Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/183

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173
Mr. Webster on the Strata lying over the Chalk.
  1. Chalk formation.[1]
  2. The lowest marine formation over the shell, including the plastic clay and sand, together with London clay.
  3. The lowest freshwater formation.
  4. The upper marine formation.
  5. The upper-freshwater formation.
  6. Alluvium.

Fortunately the complete series of these alternations may be seen at one place in the Isle of Wight, which leaves us no room to doubt their superposition; and when their characters have been studied in this spot, it is more easy to become acquainted with them in other places.

§ 1. Chalk Formation.

The south-east coast of England and that of the Isle of Wight afford us many excellent opportunities of examining the chalk. In numerous natural sections formed by cliffs, as well as in chalk pits, I have observed it as distinguished into at least three strata, each of which has peculiar and distinctive characters.

The lower stratum is more or less argillaceous, and constitutes what is called the chalk marl. Together with the other strata it frequently forms cliffs of considerable height, and though differing little from them in colour is easily distinguished by its constantly shivering with the frost, which always pulverizes a mass of it when exposed to the air for a few months; whereas the others resist the weather in a much greater degree, and are often even employed as a material for building.

  1. In using the word formation I have followed the example of M. M. Cuvier and Brongniart, who have employed it to express an assemblage of beds of the same nature, or of a different nature but formed during the same epoch.