26 M. D'Omalius D'Halloy on 



portions rest upon the older chalk ; but it is extremely diffi- 

 cult to say where the common chalk ends, since the passage 

 of this rock into the chalk with pale flints is so gradual, 

 that it cannot positiyely be determined where it takes place. 

 I conceive nevertheless it may be admitted, that in the 

 western part of the basin, the countries to the south-west of 

 Chartres, Courville, Verneuil, &c. belong to the chalk with 

 pale flints, which afterwards forms the belt of the Parisian 

 formations to beyond the Loire. 



This chalk differs but little from that with dark flints, 

 and sometimes contains subordinate beds, which do not at 

 all differ from it ; it is generally of a coarser grain, of 

 slighter cohesion, and contains a greater quantity of sand, 

 sometimes clay, and even chlorite in the inferior beds ; it is 

 often advantageously employed to manure lands. The flints 

 are generally more abundant than in common chalk ; there 

 are even situations where their mass surpasses that of the 

 chalky matter ; their colour is commonly light or yellowish 

 brown, sometimes ash grey, rarely black. They now and 

 then lose their mineralogical character, and pass by insensi- 

 ble shades into jasper, calcariferous sandstone, and breccia 

 or puddingstone, which notwithstanding their clastoide ap- 

 pearance, clearly shew an origin analogous to that of the 

 other siliceous nodules. 



The passages of the chalk with pale flints into, and alter- 

 nations with, coarse chalk, and the sands of the chalk, 

 render it also extremely difiicult to trace a limit between 

 these two rocks. But the predominance of the sandy beds 

 to the west of the band of chalk with pale flints, which I 

 have just noticed, forms a sandy country, which may be con- 

 sidered as divided into two small regions by a point in the 

 shape of a cape, formed by the ancient limestone in the 

 neighbourhood of la Ferte-Bernard (Sarthe). One of these 

 regions known by the common name of Perche, is a thicket 

 country, furrowed by numerous small valleys, and extends 

 from the environs of Aigle (Orne), towards Montdoubleau 

 (Indre and Loire) ; the other comprehends the arid plat- 



