282 M. D'Halloy on the Geographical Extent [April, 



side of Champagne, the other beds of the older chalk are not 

 entirely wanting. We find the same tuffeau containing chlorite, 

 particularly at Autry, in the department of the Ardennes ; and it 

 is a remarkable fact that the older chalk of Champagne differs 

 from that of the other parts of the basin in the very same cha- 

 racter which is peculiar to the true chalk of this region. 



To the north of Champagne the limits of the chalk extend too 

 far from the Paris basin to be described in this memoir ; but near 

 to this basin, and even within a short distance of Paris, is a very 

 small region, in which we see not only the older chalk but also the 

 lias, formation, or at least its later members. This canton, com- 

 monly called the pays de Bray, is situated on the confines of the 

 departments of the Oise, the Lower Seine, and the Eure. It 

 resembles an island, and may be considered as the summit of 

 a mountain buried by the great deposit of chalk. 



The parts of this deposit nearest to the pays de Bray have the 

 characters of the older chalk. Between Argeuil and St. Sansom 

 we see the substance penetrated by grains of chlorite of a blackish 

 green colour in great quantity, and another modification of a 

 coarse texture, which passes into the state of a sandy marl, and 

 contains, instead of real flints, nodules of greyish calcareous 

 sandstones. It appears that the sands and marly clay which 

 form the particular character of the country come out from 

 underneath this coarse grained chalk. I say only that it appears 

 so, because the disintegrated nature of these deposits, and the 

 labours of agriculture, conceal the order of superposition, and 

 because also the vicinity of the sands and plastic clay of the 

 limestone with cerithia of the Paris basin might offer the suppo- 

 sition that those beds extended to the pays de Bray. The 

 presence of the lias, however, in the central parts of this region, 

 at Menerval, Cuy-Saint-Fiacre, &c. leaves no doubt that at least 

 the greater part of the clays of this canton belong to the forma- 

 tion intermediate between the chalk and the lias. 



This last mentioned limestone (the lias), generally of a yel- 

 lowish white or greyish yellow colour, is remarkable for its 

 hardness, for the abundance of spathose matter it contains, and 

 particularly for the great number of small oysters found in it, 

 although there are some beds of it quite compact, and without 

 any organic remains. It is difficult to decide on the position of 

 the principal mass of this with respect to the clay ; but we may 

 see distinctly that there are beds of the two formations which 

 alternate with each other. 



These characters are sufficient to point out in this limestone a 

 small formation, very remarkable for the constancy of its mine- 

 ralogical and geological characters in countries situated at consi- 

 derable distances from each other, such as Berry, Lorraine, the 

 Boulonais,* the coast of Calvados, See. This limestone is distin- 



• The greater part of the Boulonais is formed of the same beds as those of 

 Bray. It is in the northern portion oivly that we see the successive appearance 



