1818.] the Strata of the Environs of Paris. 91 



the marls of fresh-water formation are employed with advantage 

 to render the chalky lands fit for the growth of the vine. 



Lastly, the elevated plains of the formation of the buhr-stones 

 are generally covered with forests or heaths, which, by the large 

 rocks that are found scattered over the surface, recal to mind 

 primitive countries. 



The limits of the Paris beds in the part of the basin situated 

 to the south of the Seine do not long maintain that decided 

 physical demarcation which characterises those of the north. 

 This arises from the changes of the geological constitution of 

 the soil — changes which I am about to point out in a general 

 manner. 



General Considerations on the Disposition of the Strata of the 

 Paris Basin : their Division into four principal Formations. 



Although we are in the habit of considering the different 

 formations of the Paris basin as being placed horizontally over 

 each other, and they appear to be actually so in the central 

 parts of the basin, we should observe, that in taking a general 

 view of the strata, they have a decided inclination towards the 

 south ; and, to a certain extent, resemble a set of wedges placed 

 like the tiles of a roof, with this peculiar circumstance, that the 

 lowest wedge attains the greatest height. 



1. First murine Formation. — This first series (etage) of beds is. 

 as is well known, the lime-stone with cerithia. Its most elevated 

 point appears to be at the northern extremity of the basin, in the 

 hills of Laon, which are about 300 metres above the sea,* and 

 where this lime-stone is not covered by any of the other for- 

 mations. From the summits of these hills the level of this stra- 

 tum gradually falls. It dips under the other deposits and 

 disappears entirely on the south of the Marae and the Seine. 



2. First fresh-water Formation. — The second series (etage), or 

 first set of beds of fresh-water formation, which, as I have said 

 above, I consider to be composed of the siliceous lime-stone, the 

 gypsum, and the first fresh-water lime-stone and marls, does 

 not begin to appear till some distance to the north of the Marne 

 and the Seine. In the environs of Paris it does not rise above 

 150 metres, but it probably attains a greater elevation on the 

 eastern side, and particularly towards the borders of Champagne. 

 This formation prevails throughout a considerable space of a 

 triangular shape. It dips under the later formations in the same 

 manner as the preceding, and disappears near to a line drawn 

 in the direction from north-west to south-east, from Houdan 

 through Arpajon to Nemours. In the greater part of this sur- 

 face, that is to say, in the part covered by the siliceous lime- 

 stone, the lime-stone with cerithia is entirely wanting, as was 

 remarked by MM. Cuvier and Brongniart. Its place is to be 



* Lemaitre, Journal dcs Mines, No. 35, p. 8.53. 



