Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/198

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Mr. Webster on the Strata lying over the Chalk.

persed through it in smaller quantities. Of these the chief is sulphuret of iron, which is frequently the mineralizing matter both of the vegetable and animal remains included in the blue clay.

Selenite is also very abundant; and sulphat of iron frequently effloresces when the clay is exposed to the air, from the decomposition of the pyrites contained in it. Phosphat of iron is also sometimes found.

On account of these salts, the water which is contained in, or which passes through this stratum, is not fit for domestic purposes.[1] Wells are therefore generally sunk entirely through it to the sand below.

In the country round London, this sand which belongs to the plastic clay, is the great reservoir of water, which generally bursts out with great violence when broken into.

Sulphat of magnesia has long been known in the springs at Epsom, which has given its name to this salt. Its origin however does not appear to have been clearly ascertained, although from its situation it may be supposed to belong to some of the beds above the chalk. I derive from Mr. Tennant the information, that the London clay abounds in Epsom salts. The bricks of old buildings in London, after fine dry weather, are covered with an efflorescence of this salt. This may be seen in the walls of the Temple.

In the Isle of Wight, as well as the London basin, this stratum occupies the same situation, and does not appear to differ materially in each of these places. In Alum bay it is seen forming the most northerly of the vertical strata. On the opposite shore of the Solent it occurs in a horizontal position. Sections of it are formed by the cliffs between Lymington and Poole. Of these, Hordwell cliff,

  1. Hence the country on the northern side of London is so thinly inhabited.