254; M. Desnoyers on the Chalk of the Cotentin. 



■ Reduced to the beds which alone belong to the baculite 

 limestone, this rock appears to me to be as easily determinable 

 by its mineral structure and organic I'emains as any known 

 formation. Always, however, rejecting the solid calcareous 

 beds, containing milliolites, and those with pisolite concretions, 

 which under the name o'i fahlun have been confounded with 

 the friable beds of the calcaire a Cerites, and perhaps a chlo- 

 ritic limestone with and without nummulites. 



Thus limited, this formation is composed of a calcareous 

 rock, (the true baculite limestone,) commonly white or yellow, 

 solid, heavy, homogeneous in appearance, and nearly as com- 

 pact as the hardest of the Jura limestones *: it often presents 

 spathose laminae, and its texture is sometimes slightly granu- 

 lar. When the spathose solution has not uniformly penetrated 

 the mass, which most frequently occurs, the cause of this gra- 

 nular and false oolitic appearance is discovered in numerous 

 rounded fragments of corals and shells, which sometimes form 

 irregular nests, the destruction of which, joined to the absence 

 of tiie spathose cement, often produces cellular cavities in the 

 midst of the most compact beds. This first and most common 

 state completely represents the coarse chalk of Saintonge and 

 Perigord. A penetration of siliceous matter often renders this 

 structure complicated, by producing a complete mixture with 

 the calcareous paste, a sort of siliceo-limestone ; or forms in 

 the upper part, as at Freville, isolated nodules of pale chert 

 covered by an earthy white calcareous substance, resembling 

 those of Maestricht,and the ancient chalk. Traces of this double 

 precipitate may be observed in the thin and irregular veins 

 of these two substances, which cross and vertically traverse the 

 strata. 



When there is a mixture of the debris of more ancient rocks, 

 especially of sandstone and quartz, the beds take an arenaceous 

 structure, become a kind of sandstone, always, however, hard- 

 ened by the same calcareous cement : this is to be observed at 

 Bonne Ville and Orglandes in the upper beds. When on the 

 contrary there is no mixture of the paste or fragments of fo- 

 reign substances, beds alone formed of very attenuated marine 

 bodies, or small calcareous fragments, sometimes white and 

 staininf^, {fahlun craymx of Nehou,) commonly united, varying 

 in size from the finest powder to that of a nut, then constitute 

 incoherent marls, worked for agricultural purposes, as are 

 those of the calcaire grassier, which have been confounded 

 with them. The latter can however be distinguished from 

 them by the presence of Miliolites, Rotalites, and other mi- 



* Equivalents of the Oolites. — Trans. 



croscopical 



