1818.] the Strata of the Environs of Paris. 95 



tongue of the fresh-water lime-stone that is formed along the 

 river from Gien to Cosne. Thus the Parisian strata which, 

 through so large a space cover the chalk, and are afterwards 

 confounded with it on the same level, at length terminate in a 

 valley lower than the chalk hills which surround it. 



Taking up the limits of the Paris basin from this south- 

 eastern extremity, we shall see that they are not much more 

 clearly to be traced along the canal of Briare than in the neigh- 

 bourhood of the Loire, but that they reassume their character to 

 the north of Montargis, and particularly about Nemours, where 

 the stand-stones without shells reappear. 



Such is the extent of the Paris basin considered as a whole ; 

 for the different formations composing it are prolonged by rami- 

 fications of greater or less extent beyond these limits. The 

 fresh-water beds in particular extend to considerable distances. 

 I have already shown that they extended along the banks of the 

 Loire and the Allier, and to the elevated plains of the Berry.* 

 We know, moreover, that it forms another basin of considerable 

 extent in the Dimagne of Auvergne. We again find them, 

 though in smaller quantity, towards Tours and Le Mans. 



The second marine formation forms also some isolated deposits 

 beyond the limits of the basin ; at least I think I can refer to 

 this formation the deposits of white sand-stone found in the 

 plains of Picardie, and as far as the departments of the north 

 end of Jemappe, as well as those which are found in the chalk 

 country between the Seine and the Loing. 



The plastic clay, in the form of black pyritous earths, is also 

 found in detached parts on the chalk to the north of the Seine ; 

 and it appears that there are other clay deposits at a considerable 

 distance from the basin of Paris, which, although resting on 

 •strata older than the chalk, may be referred to this formation. 



The lime-stone with cerithia, properly so called, appears on 

 the contrary to be confined to the limits of the Paris basin ; and 

 this is a very remarkable circumstance in the history of this 

 formation ; at least I have not yet observed it elsewhere, 

 although I have made a point of examining the places where, 

 from mineralogical resemblances, the existence of this bed was 

 supposed, and where I found the calcareous formations older 

 than the chalk, properly so called, having the coarse textnrr- 

 and yellowish colour of the building stone of Paris. 



* Journal t\c Min s, vol. xv\i ; . p. 43. 



