Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/210

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Mr. Webster on the Strata lying over the Chalk.

The counties of Suffolk and Norfolk have been little explored, and still present a wide field for research. They will I have no doubt amply reward those members of the Society who have opportunities of examining them.

I shall now proceed to point out those particulars in which I have observed the marine formation which we have been examining to be analogous to some of the strata in the basin of Paris.

The plastic clay of the Paris basin is described as sometimes consisting of two beds separated by a bed of sand, The lower bed is properly the plastic clay. It is unctuous, tenacious, containing some siliceous but no calcareous matter; and absolutely refractory in the fire when it has not too great a portion of iron. It varies much in colour, being very white, grey, yellow, grey mixed with red, and almost pure red. This clay is employed, according to its quality, in making coarse or fine pottery and porcelain.

The corresponding beds of clay in this country agree well with this description. The clays of Dorsetshire are extremely pure, and are much employed in the potteries of Staffordshire and other parts of England. In Alum bay we see clays of all the colours just described. Some of them appear very promising, so much so that the late Mr. Wedgewood had pits opened there; but although extremely refractory, they were found upon trial not to burn sufficiently white for the purposes required. The deep red clays we have seen are very common in many parts of the country over the chalk.

No fossil shells have been found in this bed of clay in the French basin, nor in the clay pits of Dorsetshire; nor are there any in the pure clays of Alum bay.

The uppermost bed of potters' clay in France is sandy, blackish, and contains sometimes fossil shells of the genera cytherea and turritella, and the sand is often coloured red or bluish grey.