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



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 w^ith 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,* 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 consoli- 

 dated before the deposition of the clay which covers it : a circum- 

 stance 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 be- 

 tween the chalk and the plastic clay. This circumstance, together 

 with the irregular form of the bottom of their basin, seems to indi- 

 cate a considerable action of water upon its surface, so as to render 

 it now difficult to ascertain what might have been the last deposi- 



* A singular circumstance is mentioned respecting the chalk of France io 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 vege- 

 tables, on which are pastured vast quantities of sheep. The chalk of France is said 

 to contain 11 per cent, of magnesia, to which the barrenness may be owing. 



