88 Mr. De la Beche on theGeographical Distribution of Organic 



is, however, not nearly so well pronounced ; the beds corre- 

 sponding with the Oxfoi-d and Kimmeridge clays are but 

 rarely observed in this part of France, and appear to be re- 

 placed by marly limestone : this is most commonly the case 

 between the middle and superior systems. 



" The numerous subdivisions noticed by the English geolo- 

 gists are but very imperfectly seen in the secondary basin under 

 consideration ; there are nevertheless some sufficiently constant. 



" The lower is the only part of the oolite which appears on 

 the eastern extremity of the basin ; it forms a considerable 

 mass on the N. of the department of the Herault, which ad- 

 vances into the sea near Montpellier and Cette. 



" The oolite formations are greatly developed on the W. of 

 the basin ; from Cahors to the ocean they form a chain with a 

 mean breadth of twelve leagues : they are more than twenty- 

 five between the mountains of Limousin and La Vendee. 



" Between the two extreme points we have named, the three 

 systems of the oolite may nearly always be seen ; the inferior 

 frequently resting on the marls and limestone which we have 

 referred to the lias. 



" In some localities (Milhau, near Villefranche, &c.) the 

 inferior system presents micaceous marls containing Gryphaea 

 cymbium, belemnites, &c. which may be compared to the sand 

 of the inferior oolite : we also find sublamellar limestones, 

 compact limestones containing beds of oolitic iron, and beds 

 of a white oolite, furnishing excellent building-stone ; the lat- 

 ter beds, well developed only at Mauriac, in the Aveyron, re- 

 present the great or Bath oolite. They are associated with 

 polypifers as at Caen. In other parts of the basin, principally 

 in the east, this system is composed of compact limestones of 

 a yellowish-graj' colour, containing an abundance of silex, 

 (environs of Nontron, Poitiers, &c.) and beds of slightly oolitic 

 limestone. The beds of compact earthy limestone containing 

 many ammonites and terebratidae appear to form the upper 

 part of this system : by comparing these fossils with those 

 found in the cornbrash in England, we are led to assimilate the 

 rocks. The lower system is the thickest ; it forms by itself 

 more than three quarters of the whole depth ; it occupies more 

 than twelve leagues of the fifteen or sixteen which this forma- 

 tion covers between the Sables d'Olonne and Rochefort. 



" The middle oolite system is in a great measure composed 

 of marly limestone beds : yet in many places (Marthon, forest of 

 La Braconne, Pointe de Duche, Pointe d'Angoulin, &c.) con- 

 siderable masses of polypifers are associated in it with thick 

 beds of irregular and earthy oolite. The prodigious abundance 

 of polypifers, the nature of the oolite, and the occurrence of 



many 



