1. Diluvial detritus consisting of flint and gravel, sand, and
loam, occasionally concreted by iron,

2. Loam with a few marly concretions, but no septaria, peb-
bles, or shells,

3. Blue clay with septaria and shells, hardest in the lowest
layers,

Feet.

.

10

30

65

105

The calcaire grossier of the Paris strata, which lies immediately under the surface of the soil and diluvium, corresponds most nearly in its fossils, and its geological relations, with the London clay of the English series. It is used in that capital as a building stone. No beds have been discovered in the London basin which make a nearer approach to the French lime-crag in chemical and external characters than the calcareous sandstone patches above mentioned.

Whenever the London clay occurs in a cliff, or has been laid open in sinking wells, it has been found uniformly to contain nearly horizontal layers of ovate or flattish masses of argillaceous limestone. These have been called septaria, because they generally exhibit the appearance of having been traversed in various directions by fissures, afterwards filled up wholly or partially by calcareous spar or sulphate of barytes. They were considered at one time as characteristic of the London clay; but they are not uncommon in the superficial clay beds of coal districts. They abound in such a degree in the lias at Whitby, as to have acquired for the stratum in which they lie, the name of cement stone, as already mentioned under that head.

The London clay, therefore, is a vast argillaceous deposit, containing subordinate beds of calcareous

FOSSILS OF THE LONDON CLAY.

299

concretions, which sometimes form solid rocks, but these partial changes, never affect the general character of the formation.

Interspersed through it, we find sulphuret of iron, selenite, and sometimes phosphate of iron, substances which impregnate any water which issues from this stratum, and make it unfit for domestic use. The mineral copal or resin which was discovered in excavating Highgate archway, though manifestly a vegetable product, may, in its fossil state, be classed among the contents of this formation. In the gravel about London, amber has also been found, as also in the cliffs of brown clay on the coast of Holderness, in the south of Yorkshire. It may most probably be assigned to the London clay, since it occurs in the contemporaneous formations of Europe, in Italy, France, and especially Prussia, associated with a vast quantity of vegetable remains.

Few mineral strata claim a greater interest than this formation does, by its organic remains. Among the amphibious class of the higher orders of animals, we find here crocodiles and turtles; proving that the shores of some dry land, where these creatures could deposit their eggs, must have lain within an accessible distance of their abode. Several species of vertebral fish, are found in fine preservation, which merit the study of the comparative anatomist. Among crustaceous fish we may enumerate species of the crab and the lobster.

The testaceous exuviæ are very numerous, and so well preserved, as to have the freshness of recent shells. With respect to generic character, there are few existing shellfish which are not represented

by types in the London clay; but in specific character there is a frequent difference. Of the extinct genera which are so common in the deeper seated strata, few specimens occur in this, showing the intermediate epoch of its formation. Thus though nautilites resembling those of the Indian seas are still common, ammonites and belemnites are so rare, as to render it doubtful if any be really buried in the clay. Ammonites and belemnites may occur superficially which have been detached and drifted from the inferior oolites. Echinites so common in the chalk, are seldom seen in this superior

stratum.

M. Defrance has collected in the corresponding geological formation at Grignon, near Paris, 800 different species of shells, besides serpulæ, siliquaria, dentalia, and a few echinites and madrepores. It is probable that if an equally skilful and zealous conchologist were to explore the Hordwell cliff in Hampshire, he would find corresponding species, and numbers of shells. In the lower beds of the French calcaire grossier at Liancourt, Mr. Webster observed an exact accordance in organic remains with the beds of the London clay at Stubbington.

Zoophytes and encrinites are likewise very rare. Portions of wood occur, which are fibrous at one time and charred at others. These exhibit the perforations, and even contain the casts of an animal analogous to the teredo navalis, or borer, which now commits great havock on the planks of ships in tropical seas. In the heart of the nodules of argillaceous limestone, which resemble septaria, a wooden nucleus is frequently found. These masses

TROPICAL FRUITS IN THE LONDON CLAY. 301

also contain shells, that possess their fresh pearly lustre.

The most curious vegetable remains of this formation, are those of fruit and ligneous seed-vessels found in vast profusion and variety in the Isle of Sheppey. Mr. Crowe of Faversham, has made a collection of seed vessels from this locality, amounting to no less than 700 different varieties, of which very few agree with any existing seed vessels known to botanists.

A few of the same vegetable remains have been found likewise on the opposite shore in Essex, as also at Kew. Among Mr. Crow's specimens are many which belong to tropical climates. Some are evidently a species of cocoa nut, and others are varieties of spices.

"The evidence of a neighbouring region of dry land seems attested by these vegetable remains, (which from the state in which they are found can scarcely be supposed to have been drifted from any great distance,) as well as by the occurrence of the amphibia before mentioned. We can scarcely resist the temptation of asking, What was that ancient land? had any part of England then raised its head above the waves? Does it not sound extravagantly, even to inquire whether its oldest and highest mountain tracts then formed a groupe of spice islands frequented by the turtle and crocodile ? Speculations like these, though unavoidably sug

gested, almost give the features of romance to the sober walks of science."-Conybeare, Geology of England, p. 30.

Wherever the sea has broken up the solid clay, numbers of its imbedded fossils come into view; as in Harwich cliffs in Essex; in those on the north of the Isle of Sheppey; and especially those between Hordwell and Christchurch on the south-west of Hampshire.

This clay forms the upper stratum of the London chalk basin, except where it is covered with the sands of the upper marine formation, as on the surface of Highgate-hill, Bagshot, Frimby, Purbright heaths, &c.; or by alluvial and diluvial gravel, and loam. The London clay stratum constitutes a very considerable part of the soil of Suffolk, nearly the whole of Essex, including Hainault and Epping Forests, down to the sea, the entire surface of Middlesex, with portions of Surrey, Berkshire, and Kent. Here it is seen on the northern side of the Medway; it constitutes the Isle of Sheppey; the cliff from Whitstable to the Reculver north of Canterbury, and thence westward to Boughton Hill. It is also very extensive in the chalk basin of the Isle of Wight. The whole coast from Worthing in Sussex to Christchurch in Hampshire is formed of this clay bed. Thence it extends inland by Ringwood, Romsey, Fareham, to Worthing, composing the surface soil except in a few places where it is covered by sand or alluvium. In the Isle of Wight, it extends in a dislocated upright position, all along from Alum Bay on the west, to Whitecliffe Bay on the east. Its two planes therefore front to the south