4i Cn the Mincraloglcal Geography 



some of the -geodites of strontian found in the green marles 

 of gypsous formation ; i)ut we have not as yet seen any of 

 them so large and so well defined. 



Article II. Formation of Plastic Argil, 



Almost the whole surface of the mass of chalk is covered 

 with a stratum of plastic clay which has some very remark- 

 able common characters, although it presents sensible dif- 

 ferences in various points. 



This clay is unctuous, to the touch tenacious, and con- 

 tains silex, but very little lime; so that it does not effer- 

 vesce with the acids. It is even absolutely infusible in a 

 porcelain furnace when it does not contain too great a 

 proportion of iron. 



It varies considerably in colour :— at Morct in the forest 

 of Drcux, it is very white : at Montereau, Houdan, and 

 Conde, it is gray : pure slate grav, slate gray mixed with 

 red, and almost pure red, throughout the whole south of 

 Paris, from Gcntilly to Meudon. 



This plastic clay is, according to its various qualilie?, 

 cmploved in makmg porcelain, stone- w-are, crucibles, or 

 common red earthenware, it is never effervescent nor fu- 

 sible. The red colour, the pyritous grains, the portions of 

 silex, the small fragments of chalk, and the crystals of 

 sclenite which it sometimes contains, are the only defects 

 found in it. 



This stratum varies much in thickness : in some parts 

 it is 16 metres (52-^ feet) and upwards; in others it forms 

 only a thin covering of one or two decimetres (4 or 8 inches). 



It seems almost certain, that no marine nor terrestrial 

 fossil is found in this clay, at least we have not seen any, 

 either in the different strata we examined in their respective 

 positions, or on the large heaps which we repeatedly ex- 

 amined, in the numerous manufactories in which it is 

 used ; besides which, the workmen who dig up this article 

 in the south of Paris assured us, that they never found in it 

 any shells, bones, wood, or vegetables. 



Dolomieu, who discovered this same bed of clay be- 

 tween the chalk and the coarse limestone in the elbow 

 formed by the Seine in front of Kolleboise, says indeed 

 that fragments of bituminous wood have been found in it, 

 and thai they have been even taken for coal ; but he re- 

 marks, that these small woody portions have been found ia 

 pieces rolled down from the bank, which may have en- 

 veloped them at a period posterior to the primitive deposit 

 of this clay. 



The 



