244 M. Brongniart on the 



§ III. Chalk of the environs of Perigueux and Bayonne. 



The chalk has been observed to end, on the south of 

 Paris, at the southern confines of the department of the 

 Indre. It really ceases there, for the rocks which succeed 

 it are inferior to it; but when, proceeding to the S.W. these 

 older rocks are passed over, the craie tufau is again found in 

 the department of the Dordogne, in the environs of Peri- 

 gueux, and especially on the west of that town. The high 

 and steep hills which border the river Lillefrom Perigueux 

 to the place named La Massoulie, are composed of sandy 

 and often micaceous grey chalk, i. e. of craie tufau which 

 occurs in an immense mass without any distinct strata 

 throughout the greater part of its extent ; stratification is 

 however indicated by seams of black flints which divide it 

 into numerous beds. These flints, which belong rather to 

 the variety we have named silex comes (splintery horn- 

 stone) than to true flints, are, as we have elsewhere stated, 

 characteristic of the craie tufau^ into which they seem to 

 pass. 



The shells which this chalk contains are numerous in some 

 places, and although I observed this hill very rapidly,* I 

 was able to collect the following species : 



List of some shells of the craie tufau in the environs of 

 Perigueux. 



Nautilus Pseudopompilius. Schlotheim. 



rr, 1 (Indeterminate inte- 



Trochus ■{ 



I nor casts. 



The individuals are 

 smaller, & resemble 



Ostrea vesicularis Lam. (pi. iii. fig. 5.).| those of Luzarche, & 



the latter the small in- 

 ^dividuals of Meudon. 

 Gryphaea auricularis, A. Br. (pi. vi. fig. 9.) 



{Although I have only- 

 seen the interior sur- 

 dTno^t'dourtX^c". 

 rectness of this deter- 

 mination. 



* I visited this place in 1808, and then observed the characters of the 

 craie tufau formation. 



