Chippenham, Calne, &c., to which we shall allude hereafter.*

10. SUBDIVISION OF THE OOLITE:-PORTLAND OOLITE.† -Beds of limestone having the roe-like structure above described form a principal lithological feature of the calcareous portion of the Oolite; but the uppermost deposits consist of shelly freestones, of variable structure. From the great employment of certain beds of this stone for architectural purposes, and the extensive quarries that have for centuries been worked in the Isle of Portland, this upper group is called the Portland stone. In the south of England, as we have already had occasion to mention (p. 397),‡ the Portland beds are covered by the Purbeck (Wealden) strata, including in some places layers of vegetable mould and petrified upright trunks of coniferous trees. The lower part of this group is composed of a bed of sand (Portland sand) from 50 to 80 feet thick, which gradually passes into the underlying clay. The fossils of the Portland beds are very numerous; large ammonites, pleurotomariæ, cerithia, trigoniæ, pectens, oysters, &c., and bones of saurians, with drifted coniferous wood, are among the prevailing organic remains.§

* See Medals of Creation, vol. ii.; Geological Excursions to Clifton, p. 864; and, for the section exposed by the Birmingham and Derby line, Excursions to Matlock, p. 867.

Mr. Hugh Miller contrasts the appearance of these railway-sections, in which the strata have a low angle of inclination (as usual with the secondary rocks of England), with those of the line from Glasgow to Edinburgh. There every few hundred yards in the line brings the traveller to a trap-rock, against which the strata are tilted at every possible angle of elevation. See "First Impressions of England and its People."

For details of the geological phenomena exhibited by the beds below the Chalk and above the New Red Sandstone, in the south-east of England, Dr. Fitton's elaborate memoir, in the Geol. Trans. second series, vol. iv., should be consulted.

See also Geological Excursions round the Isle of Wight, &c. p. 286.
As a British locality exhibiting the Portland strata and their cha-

11. THE KIMMERIDGE CLAY.-This argillaceous deposit consists of dark-bluish and grey clay, which in some parts passes into highly bituminous shale, known as "Kimme. ridge Coal;" the name is derived from Kimmeridge in Dorsetshire, where some of the layers are sufficiently combustible to be used as fuel.* This deposit is in some places 300 feet in thickness, but thins out very considerably in the northern counties of England. It extends into France and Switzerland.

Near Hartwell, in the vale of Aylesbury, in Buckinghamshire, this clay is largely developed, and abounds in organic remains of great beauty and interest, especially Ammonites, Pleurotomariæ, Thraciæ, Pernæ, &c.; the nacreous or pearly coat of the shells of the ammonites is often as perfect and splendid with iridescent colours as in a polished shell of a recent nautilus. Many of the shells of which casts only occur in the Portland rock above are found preserved entire in the clay. A flat oyster of a deltoid form (Ostræa deltoidea) is very abundant, and is characteristic of the Kimmeridge clay.

Bones of Ichthyosauri, Plesiosauri, and Cetiosauri are occasionally found, also scales, teeth, and other remains of fishes, among which are mandibles of the Chimæroids.†

12. THE OXFORD OOLITE, or CORAL-RAG.-The Kimmeridge clay rests on beds of coralline limestone, termed Coral-rag, many of which are really petrified coral-reefs, and racteristic fossils, Swindon in Wiltshire is one of the most interesting. See Geological Excursions in the Medals of Creation, vol. ii. p. 862.

*See Excursions round the Isle of Wight, p. 265. This bituminous shale contains, according to the analyses of some chemists, 25 per cent. more illuminating power than Newcastle coal, when applied to manufacturing purposes. It occurs in several layers, varying from 2 to 7 feet in thickness, interstratified with dark-coloured sandy calcareous beds.

