Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/211

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201
Mr. Webster on the Strata lying over the Chalk.

In the stratum of blue clay, next to the deep red clay that adjoins the chalk of Alum bay, there are septaria with fossil shells, among which I found cytheræ and turritellæ; the rest were too much mutilated. Hence it would appear, that no fossil organic bodies are disseminated through the pure plastic clays of either basins, but that they are to be found in such beds of this clay as are impure.

A species of imperfect coal also occurs in the lower strata of the Paris basin, and is probably analogous to that of Corfe, Alum bay, and Newhaven.

The French sands are of a great variety of colours. The sands of Alum bay may correspond to that between their chalk and the plastic clay, which is described as very pure, though often coloured red or blueish grey. It is refractory, and often in very large grains.

In the lower marine formation of the Paris basin, the most remarkable and best characterized stratum is that of the coarse shelly limestone or calcaire grossier. This is generally separated from the chalk by the clay and the sand just described; or, when that is wanting, it rests immediately upon the chalk. It is sometimes, also, separated from the clay by a bed of sand, more or less thick, in which no shells have been found.

The description given by the French of this calcareous formation is extremely precise, and corresponds to its importance. From this it appears to be composed of alternate beds of limestone, more or less hard, of argillaceous marl, of laminated clay, in very thin beds, and of calcareous marl. These are of great extent, and preserve constantly the same order of superposition, although all the beds are not continuous. Each bed is also characterized by its peculiar fossils. The lower are often more sandy than calcareous, and when solid they fall to pieces on exposure to the air. They frequently contain a considerable quantity of green earth,

Vol. II.2 c