SECOND FORMATION. 2#7 



iibres, disposed somewhat like the reticular texture in 

 bones. These cavities are sometimes lined or filled with 

 red ochre, clay marl, or clayey sand, and they have no 

 communication with each other. Most of the millstones 

 found around Paris have a red or yellowish tint, but the 

 rarer and most esteemed varieties have a bluish shade of 

 colour. The bluish variety is the most highly prized, 

 because it affords the whitest coloured flour; and a mill- 

 stone of this kind, six feet and a half in diameter, sells at 

 1200 francs. We never observe ID 'its cavities any silice 

 ous stalactites, orcry stallized quartz ; and this character 

 enables us to distinguish, in hand specimens, this millstone 

 from that found in the siliceous limestone. It is some- 

 times compact. It has been analyzed by Hecht in the 

 Journal des Mines, No. xxii. p. 333, and appears to be 

 almost entirely composed of silicea. Another geognos- 

 tic character of the millstone, properly so called, is the 

 absence of all fossil animal and vegetable productions, 

 whether of fresh or salt water origin. 



It often rests on a bed of clay marl, which appears to 

 belong to the gypsum formation ; in some places it is 

 separated from it by a bed varying in thickness, of sand- 

 stone or sand without shells. It is sometimes imme- 

 diately covered with vegetable earth, but in other in- 

 stances it has resting on it the upper fresh water forma- 

 tion, or the alluvial formation.^ 



* The most extensive mass of this millstone occurs in the plateau 

 which extends from La Ferte sous Jouarre (on the Marne, 16 leapies 

 oast from Paris) nearly to Montmirail ; and here, near the first town, it 

 has been quarried upwards of four hundred years for the excellent mill- 

 stones it affords. The lower part of the plateau is marine limestone ; 

 the upper part, on the edges, and toward the Marne, of marl and gyp- 

 sum ; hut in the middle, of an iron-shot and clayey sand, which forma 

 a bed upwards of 60 feet thick. The millstone occurs in this great bed 



