146 M. Charbaut on the 



nites, planulites, and nautilites, disposed parallel to the beds, 

 forming a kind of nodule easily separated from the marl. 



A bed of oolite, composed of small grains of oxide of iron 

 cemented by marly limestone, regularly rests on this enor- 

 mous mass of marl beds ; the grains are yellow and red, 

 their form resembles that of gunpowder; the cement is often 

 ferruginous, its colour is then reddish brown; at other times 

 it is grey, like that of the inferior marls. This oolite has 

 not much tenacity, and the action of the air reduces it to 

 powder, but it is more durable in proportion to the oxide of 

 iron it contains. Its richness is sometimes so great that it is 

 worked as an iron ore.* 



The common thickness of this bed is two metres [about 6 

 feet 6 in.] ; it varies to five [about 16 f. 6 in.]. 



Among the numerous shells it contains, I may notice 

 planulites, belemnites, ammonites, terebratulae, pectens, 

 trochi, donaces, nautili, and entrochi ; the two first are the 

 most abundant. + 



Crystals of sulphate of strontian occur in the geodes of 

 calcareous spar existing sometimes in the middle of these 

 shells, especially in the last whorls of the planulites. 



Above this ferruginous bed some beds occur still separated 

 by marl strata, forming the passage from grey compact marly 

 limestone into yellowish siliceous limestone ; the marl after- 

 wards disappears entirely ; the passage is even sudden where 

 the ferruginous bed is thick. Here commeuces that long 

 suite of oolitic granular or compact limestone, composing the 

 escarpment of the first platform of the Jura. 



From the idea attached in mineralogy to the denomination 

 of granular limestone, it may appear surprising to see this 

 rock among the secondary formations ; it is nevertheless 

 certain that limestone beds, composed of small lamellar 



* This is evidently the inferior oolite. (Trans.) 



+ For a detailed list of fossil shells contained in the English inferior 

 oolite, consult Messrs. Conybeare and Phillips' Outlines of the Geology 

 of England and Wales, p. 239, &c. (Trans.) 



