SPRINGS AND WELLS IN THE GREAT OOLITE. 263

Cheltenham, 1134 feet above the level of the sea; Broadway, 1086; Stow in the Wold, 883; and Landsdown near Bath, 813.

Thus the hills which descend with an abrupt escarpment to the north-west, slope with a gentle declivity to the south-east. This peculiarity belongs to the chains formed by all the beds above the new red sandstone, and proceeds from the cropping out of their strata to the north and west, against the older rocks that lie in these lines of direction. The Thames which has its source on the back of the Cotteswolds on the high grounds between Cirencester and Cheltenham, wanders to the east through a course of more than 150 miles before it reaches the tide level at Richmond; while on the western side of these hills, the Severn, within twelve miles of the origin of the Thames, already feels the influence of the tide. This oolitic range, like the chalk, is completely broken through by many vallies.

The thickness of these beds varies from 250 to 400 feet. The great oolite, in common with all the beds of this formation, has a gentle dip towards the south-east. The Cornbrash forms an excellent arable soil; but that over the great oolite, is a loose, absorbent, and unproductive stonebrash.

The clays which lie under the Cornbrash and the Forest marble generally keep up the water beneath these strata, affording a ready supply of springs. Hence, as has been happily observed by Mr. Smith, the course of these beds is distinguished by a denser population, than of those of the inferior oolite, where water cannot be obtained unless from deep wells sunk at a great expense. In order to procure this

necessary of life, wells have been sunk 130 feet through the rock, till its junction with the fuller's earth, which intercepts and throws out its springs, forming a weeping soil round the escarpments of the oolitic hills; circumstances very observable near Bath. When there is a failure of this intervening clay, the springs escape downward, and are lost. This is particularly the case with the Forest marble, which has numerous swallow holes. These absorb the springs filtered through the Cornbrash. Thirty of them may be counted, within the compass of half a mile, round Hinton.

§ VII. MIDDLE DIVISION OF OOLITES.

1. Oxford Clay.-Clunch clay of Smith.

This clay forms the separation, between the middle and inferior assemblage of oolites, including subordinate beds of limestone called the Kelloway rock.

This formation is composed of beds of immense thickness of a tenacious dark blue clay, which becomes brown on exposure to the air. It contains argillo-calcareous geodes and septaria (balls and cakes) termed turtle stones. These geodes afford a coarse marble. The argillaceous strata are frequently blended with calcareous or bituminous matter, producing in the latter case an inflammable shale. Irregular beds of limestone occasionally appear in the lower part of this formation, which are merely subordinate. These are the Kelloway rock, in irregular concretions, hard and of a bluish colour. They consist almost entirely of a congeries of organic remains, among which several species of

OXFORD CLAY AND KELLOWAY ROCK. 265

ammonites predominate. The beds of clay immediately over this rock abound in selenite; below are found a brown aluminous earth and bituminous wood. Beds of clay separate the Kelloway rock from the inferior oolites.

Iron pyrites and selenite occur abundantly in this, as in all argillaceous formations. The association of sulphur with the clay strata affords a curious subject of chemical inquiry.

The organic remains of the Kelloway rock, are different from those in the clay beds.

1. In the clay, are a few bones of the ichthyosaurus, of a different species from those in the lias. Here also among testaceous remains, are ammonites, rostellaria, serpula, patella, ostrea, gryphæa, and

perna.

2. Shells of the Kelloway rock.-Ammonites of different species from those in the clay, nautili, and belemnites, rostellaria, cardita, chama, gryphæa, pecten, plagiostoma, avicula, terebratula. The most characteristic shell is the gryphæa dilatata,

See Plate IV.; and for the range and extent of this formation, consult Conybeare and Phillips, p. 197. This stratum is for the most part extremely low.

Its thickness probably exceeds 500 feet, being about 700 in the midland counties. It is nearly conformable with the strata already described; its general dip being to the east and south-east under a very small angle. It is commonly necessary to sink through this dense mass before water can be found. This is a serious undertaking, a well having been sunk at Boston to the depth of 478 with

out success.

§ VIII. CORAL RAG,

The Superior Oolite, Oxford Oolite or Pisolite of some authors.

This formation comprises a series of beds, in all from one to two hundred feet thick; in the upper part, which contains the oolite, the calcareous matter prevails; and in the lower, the siliceous matter. The coral rag occupies the middle of the series.

The sandy, or siliceo-calcareous beds, consist of a thick deposit of yellow coloured quartzose sand, traversed by irregular strata of hard gritstone. These rest immediately on the Oxford clay. The fossils of this formation are found most abundantly in the calcareous grit of this sand, especially beneath the coral rag beds. The ostrea gregarea, characterises the sand. Iron is diffused through it in such plenty, as to cause the appearance of the iron sand found in a superior bed.

The remains of vertebral animals are scarce; but a few vertebræ of the ichthyosaurus have occurred in the calcareous grit.

The shells are numerous there; ammonites, nautilus, belemnites, melanea, turbo, helix, trochus, ampullaria, serpulites, ostrea, pecten, chama, trigonia, lima, lithophaga, mytilus, modiola. Many beautiful echinites are found in this formation; of the cidaris and clypeus genera. It is the first formation, in descending from the surface, which af fords any considerable number or variety of madrepores. Several species of the caryophyllia and astrea are also observed. Fossil wood occurs frequently in the calcareous grit. See Plate IV.

The coral rag, which reposes on these siliceo

calcareous beds, consists of a loose rubbly limestone, often almost entirely composed of a congeries of several varieties of aggregated and branching madrepores. There are two or three irregular courses of this rock between the freestones and the inferior sandy beds.

upper

The calcareous beds are; a calcareous freestone of tolerably close texture, full of comminuted shells. It is oolitic, often with large ovoid particles, whence the name Pisolite. Colour, pale yellowish white. As a building stone, it is apt to scale off in large flakes from exposure to the weather, a defect from which the city of Oxford has suffered.

The thickness of the calcareous sand and coral rag may be estimated at about 100 feet for each. The inclination of the beds is very slight; but care must be taken in observing it, not to mistake the lines of the cleavage of the rock, for those of stratification.

From the porosity of these strata, it is necessary to penetrate to their junction with the subjacent clay, before any considerable supply of water can be had.

$ IX. UPPER DIVISION OF THE OOLITIC SERIES.

1. Kimmeridge clay. 2. Portland oolite. 3. Argillo-calcareous beds of Purbeck.

1. Kimmeridge clay. These beds consist of a blue slaty or grayish yellow clay, which contains selenite, with occasional strata of highly bituminous shale, called Kimmeridge coal, because it is