The extensive Museum of Dr. Lee of Hartwell contains a fine col lection of fossils from this neighbourhood; and strangers are allowed free access by the learned and liberal proprietor.

consist of coarse limestone, composed of stony corals, chiefly of the Astroido family, and having the interstices filled up

with shells, echinoderms, sand, and pebbles; the whole is more or less consolidated by calcareous, and in some in

stances by silicious, infiltrations. So obvious is the origin of these rocks, that the most incurious observer who travels through the districts where these deposits prevail cannot fail to remark the blocks of coral which everywhere meet his view in the quarries and on the road-side. In many parts of Wiltshire, Berkshire, and Gloucestershire, the Coral-rag is extremely rich in organic remains; corals, shells, and echinites occurring in almost every mass of stone. From the pits near Farringdon * hundreds of specimens may be collected in the course of a few hours; and the quarries near Calne, in Wiltshire, abound in echinites of that beautiful species popularly called " Fairies' night-caps" (Cidaris, Lign. 117, fig. 3), which are often surrounded by their spines in as perfect a state as if they had just sunk down into soft sand or mud; detached spines of these animals are found (Lign. 117, fig. 5) in immense numbers.†

Many species of the bivalves called Trigonia,‡ of which only three species, inhabitants of the Australian seas, are known living, occur in these beds in great perfection and abundance; two species are here figured (Lign. 117); limestone-casts of these shells are very frequent in the Portland stone, and are generally accompanied with a Cerithium.

13. OXFORD CLAY.-The Coral-rag rests upon a bed of clay, in many places 300 feet thick, which is characterized

* Most of the heights around Farringdon are capped with shelly sand and sponge-gravel, referable to the Lower Greensand, overlying Coralrag. Stanford pit, three miles south-east of Farringdon, contains:-1. Uppermost, Coral-rag, 3 feet; 2. Limestone, with immense numbers of shells, 4 feet; 3. Sand, 3 feet; 4. Clay. These beds abound in trigoniæ, gervilliæ, terebratulæ, ostrææ, belemnites, and ammonites; in a slab of coarse sandy limestone, four feet square, I counted above fifty gervilliæ and many trigoniæ. Between Watchfield and Shrivenham the Coral-rag is seen in openings on the road-side. See Medals of Creation, vol. ii. p. 861.

† See Medals of Creation, vol. i. p. 311, for a particular account of these fossils. Ibid. p. 412.

by the abundance and variety of its organic remains. Some localities in Wiltshire are celebrated for the state of perfection in which many species of extinct Cephalopoda occur. At Christian Malford, near Chippenham, the cuttings for the Great Western Railway brought to light specimens of the soft parts of the animals allied to the Sepiade or Cuttlefish, whose shells are so abundant in the argillaceous deposits of the Oolite. The Belemnoteuthis (of Channing Pearce),* an extinct cephalopodous mollusc allied to the Belemnite, but generically distinct, has been found with its arms entire, and the acetabula or suckers and spines attached. In the works for a branch-railway through Trowbridge, the Kelloways rock, Oxford clay, Cornbrash, and Bradford clay were largely exposed, and yielded innumerable specimens of the usual fossils, and some species of ammonites and other shells not previously observed. The members of the upper part of the oolitic system occur

* See London Palæontological Journal, No. 2, Pl. XV. XVI., for beautiful figures of some remarkable specimens of these extinct Cephalopoda. Medals, vol. ii. p. 447, and p. 460. Phil. Trans. 1850, p. 393.

For a systematic account of the Belemnites and other members of the Cephalopodous family, see Woodward's "Manual of the Mollusca."

Mr. Reginald Neville Mantell made a large collection of the fossils brought to light by the cuttings and excavations in this locality. It comprised very large and fine specimens of Ammonites; Nautili; Belemnites with the phragmocone and traces of the soft parts; the cartilagin ous base of the Belemnoteuthis; innumerable small gasteropodous shells; Ostrææ deltoidea; Gryphææ, Terebratulæ, &c.; masses of coniferous wood and lignite; bones of Ichthyosauri, Plesiosauri, Teleosauri, &c. ; and a few relics of fishes. The profusion of fossil shells dug up in the comparatively small area traversed by the railway, some of which were inhabitants of deep and others of shallow water, here and there intermingled with drift-wood, attests the effects of sub-marine currents by which the remains of molluscs of such different habitats were accumulated and spread over this area of the sea-bottom, with the spoils of the land transported from a distance by rivers. Quart. Journ. Geol. Soc. vol. vi. p. 315.