Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/184

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Mr. Webster on the Strata lying over the Chalk.

This chalk marl is never quite so white as chalk, having generally a tinge of yellow, and sometimes of grey and brown. It also contains nodules and beds of a more indurated marl, which is usually called the grey chalk from its dark colour, which varies from a light to a dark grey and brownish grey. Like all argillaceous limestones it possesses in a considerable degree the property of setting under water when calcined and made into mortar; and it has been used with great success for this purpose in building the London Docks. The part easily reducible to the pulverulent state by the moisture and frost is a most valuable manure when employed judiciously in certain soils. This stratum contains no flints.

The middle and upper strata consist of chalk of extreme whiteness and purity, and are chiefly distinguished from each other by the upper one containing layers of flint nodules which do not occur in the lower. The chalk without flints is most frequently somewhat harder than that with flints, and hence they are sometimes distinguished as the hard and soft chalk; but from some observations which I have made in Dorsetshire, it will appear that the hardness, or degree of induration, does not always mark a particular bed, the flint chalk being in some places much harder than that without flints in others.

On the subject of the nodules and laminæ of flint in the upper chalk, the observations of Sir Henry Englefield have thrown great light, and will be mentioned in his intended work on the Isle of Wight.

The several beds of these chalk strata, which vary in thickness from a few inches to several feet, are frequently separated from each other not only by layers of flint nodules but frequently also by a marl containing a considerable proportion of clay, and this substance also sometimes fills up the diagonal fissures which cross the strata.