1818.] of the Strata of the Environs of Paris. 277 



noticed, but with this difference, that frequently one or two of 

 the series acquiring an unusual thickness, give their own cha- 

 racter to the district, while the rudiments only of the remaining 

 ones exist, and are hidden by the preponderance of the others. 

 It thus happens that in different districts of this formation the 

 country assumes a calcareous, a sandy, or a clayey character. 

 There is also this difference, that on the east side, from the Oise 

 to the Yonne, these strata form only a narrow band ; while to the 

 south, and more particularly to the south west, they occupy a 

 considerable space. The cause of this circumstance appears to 

 be, that the beds, notwithstanding they seem to be horizontal, 

 have on the east side an inclination determined by that of the 

 beds on which they rest. This is considerable, and consequently 

 circumscribes the superficial extent of the strata we are speaking 

 of; while, on the other hand, towards the south west, the lower 

 beds, being more horizontal, allow the edges of the strata which 

 form the older parts of the chalk formation, where they rise to 

 the day, to extend over a greater surface. 



The strata of the Paris beds are not placed exactly in the 

 middle of the great chalk basin, as their southern part rests on 

 the older chalk. It is, however, very difficult to say where the 

 common chalk ends, for its. passage into the chalk with pale flints 

 takes place by insensible degrees ; but I think it may be stated, 

 that on the western side of the basin the country to the south- 

 west of Chartres, Courville, Verneuil, &c. belongs to the chalk 

 with pale flints, which there forms the belt containing the Paris 

 strata, and extends beyond the Loire. 



This chalk differs but little from that with dark flints, and 

 sometimes contains subordinate beds agreeing with the common 

 chalk in all respects. It is generally of a coarser grain, less 

 coherent, and contains a greater quantity of sand, and sometimes 

 of clay, and even of chlorite in its lower parts. It is often used 

 as a manure with much advantage. The flints are generally in 

 greater abundance than in the common chalk, and there are 

 some places where their quantity is greater than that of the cal- 

 careous matter ; their colour is generally bluish white or yellowish 

 brown, sometimes a cinereous grey, rarely blackish. They occa- 

 sionally lose their mineralogical character, passing by insensible 

 •_; nidations to jasper, to calcareous sandstone, and to breccia, or 

 |mdding-stone, which have manifestly an origin analogous to that 

 of the other siliceous nodules. 



The alternations of the chalk with pale flints with the coarse 

 chalk and the sands of the chalk, and the passing of each into 

 the others, add also to the difficulties of tracing the limits of these 

 beds. But the predominance which the sandy beds acquire to 

 the west of the band of chalk with pale flints, which I have just 

 pointed out, occasions the existence of a sandy country, which 

 may be considered to be subdivided into two small regions by 

 i point, or cape of the lias near to La Ferte Bernard. One of 



