-metal. The compact limestones are sparingly fossiliferous, but the intervening shaly or marly beds afford a number of species, many of which have been obtained at Tadmarton.

In Northamptonshire the limestone beds are worked for building-purposes and lime-burning, at Kettering, Higham Ferrers (Stanwick Ragstone), &c.

At Alwalton, near Peterborough, the stone takes a good polish, and is termed the Alwalton Marble. Flaggy beds occur also at Alwalton, Castor, &c.

Great Oolite Clay.

This clay, termed Blisworth Clay by Mr. Sharp, and Great Oolite Clay by Mr. Judd, occurs at the top of the Great

Oolite series in Northamptonshire, resting upon the white marly limestones (Great Oolite Limestone). Mr. Sharp has estimated its thickness at from 2 to 20 feet. He considers that the clay is not represented in Yorkshire, but doubtless answers to the Forest Marble and Bradford Clay of the southwest of England.

Mr. Judd describes it as a series of variegated, blue, green, yellow, and purplish clays, often containing bands of irregular whitish or pale-green calcareous concretions, and sometimes septaria.

It contains Ostrea subrugulosa, O. Sowerbyi, Placunopsis socialis, &c. In part the beds seem to be of estuarine


The clay is employed for brick-making at Bedford Purlieus, and New England, near Peterborough. At Bottlebridge, near Overton Longville, some ironstone has been raised. The soil is by no means fertile.


The Forest Marble, so termed by William Smith from . the Forest of Wychwood in Oxfordshire, consists of flaggy or fissile oolite, shelly limestone, and beds of clay, sand, and grit, displaying much false bedding. Prof. Hull remarks that owing to the rapid enclosure of Wychwood Forest many quarries have been opened, showing the lower beds (resting on the Great Oolite) to consist of false-bedded shelly oolite, splitting into slabs and flags, and composed of enormous quantities of broken oyster shells; they are about 30 feet in thickness. The higher beds consist of bluish clays and marls, with thin flagstones and roofing slates, from 20 to 30 feet in thickness.

Ripple-marks and worm-tracks abound on the surfaces of some of the beds. Fossil-wood occurs in a few localities.

Among the fossils are Terebratula maxillata, Rhynchonella concinna, R. obsoleta, Pecten vagans, Ostrea acuminata, O. rugosa, Avicula echinata, &c.; but none of these are peculiar to the formation.

In Dorsetshire the Forest Marble (according to Mr. Bristow) comprises beds of fissile limestone, divided by seams of clay, which attain a thickness of 450 feet. This disproportionate thickness was considered by Conybeare to be due to the Great Oolite being probably represented in the series.

The following divisions of the beds in Somersetshire and Wiltshire were made by Lonsdale :


(6. Clay with occasional laminæ of grit





[ocr errors]
[ocr errors]




[ocr errors]


5. Sand and Grit Hinton Sand and Sandstone of W. Smith, at Charterhouse Hinton, near Bath Marble. 4. Clay, with thin slabs of stone, and laminæ of grit. 3. Shelly limestone or coarse oolite

2. Sand or sandy clay and grit.
1. Bradford clay.

Between Cirencester and North Leach the beds have a thickness of about 40 feet. In Oxfordshire the thickness averages 25 feet. At Blenheim Park the thickness is 14 or 15 feet. The Forest Marble also occurs near Peterborough. Some of the beds furnish coarse roofing-slates and flagstones, as at Fairford and Chavenage.

Near Milborne Port the beds are worked at Bowden, and known as Bowden Marble; they are also quarried at Wincanton.

Mr. Bristow informs me that at Long Burton in the neighbourhood of Sherborne (Dorset) beds of Forest Marble are sometimes polished for ornamental purposes.'

1 Sometimes called Yeovil marble.

Prof. Buckman remarks that slabs of Forest Marble are of great value for forming the sides of piggeries and cattlesheds. The smaller pieces broken up form a very durable material for road-making. Some of the thicker blue-centred slabs are used for building purposes, for which (especially in the shape of rough ashlar) they are admirably adapted, as they readily form even courses of a somewhat regular thickness. As a soil, the Forest Marble is usually poor, but capable of great improvement by draining and cultivation.

Some of the flaggy beds are used for paving; and at Brotherhampton, south of Bridport, the more calcareous beds have been burned for lime.

Bradford Clay.

This is a deposit of pale-grey clay, slightly calcareous, containing seams of tough brown limestone and calcareous sandstone, identical in character with beds sometimes interstratified with the lower portion of the Forest Marble.

It is, however, of sufficient importance locally to receive a separate name from its occurrence at Bradford-on-Avon.

The greatest thickness of the Bradford Clay seems to be near Farleigh, where it is roughly estimated at from 40 to 60 feet. Where the Great Oolite has disappeared south of Frome, the Bradford Clay and Fuller's Earth come together, and it is not easy to separate the two; while, on the other hand, where the Bradford Clay is wanting it becomes difficult to distinguish the upper beds of the Great Oolite from the lowest of the Forest Marble.

A characteristic fossil, but one which also occurs in the Forest Marble, is the Bradford or Pear Encrinite, Apiocrinites rotundus (or Parkinsoni). Terebratula digona, T.

1 The fragments of the stalk and body of this Crinoid are called 'Coach-wheels' by the quarrymen.

« PreviousContinue »