the Environs of Paris. S5 



the common chalk with dark flints, from which it is some- 

 times not sensibly distinguished ; that, on the contrary, the 

 argillaceous rocks are the first of the formation, and there is 

 even a part of them which belongs rather to the old hori- 

 lontal limestone, than to the older chalk. , •' 



Fossils are very abundant in these different systems ; some, 

 such as the echinites, are the same as those in common 

 chalk ; others, such as ammonites, resemble those iu alpine 

 limestone ; there are those, such as belemnites, terebratulae, 

 &c. that are common to them, the chalk, and alpine lime- 

 stone ; those that may be cited as characteristic, as much 

 for their abundance in these systems, as for their rarity, and 

 even perhaps their total absence in other formations, are the 

 orbicular gryphite, and a large shell referred to the genus 

 spondylus. 



The immense chalky basin which extends as a gulf into 

 the north west of France, presents these different modifica- 

 tions of the older chalk throughout its contour, except towards 

 the English channel, where the true chalk extends to the 

 sea-shore. Every where else the four systems I have noticed 

 may be recognized, with this difference, that one or two of 

 the systems, being often considerably developed, mask 

 the others, scarcely existing, and alone determine the cha- 

 racter of the country. It is thus, that calcareous, sandy, 

 and clayey countries, occur within the range of this forma- 

 tion. There is also this general difference, that this rock 

 forms but a narrow band on the east, from the Oise to the 

 Yonne, whilst on the south, and especially on the south-west, 

 it occupies a considerable space. This appears to arise from 

 the beds, notwithstanding their horizontal appearance, 

 having an inclination on the eastern part proportioned to 

 the rapidity with which the formation on which they rest, 

 rises ; hence it follows, that the same system cannot long 

 appear on the surface. In the south-west where the surface 

 is lower, the beds, being on the contrary more perfectly 

 horizontal, extend over a greater surface. 



The deposit of the Parisian formations is not placed ex- 

 actly in the middle of the great chalk basin, for the southern 



