224 Mr. Wkbster on the Strata lying over the Chalk, 



In the Paris basin the marine marls arc covered by a very ex- 

 tensive and thick bed of sand, entirely without animal exuvise, 

 which has nearly filled up all the cavities which still existed on the 

 surface of the basin, covering the partial formations of marine 

 gypsums, the lower freshwater marls, together with all the other 

 strata, and reducing the surface nearly to a level. 



This sand frequently contains beds of sandstone of the same 

 nature with itself, which frequently form the flanks of the hills in 

 the neighbourhood of Paris ; and great blocks of these have fre- 

 quently rolled down into the valleys, the sand having been carried 

 away ; such are the sandstones in the forest of Fontainebleau, 

 Palaiseau, &:c. The sand of this stratum is often an extremely 

 pure white quartz, and is much used in the arts ; but sometimes it 

 is coloured by oxides of iron, or impregnated by carbonat of lime. 

 It forms all the soil of Beaucc. The sandstone is very hard, pure, 

 and homogeneous. 



This bed is usually covered only by the burr stones without 

 shells, or by the upper freshwater formation. Over it however 

 there is in some parts a sandstone containing marine shells, agree- 

 ing nearly with those of the middle beds of the calcaire grossier, 

 or rather with those already observed in this upper marine forma- 

 tion ; and in other places a calcareous stratum with shells. 



The London clay is in many places covered by an extensive bed 

 of sand, usually called the Bagshot sand. It extends over Bagshot, 

 Frimby, and Purbright heaths, in the county of Surrey, and that 

 on Hampstead heath belongs also to it. 



At Purbright, and many parts of the surrounding country, loose 

 blocks of a stone is found similar to what has been called the grey 

 iveathers. This stone, composed of siliceous particles cemented 

 together without any intervening substance, may be considered as 



