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MILL-STONE AND FRESHWATER LIMESTONE. 



The Upper Marine Sand and Sandstone contains numerous ma- 

 rine shells ; it has frequently a reddish colour ; it is a thin bed, com- 

 pared with the sandstone without shells, and is not of general occur- 

 rence. It may be studied at Montmartre. Whether any analogous 

 beds have been found in England, is not well ascertained, but the 

 beds of sand at Bagshot Heath, and in other situations resting on 

 London clay, have been generally classed with the upper marine 

 sandstone of the Paris basin. The Bagshot sands consist according 

 to Mr. Warburton, of ocherous sand, foliated green clay, with green 

 sand, and various coloured marls ; a few marine shells have been 

 found in this sand. The Crag of Norfolk has been often classed 

 with the upper marine sand, but it probably belongs to a more recent 

 series, and will be noticed at the end of the present chapter. 



The marine sand and sandstone is, in some parts, covered with a 

 bed of argillaceous and ferruginous marl, from three to fourteen feet 

 in thickness, in which are imbedded irregular layers of compact si- 

 lex or hornstone, full of pores and cavities, which give it a corroded 

 and cellular appearance. It is this asperity of surface that renders 

 this stone peculiarly fitted for mill-stones. The substance of mill- 

 stone, when unmixed, is pure silex ; it has, generally, a reddish or 

 yellowish colour, but that of the best quality is nearly white. All 

 the best mill-stones used in England are brought from this bed, and 

 are known by the name of Burrh stones. There are no shells or or- 

 ganic remains in this bed. 



Upper Freshwater Formation. — This formation, though exten- 

 sively spread over many parts of the Continent, is scarcely known in 

 England : it occurs in the Isle of Wight. In the Paris basin, it cov- 

 ers all the other tertiary strata, and is itself covered with vegetable 

 soil. The upper freshwater formation is so called, because all the; 

 shells which it contains are analogous to freshwater shells : it con-/ 

 sists, principally, of calcareous earth, and siliceous earth, sometimes - 

 separated, and sometimes intermixed. Calcareous earth, in the state 

 of pure limestone, is the most common : large masses of freshwater ; 

 silex are more rare. The silex occurs, sometimes, as a pure trans-, 

 lucent flint, and sometimes, opaque, with a resinous fracture ; some- 

 times, it approaches to the state of jasper, and sometimes, it has ali 

 the characters of mill-stone. 



Freshwater limestone, in the vicinity of Paris, has generally, a 

 greyish white, or a yellowish colour; it is sometimes, as tender as 

 chalk, and sometimes, hard and compact, with a fine grain and con- 

 choidal fracture ; in the latter state, it is brittle, and breaks into 

 sharp-edged fragments like flint. Some of this limestone, at a dis- 

 tance from Paris, particularly that of Chateau Landon, presents the 

 character of a transition marble, and will receive a fine polish. Sev- 

 eral of the basins with^e^^ d^eau in the gardens of the Thuilleries are 

 made of this marble. Many of the harder freshwater limestones, 

 however, rapidly disintegrate on exposure to air and moisture, fall to 



