1818.] the Strata of the Environs of Paris 87 



rest upon the chalk, which forms, as MM. Cuvier and Brong- 

 niart have observed, a vast belt around the Paris basin. (See 

 PI. LXXVII.) 



That part of these limits north of the Seine is easily deter- 

 mined, being marked by its physical as well as its geological 

 character. The Paris strata have, throughout, the form of a 

 chain of hills, more or less indented, which rise above the chalky 

 plain. This latter, as it approaches the foot of these hills, be- 

 comes lower, and of a more even surface than usual. 



MM. Cuvier and Brongniart have described a great number 

 of these chutes of the Paris strata towards the plain of chalk ; 

 but as they have not mentioned that near Damerie and Rheims, 

 I will here describe it. 



The chalk, which to the east of Paris is concealed by the 

 strata of a posterior formation, begins to appear in the valley of 

 the Maine below Dormans, and rises, as we proceed up the 

 valley, so that, on arriving at the plain of Champagne, we see it 

 forming the bases of the hills to the height of some metres above 

 the level of the plain. This fact, which occurs also in several 

 other parts of the borders of the Paris beds, proves, that apart of 

 the valley of the Marne has been hollowed in the chalk, and 

 indicates also that the low plain which occurs round the hills of 

 the Paris strata has not merely arisen from its being the acci- 

 dental form of the chalk originally, but that, to a certain extent, 

 it is owing to the same cause as that which has worn the outer 

 edges of the hills into the great number of irregularities which 

 are every where met with. 



1 have not met with the plastic clay formation in this part, but 

 from the observations of M. Desmarest, jun. it appears in the 

 form of blackish earth, often of sand, sometimes of clay, and 

 almost always impregnated with carbonaceous matter. These 

 black strata, on which M. Desmarest proposes to publish, have 

 much resemblance to those that are dug for the purposes of pre- 

 paring sulphate of iron, and which are very common in the 

 northern part of the Paris basin, and even in the chalky plain, 

 vvhere they constitute isolated deposits in the form of islands or 

 small basins. The agreement of these black pyritous earths 

 with the plastic clay will doubtless add much to the history of 

 that formation of which it considerably increases the extent, at 

 the same time that the occurrence of some of the fossils charac- 

 teristic of the lime-stone with cerithia, in some of the beds of 

 these black earths,* shows that there is a great connexion 



* I have my self observed only two of (he deposits of strata containing cerithia. 

 One is at St. Marguerite, near Dieppe, where they form a small basin in the 

 chalk. This consists of a series of beds of sand and clay, of which the first alter- 

 nate with beds strongly impregnated with carbonaceous and pvritous matter, 

 which are dug for making sulphate of iron, and beds of shells more or less broken, 

 in which are distinguishable cerithia, and some bivalves, which I believe to belong 

 to the genus cytherea. The other deposit is near Chateau-Thierry ^Dep. of Aisne), 

 where the valley of the Mai or presents some blackiih clay full of shells, among 



