PLASTIC CLAY FORMATION.

III. Fresh Water formations.

293

IV. Upper Marine formation; Crag and Bagshot sand.

§ I. PLASTIC CLAY FORMATION.

Viewed as a whole this formation consists of an indefinite number of sand, clay, and pebble beds, in irregular alternation. In England, the sands are the most extensive of these deposits; among which the clay and pebbles are subordinately placed at unequable intervals. On attentive investigation over a wide surface, these alternate layers above the chalk, will be found to form only one nearly contemporaneous series, between the chalk and the London clay. The plastic clay of the Paris chalk basin consists commonly of two beds separated by a stratum of sand, of which the lower is properly the plastic clay. It is unctuous, tenacious, with some siliceous matter, but no calcareous, and quite refractory in the furnace, provided that there be little iron intermixed. Its colour is very variable; white, gray, yellow, gray mixed with red, or almost pure red. The clay is used according to its quality, for fine or coarse pottery, The French sands have a great variety of colours. In the lower strata of the Paris plastic clay bed an imperfect coal occurs.

In England the sands of this formation are in some places of an almost infinite variety of colours, as at Alum Bay on the coast of the Isle of Wight, and it is sometimes aggregated into sandstone. The clays differ in colour and quality, and are sometimes laminated. They are called fire-clay, brickclay, pipe-clay, and potter's-clay; the two latter occur at Poole and the Isle of Wight. Beds of

wood coal have been observed in the Isle of Wight, - in Dorsetshire, and at Newhaven ; and fuller's earth in the beds at Catsgrove near Reading, and on the Edgeware road. In all these particulars, a close analogy exists between the French and English basins.

Pyrites, selenite, fibrous gypsum, and tubular ironstone, are the mineral contents of this formation. They occur in veins. There are also nodules of a dark coloured limestone.

Among the organic exuviæ of the plastic clay beds, we have, ostreæ, cerithiæ, turritellæ, cythereæ, cyclades, &c. along with the teeth of fish, woody fibre passing into coal, occasionally exhibiting unchanged branches and leaves of plants. Fossil bones have been also noticed in it, near Margate. These exuviæ are as irregularly distributed, as the constituent layers of the formation itself; for they appear sometimes in the clay, and sometimes in the sand and pebbles, and are often absent. Thus, for example, we find no vestige of animal or vegetable bodies in any of the strata of the plastic clay at Reading in Berkshire, except in the occasional green sand. The same barrenness is to be observed in the purest plastic clay of Paris, and of the Isle of Wight and Corfe Castle.

Mr. Webster has ascertained that the vegetables in this formation at Newhaven, agree with those found in the Paris basin. One of them was the fruit of the palm-tree, furnishing another instance of the presence in our cold strata of the exotics of a hot climate. This formation visibly discovers itself as superjacent on the London chalk, and is seen to skirt the whole strata of the London clay which

SECTION OF CATSGROVE-HILL, NEAR READING. 295

covers it. For the range and extent of the English plastic clay, see Geol. of Eng. p. 39. The thickness of this formation does not seem to have been ascertained in many points. Chalk has been found beneath its surface near Wormley Bury in Hertfordshire, just beyond the boundary of the clay, at 100 feet below the surface, which appears to be its thickness also near London. Round Woolwich it sometimes amounts to nearly 200 feet. In the Isle of Wight, the apparent thickness is no less than 1100 feet, which is probably from being squeezed up by violent lateral pressure, into a vertical position, a result quite compatible with the loose state. (Fig. Conyb. pl. 2, fig. 6.) In dip, the plastic clay usually conforms to the subjacent chalk. The following downward section of strata at Catsgrove hill near Reading, will give an accurate idea of the tertiary formations, and the plastic clay beds in that locality.

SECTION OF CATSGROVE HILL, NEAR READING.

1. Alluvium of clay, sand and gravel, covered with vegetable mould. 2. Soft loam, of soft sand mixed with flakes of ash-coloured clay,

Feet.

with much iron shot and pyritous nodules in the lower portions, 11 3. Dark red clay partially mottled and mixed with gray clay, 4 4. Light ash coloured clay, mixed with fine sand; used for bricks, 7 5. Fine micaceous sand, laminated, with clay and iron-shot; used

for making tiles,

6. Bed called the White vein. An ash-coloured sand, mixed with a little clay; used for bricks,

5

7. Dark red clay mottled with blue, with a little iron-shot; used for tiles,

6

8. Lowest brick clay, of a light gray colour, mixed with fine sand,

11. Yellowish quartzose sand, with a few green particles,

5

12. Siliceous sand mixed with particles of green earth, containing chalk flints, both angular and rolled, oysters, with many small and nearly cylindrical teeth of fish, from an inch to 1-12th in length,

13. Chalk containing the usual fossils and flints, depth unknown. The red clay of Reading (No. 7.) is quite identical with that of Meudon in France. The oyster shells of No. 12 are remarkably perfect when first laid open, and seem to have suffered no mineralisation; but they soon fall to pieces on exposure to air, having lost their albuminous cement. The angular flints seem to have proceeded from the partial destruction of the immediately subjacent chalk, the upper surface of which in contact with the sand is considerably decomposed to the depth of about a foot. Its fissures and numerous small tubular cavities, probably caused by the decay of organic substances, are filled with granular particles of the green earth and siliceous sand of the incumbent stratum. Many of the subdivisions noted in the above section, do not appear in other places, showing the irregularity of the interior distribution of the strata. Several other instructive sections will be found in the Geology of England, p. 44, et seq.

§ II. LONDON CLAY.

This great argillaceous formation is highly interesting from the variety of its organic remains, both animal and vegetable, and from the conclusions which these warrant, that as their species can be completely identified in but a few instances with recent analogues, the London clay is therefore a much more ancient deposit than the beds imme

SECTION OF LONDON CLAY..

297

diately over it, which do contain specimens of existing species. The same inference follows from the London clay having been raised into a vertical position in the Isle of Wight, while it lay uncovered; for the upper beds now repose horizontally on its truncated upright edges. It must have lain a sufficient time to acquire solidity before being so lifted up, and covered with the newer horizontal deposits.

This formation is composed almost wholly of bluish or blackish clay, of a very tough quality. A few of its strata, indeed, are sometimes of a marly nature, effervescing somewhat briskly when immersed in an acid. It contains a great deal of green earth in the Isle of Wight; considerable beds of sandstone occasionally occur, as the Bognor rocks, the Barns rocks, the Roundgate and Street rocks, and Mixen rocks to the south of Selsea. Beds of stratified limestone appear in the clay cliffs near Harwich in Essex. The following section of a well at Twyford near Acton in Middlesex, will show the interior structure of this formation.

SECTION OF LONDON CLAY.

1. Yellow clay,

2. Lead coloured clay, containing some fossil wood at 188,

and shells at 200 feet from the surface,

170

2

32

The section at the Highgate tunnel, disclosed the following strata.