MODE XI. CHALK. 507 



seem more ancient than those in many lime- 

 stones ; and Brongniart has observed that they 

 are chiefly pelasgian. The shells are often 

 changed into flint, particularly the echinites. 

 Sometimes unaccountably the shell retains its 

 original appearance, and even lustre when 

 broken; while the interior is a mass of solid 

 flint, which has also been found to pass through 

 the shell without affecting its texture, which 

 could not have happened if the flint had been in 

 a state of fusion from. heat. 



Patrin informs us, from ButTon, that a chalk 

 region is also found in Poland ; but he adds par- 

 ticularly in the territory of Sadki, where it is 

 only found above an iron mine, with beds of 

 other substances. This may perhaps be a soft 

 white limestone, like that sometimes used at 

 Lyons, and which has also been employed in 

 building the famous bridge of St. Esprit over the 

 Rhone. Some regard this last as a highly in- 

 durated chalk, which may be cut with a common indurated, 

 saw, and becomes, like most other stones, more 

 solid by exposure to the air, 'losing what the 

 quarriers call the rock water. 



There is, on the other hand, a kind of chalk, 

 which may be regarded as crude and imperfect, 

 often consisting merely of comminuted shells; 



