10 M. D'Omahus D'Halloy on 



I conceived that the determinatioa of these limits through- 

 out the extent of the basin would afford some interest, and I 

 have with this design undertaken several excursions, the 

 results of which I here present, I should however acknow- 

 ledge that a part of this work has already been executed by 

 M. Desmarest, sen., who had carefully determined the limits 

 of the Chalk in Champagne. I have also received great 

 assistance from the mineralogical atlas of M. Monnet, a 

 work full of valuable observations, and less known than it 

 deserves to be. Lastly, I have consulted two memoirs of 

 Messrs. de Tristan and Bigot de Morogues, with advantage, 

 for that part of this basin which occurs in the neighbourhood 

 of the Loire. 



The different formations of which the country in the vici- 

 nity of Paris is composed, taken collectively, a few isolated 

 patches detached from the mass being omitted, occupy a 

 surface of about 170 square myriameters, (7100 square miles 

 English) forming an irregular polygon, elongated from north 

 to south, whose greatest axis may be represented by a line, 

 30 myriameters long (328,391 English yards), drawn from 

 Laon to Blois, the sides of this polygon pass in the vicinity 

 of the towns of Laon, La Fere, Noyon, Clermont, Beau- 

 mont, Gisors, Mantes, Houdan, Chartres, Chateaudun, 

 Vendome, Blois, Orleans, Cosne, Montargis, Nemours, 

 Nogent sur Seine, Sezanne, Epernay, and Reims. The 

 Parisian formations rest throughout this extent upon chalk, 

 which forms, as Messrs. Cuvier and Brongniart have re- 

 marked, avast belt round the Paris Basin. (See Plate 2.) 



These limits are very easily determined on the north of 

 the Seine, being both physically and geologically distinr 

 guished ; the Parisian formation every where presents itself 

 under the form of a chain of hills more or less indented, 

 which rise above the chalk plain. The latter becomes, how- 

 ever, lower and more even than it usually is, as it approaches 

 the foot of these hills. Messrs. Cuvier and Brongniart have 

 described numerous chutes of the Parisian rocks towards the 

 chalk plain ; but as they have not had occasion to spe^k of 



