Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/244

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Mr. Webster on the Strata lying over the Chalk.

buildings constructed with flints that have resisted the agency of the atmosphere for many centuries without undergoing the smallest visible alteration, or having become whitened in the least degree. It is however well known to be liable to decomposition under certain circumstances; and it must be allowed that the combined action of moisture and various decomposing causes whilst a mineral remains buried in the soil, may produce effects which we can scarcely estimate.

Upon the whole, however, it appears to be extremely improbable, that any species of imaginable action could have converted a fragment of chalk flint into a substance so very different as one of the rounded concentric pebbles of the London gravel.

To assist us in endeavouring to obtain a just idea with respect to the origin of the different accumulations which are found in our gravel, it may be useful to consider the various changes which have taken place in our upper strata. Of these, although ignorant of their causes or their extent, we yet perceive the traces written in characters sufficiently legible.

Although the chalk has been originally formed at the bottom of the ocean, yet from some change which took place either in the level of the sea, or in the state of the strata, part of it probably at an early period has been above, and part below, the surface of the water, as at present; and this before the deposition of those strata which we now see immediately superimposed upon it.

From that date, and by the same cause as we see still producing this effect, did probably the formation of rounded flint pebbles begin. The chalk itself, being easily acted upon by the waves, became disintegrated, while the siliceous nodules were better able to resist this abrasion, though yet liable to be broken and rounded by friction against each other.

This effect takes place chiefly upon the margin of the sea. In