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CONVERSATIONS ON GEOLOGY.

CONVERSATION XXVI.

Plastic Clay formation-Puddingstone-Oyster-beds-Murex-Pla

norbis-Mya.

MAT.-From my recollection of natural appearances, I have been vainly endeavouring to guess what substance will next come under our observation.

MRS. L.-We have now to consider the last division of regular Strata, containing all those between the Chalk formation, and the Alluvial deposits. These consist of various beds of Sand, Marle, and Clay, with imperfectly consolidated Limestone. In England they occupy two extensive tracts, each bounded by Chalk hills, except where broken by the sea. And because the Chalk, dipping very slightly on all sides, forms on the large scale, a sort of concave space, in which the above Strata are deposited, the tracts have been denominated basins, and distinguished by the names of the London and Isle of Wight basins.

ANNE. I have often heard these terms, and am glad to understand them.

MRS. L.-The extent of the London Basin is from the Chalk Hills south of Flamborough Head, to the north-west angle of the Isle of Thanet: running westward to Wiltshire, and eastward to the German Ocean. The boundaries of the Isle of Wight Basin are considered to be near to Winchester on the north, Carisbrook on the south, on the east, Brighton, and on the west, Dorchester: every where surrounded by Chalk, except where broken by the channel that separates the Isle of Wight from the mainland. The beds occupying these basins we have now to consider. On many accounts they are very remarkable; especially in that extraordinary phænomenon observed in the Isle of Wight Basin,

the alternating of beds, containing, separately, the remains of shells, peculiar to fresh and salt-water; as if each had successively, and by turns overflowed the space, and deposited multitudes of its inhabitants, without ever intermixing them.

ANNE. That is a curious fact: but does it occur anywhere else.

MRS. L.-In many parts, but always under similar circumstances. The extensive tract called the Basin of Paris exhibits close conformity to that of the Isle of Wight, both in the fresh-water and marine deposits; the shells of each district being in exact conformity. In many other parts of the Continent, similar Basins are observed; filled in a manner exactly similar, while in others, there is a degree of variety in the shells. We shall speak again of this phænomenon in its place, but not being the lowest of the series, it is not the first to come under our observation.

MAT. The first of this series will be of course the bed reposing immediately upon the Chalk.

MRS. L.-This lowest bed is called by geologists the Plastic Clay Formation, as distinguished from the London Clay, which is above it. This formation is composed of an indefinite number of Sand, Clay, and Pebble-beds, irregularly alternating. The Plastic Clay of the Paris Basin is unctuous and tenacious; varies much in colour, being white, grey, yellow, grey and red, or almost pure red. This clay is employed according to its qualities, in making coarser or finer pottery and porcelain. In England, this formation consists of beds of sand, clay, and pebbles. The clays are of various colours and degrees of purity, and are sometimes laminated: thus we have Fire Clay, Brick Clay, Pipe Clay, and Potters' Clay. These beds contain also Fullers' Earth; and beds of Wood Coal imperfectly formed; retaining much of the fibrous appearance of the wood, and often the impressions of the leaves and branches. Among them has been found the fruit of the Palm tree.

ANNE.-How is that accounted for in these northern

regions?

MRS. L.-It is one of many instances of the fruit of warmer climates being found fossilized, where now they will not grow. There is little reason to suppose our climates were always as they are now. Great and inscrutable convulsions have changed the face of nature and the character of its productions, since these remains were deposited in the earth.

MAT.-Have not these changes been attributed to an alteration in the position of the earth's axis?

MRS. L.-This has been the conjecture of some speculations, and we can by no means pronounce it impossible. Others rather attribute the changes of temperature to the much greater activity of volcanic fires at an earlier period, of the extent of which there is ample evidence, as we shall see hereafter.

MAT. You have yet made little mention of volcanic productions and effects.

MRS. L.-It is better first to go through the regular Strata; and speak afterwards separately of the accidental productions.

ANNE. These hidden things of the earth, as they become disclosed to us by study, increase the wonders of creation, and convict us of more ignorance than knowledge of the works of God, after all our researches. Where can these formations best be examined?

MRS. L.-Nowhere better than in the neighbourhood of London, near Reading, or in the Isle of Wight. They are very full of organic remains. The prevailing remains near Reading are oyster-shells, found in the lower beds. Teeth, apparently of sharks, are mingled in the mass. Near Plumstead, in Kent, many varieties of shells are found in good preservation, but extremely brittle, and their surfaces indented with the impression, of the minute sand. Near Bromley, stone is found formed of oyster-shells still adhering to the pebbles to which they were attached, the whole being formed by a

calcareous cement, into a coarse shell limestone. The shells are so affixed to the pebbles as to give reason to conclude they commenced their growth upon them, and lived and died there undisturbed. At Ewell in Surrey, the Clay of this formation is worked as a Fire Clay. Beds of Clay and Sand, mixed in various proportions, are manufactured into tiles and bricks for ovens, furnaces, &c. where great heat is to be withstood. Near Seaforth in Sussex, immediately against the Chalk of Beachy Head, the Breccia of the lowest bed forms a sort of Pudding-stone, composed of Sand and Chalk Flints.

ANNE. I have often heard mention of Pudding-stone, but we have not seen a specimen, I think.

MRS. L.-It is so called, I suppose, from its appearance, as resembling pudding set with plumbs. It is made up of different materials congregated together, and inclosing rounded pebbles which give it that appearance. It is therefore properly a Breccia. Fig. 1.-Plate 25, will give you an idea of it.

In Dorsetshire, Potters' Clay is found in beds between loose sands. It feels greasy and smooth, and varies from ash-grey to blue. This Clay is sent to Staffordshire, where it is mixed with ground Flints, and employed in the finer kinds of pottery. Near Poole, the white Pipe Clay is quarried. It is the intermixture and various colouring of these Sands and Clays that give to the cliffs of Alum Bay, in the Isle of Wight, so beautiful an appearance. They have been compared to the delicate striping of a Tulip, and are of colours scarcely less bright, when the cliff is newly cut. Fig. 2. is a representation of this Plastic Clay formation, as there exhibited.

MAT.-I am surprised to observe their beds drawn in a perpendicular position-I thought we were now among the Floetz, or Flat formation.

MRS. L. We are so-and these beds are generally nearly horizontal, and form low grounds. But in this instance, as in the Chalk, some disruption has evidently