Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/185

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175
Mr. Webster on the Strata lying over the Chalk.

The workmen take advantage of this circumstance in quarrying the chalk.

The description given by M. M. Cuvier and Brongniart of the chalk of France appears to agree generally with that detailed above. They mention nodules of a harder chalk as occurring in layers in a softer. They observe that Werner has enumerated grey and brown among the colours belonging to chalk: and that in a great part of Champagne the chalk contains no flint,[1] but it does not appear whether these varieties of chalk are on the continent confined to particular strata.

M. M. Cuvier and Brongniart remark that the chalk which forms the bottom of the Parisian basin, appears to have been consolidated before the deposition of the clay which covers it: a circumstance which they inferred from there being no transition of these into each other. A breccia is described as occupying the lower part of the basin at Meudon, composed of water-worn fragments of solid chalk cemented by a paste of clay, and situated between the chalk and the plastic clay. This circumstance, together with the irregular form of the bottom of their basin, seems to indicate a considerable action of water upon its surface, so as to render it now difficult to ascertain what might have been the last deposi-

  1. A singular circumstance is mentioned respecting the chalk of France to which we have nothing analogous in this country. In Champagne there are immense plains of chalk absolutely deprived of vegetation, except where patches of the calcaire grossier occur as islands or oases in the midst of these deserts. And M. Cuvier observes that many parts of this tract have not perhaps been visited for ages by any living being: no motive existing that could induce any to wander there. In England every part of our chalk is completely covered by vegetable soil, which although very thin on many parts of our downs or smooth hills, yet affords support to peculiar grasses and other vegetables, on which are pastured vast quantities of sheep. The chalk of Franco is said to contain 11 per cent. of magnesia, to which the barrenness may be owing.