19 M. D'Omalius D*Hall6y on 



form escarpments too steep for cuUivatioa : M. Desmarest, 

 jun. has observed that the fresh-water marls are advantage- 

 ously employed in rendering the chalky soil fit for the 

 growth of vines. Lastly, the platforms of the- millstone 

 formation are commonly covered by forests or heaths, which, 

 from the large blocks scattered over the surface, remind us 

 of a primitive country. 



The limits of the Parisian formations, in the part of the 

 basin south of the Seine, do not preserve this physical de- 

 marcation characterizing those of the northern portion. This 

 arises from the changes that have taken place in the geologi- 

 cal structure of the surface, changes which I am about to in- 

 dicate in a general manner. 



Although we may be accustomed to consider the different 

 rocks forming the basin of Paris, as placed horizontally one 

 upon the other, and they really appear to be so in the central 

 part of the basin, we shall observe, taking these formations 

 collectively, that they have a southern dip, sufficiently so to 

 represent to a certain degree, a set of wedges arranged like 

 tiles on a roof, with tiiis peculiarity, that the inferior wedge 

 attains the greatest elevation. 



The first series is, as is known, the calcaire a. cerites, thd 

 most elevated part of which appears to be on the northern 

 extremity of the basin among the hills of Laon, 300 metres 

 above the level of the sea,* [984 feet] exactly where this 

 limestone is not covered by any other formation. Quitting 

 the summit of these hills, the level of this rock gradually 

 becomes lower, dips under the other deposits, and disappears 

 altogether on the south of the Marne and the Seine. 



The second series, or the first fresh-water epoch, which I 

 consider from what I have above stated, as composed of 

 siliceous limestone, gypsum, and the first fresh-water lime- 

 stones and marls, first shews itself some distance to the north 

 of the Marne and the Seine ; it is not of more than 1 50 

 metres [492 feet] elevation in the environs of Paris ; but 

 it is probable that it attains a much greater height to the 



* Lemaitre, Journal des Mines, No. 35, p. 853. 



