Life and rock: a collection of zoological and geological essays / by R. Lydekker.

  • Richard Lydekker
Date:
1894
Catalogue details

Licence: Public Domain Mark

Credit: Life and rock: a collection of zoological and geological essays / by R. Lydekker. Source: Wellcome Collection.

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    explain the formation of the numerous layers of nodular flints throughout the body of the chalk. There is, however, an explanation, which will readily account for all the features presented by the chalk-flints, and requires the aid of no foreign factors in the process. This explanation is based on the phenomenon known as segregation. Now segregation is the tendency presented by a small quantity of one substance, when diffused through a much larger quantity of another substance, to collect together in nodules or strings, which generally accumulate either around some fragment of their own nature or some foreign body—especially an organic one— as a nucleus. We have well-known examples of this segregating process in the huge lenticular calcareous masses termed “ septaria,” found in the London and Kimeridge clays, and also in the iron-nodules of other formations. Premising that soluble silica has a peculiar affinity not only for any kind of silica, but likewise for gelatinous organic substances (both of which were presented by the sponges of the cretaceous seas), it will be obvious that if we can only satisfy ourselves that at the time of its deposition the chalk contained diffused among its substance a sufficient amount of soluble silica, we shall at once be able to account for the formation of its flint. Now, as we pass upwards in the cretaceous system from the lower green- sand to the upper chalk, we find a gradual change from a completelv siliceous to a calcareous rock. Moreover, while in the gault, upper greensand, and lower chalk (in which in the south of England there are no flints) there is a large but decreasing amount of soluble silica, varying from 46 per cent, in the upper greensand to 31 per cent. in the chalk-marl, when we reach the upper chalk with fliuts such soluble silica is reduced to a mere trace. As it is at the base of the white chalk that the sponges attain their greatest development, and as it is also here that flints first commence, the disappearance of the soluble silica may be safely attributed to its segregation by means of the sponges and other bodies, and its conversion into flint. Prof. Prestwich, writing on this subject, observes that in presence of the siliceous spicules of the cretaceous
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