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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, 0. 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 Congbeare 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
5. Sand and Grit = Hinton Sand and Sandstone of W.
3. Shelly limestone or coarse oolite
40 10 25 10
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 thick
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.
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.
decussata, and Avicula costata are also characteristic. Dr. S. P. Woodward obtained more than 100 species of marine animals from this formation in the neighbourhood of Cirencester.
Upper Shale and Sandstone. (Yorkshire.)
This formation (as described by Mr. Hudleston) is remarkable for the quantity of hard siliceous rock, which is sometimes bedded with it, and sometimes occurs in enormous concretions or “doggers'; it occasionally puts on the appearance of a quartzose grit. These doggers are probably the “Crowstones' of Young and Bird, who state that of this material several of the ancient rude monuments have been made.
The beds rest on the Limestone series at Scarborough ; under Wheatcroft the thickness is estimated at about 160 feet, and at Gristhorpe 80 feet.
Scarbroite (a hydrous silicate of alumina) occurs in crevices of the rocks.
Some of the clay-beds are worked near Scarborough for manufacture into tiles and drain-pipes.
The Cornbrash consists of pale-coloured earthy and rubbly limestones, which have been well described as often of a pasty or chalky consistency. The beds, where not exposed to atmospheric influences, are mostly compact, blue in colour, and sometimes sufficiently hard to be used for rough buildingpurposes, as near Malmesbury.
The thickness of the formation is about 40 feet.
lus, Ostrea Marshii, Gresslya peregrina, Lima duplicata, Avicula echinata, Myacites securiformis, Terebratula obovata, T. maxillata, T. perovalis, Echinobrissus (Nucleolites) clunicularis, Holectypus depressus, &c.
In the South of England it is never colitic; it is well developed in Dorsetshire and Somersetshire.
In Oxfordshire, near Woodstock, the Cornbrash, 6 to 15 feet in thickness, consists of shelly limestones. Prof. A. H. Green has noticed that here and there clay-beds occur which cause the formation to swell out to more than double its average thickness; these clays are irregular, and never extend beyond small areas.
The Cornbrash occurs as a rather ferruginous limestone, in the Nene Valley, near Oundle, and Peterborough ; here its thickness is about 15 feet. Rushden in Northamptonshire is a noted locality for fossils.
The Cornbrash of Yorkshire is a marly and sometimes oolitic limestone, overlaid by shale orclays of the cornbrash,' containing Avicula echinata. It is well developed in Newtondale (13 to 14 feet), where (says Mr. Hudleston) its ferruginous character tempted a speculator to work it for iron, while in a secluded nook of the same lovely valley there yet stands (1874) what was meant to have been a colliery—the shaft was sunk to a considerable depth, in the expectation of winning the real north-country coal! The shales vary from 8 to 15 feet in thickness, and seem to form a connecting link between the true Cornbrash and the Oxford Clay.
The Cornbrash is exposed at Gristhorpe and Scarborough, where it is from 5 to 10 feet in thickness.
It is a very shelly rock, and has yielded a large suite of species, for our knowledge of most of which we are indebted to Mr. Leckenby. These include Ammonites Herveyi, Pecten vagans, Lima duplicata, Modiola cuneata, Terebratula obovata, Rhynchonella concinna, &c.
The Cornbrash is used for road-mending, and for building walls, and is sometimes burnt for lime.
As the name implies, the soil in the South-West of England is well suited to the growth of corn; according to Prof. Buckman it contains more phosphate of lime than the subordinate Oolitic formations.
Clunch CLAY AND SHALE. (W'm. Smith.) The Oxford clay consists of dark-blue, yellowish or slate. coloured clay and bituminous shale. It effervesces with
. hydrochloric acid. It contains much iron-pyrites and selenite, and many septaria called turtle stones.'
Near its base is a bed of irregular calcareous sandstone of a concretionary nature and very fossiliferous, called the Kellaways (or Kelloway) Rock, by Smith, from Kellaways Bridge, near Chippenham, in Wiltshire. Although the clay derives its name from the county of Oxford, the Kellaways rock is not known in it.
The thickness of the clay varies from 300 to 600 feet; that of the Kellaways Rock is usually 8 or 10 feet in the South-west of England, and as much as 90 feet in Yorkshire.
Amongst the fossils are Gryphæa dilatata, G. bilobata,* Modiola bipartita, Avicula inæquivalvis,* Trigonia clavellata, Nucula nuda, Belemnites hastatus, B. Oweni,* Ammonites Juson, A. cordatus, A. excavatus, A. vertebrulis, A. Calloviensis,* and Ancyloceras Calloviensis.* * These belong to the Kellaways Rock.