88 On the Geographical Extent of .[Feb, 



between the plastic clay formation and that of the lime-stone 

 with cerithia, a circumstance which was already suspected by 

 MM. Cuvier and Brongniart. 



The formation of the lime-stone with cerithia, which appears 

 to rest immediately on the chalk between Damerie and Rheims, 

 does not there contain any good building stone. Its beds are, 

 on the contrary, soft and friable as at Grignon, and contain an 

 immense number of shells. It is to this series of hills that the 

 bed^ of fossils belongs, which is known by the name of Courtag- 

 non. This name is given them because M. de Courtagnon was 

 the first to form a large collection of these shells ; they are, how- 

 ever, as abundant at Fleury-la-Riviere and Arthy, and more 

 easily collected than at Courtagnon. We know that these 

 shells are in general" the same as those at Grignon. It would 

 scarcely be expected that two deposits so far distant from each 

 other could have so close a resemblance ; * for, with a few 

 exceptions, they consist of the same species disposed nearly in 

 the same manner. But in respect to the state of preservation, 

 the Courtagnon shells are superior to those of Grignon. They 

 are harder, less bleached, and have a pearly lustre, approaching 

 to that of recent shells. The coarse-grained lime-stone which 

 contains them has a yellowish tinge, more like the colour of 

 ochre than the Grignon beds. In some places it is quite friable, 

 in others the grains are adherent ; and as these different degrees 

 of cohesion are irregularly met with in the same mass, we may 

 suppose that the harder parts have acquired this property from 

 the infiltration of a sort of calcareous gluten. 



Above the lime-stone of a sandy appearance we see beds of 

 compact white lime-stone alternating in the upper parts with 

 greenish marls. I have not found any shells in situ in these 



which are many oysteri cythereae, and a cerilbium similar to that found at St. 

 Marguerite. It is very probable that this deposit is situated between the chalk 

 and the lime-stone with cerithia, since the latter constitutes the sides of the 

 neighbouring hills, and we meet with the chalk about a thousand yards higher up 

 the valley, t 



* The deposit of shells at Grignon owes its celebrity to the number and fine pre- 

 servation of the shells it contains, and the facility of extracting them entire ; but 

 the bed to which this deposit belongs is not wanting in any of the places where 

 even detached portions of the lime-stone with cerithia occur, and it perhaps ex- 

 tends to greater distances than has been suspected. This is not the place to offer 

 proofs of this opinion. Not only within the basin of Paris, but also on its out- 

 skirts, we shall find the Grignon bed wherever we penetrate through the lime-stone 

 containing cerithia. That bed, it is true, will not at first sight strike the observer, 

 except when it is friable ; and the shells are to be obtained entire from it as at 

 Perne; at Vivray, near Liancourt ; at Mount Ouen and Mount Javoult, near 

 Gisors 5 at Septeuil, to the south-west of Mantes, &c. — French Editor. 



+ I visited this spot in August, 1813. It evidently forms part of the formation 

 of the lime-stone with cerithia, and it is composed, as the author states, of plastic 

 clay, pure in the inferior parts, and above mixed with sand lignites, pyrites, and 

 the shells above noticed, also oysters in great abundance. This disposition occurs 

 in a precisely similar way, and with the same shells, above the chalk at Marly, 

 near Paris; at Vauxbuin,nearSoissone, &c— -French Editor. 



