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GENERAL OBSERVATIONS ON THE STRATA.

Whenever the stratum, originally deposited above the chalkwith flints, remains, it is either clay or sand. The sand is ingeneral extremely pure, and is of considerable thickness andextent. The clay connected with this sand is frequently fit forthe potter, and hence has been called the plastic clay: it iseither very white, grey, deep red, or mottled red and white. AtNewliaven in Sussex, and other parts of the coast betweenBrighthelmstone and Beachy head, as also at Purfleet andWoolwich on the banks of the Thames , a bed of sand twenty orthirty feet in thickness, lies immediately upon the chalk, andseparates it from the superincumbent clay. At Reading inBerkshire , the plastic clay is separated from the chalk by a bedof green sand only two feet thick, containing fossil oyster shells.In the trough of Poole, and particularly near Corfe castle, theplastic clay over the chalk is of great extent and purity, and lies,for the most part, in basins in the sand stratum.

The formation above the sand and plastic clay, in the south-east of England, has been called the London clay. It is lesspure than the former; and is characterised by its bluish blackcolour, by its frequently containing green earth, and nodules ofindurated marl called septaria, and by its abundant and beautifulorganic remains. This extensive and interesting stratum is wellknown, from the natural sections of it furnished by the cliffs ofHordwell and Stubbington in Hampshire , the Isle of Shepey,and the coast of Essex; as also by the numerous wells and otherartificial sinkings in the neighbourhood of London , Portsmouth,and other places.

The London clay sometimes forms the surface or soil, but ismore frequently covered by gravel, sand, or other mixtures