On the Silica Strata of the Lower Chalk. 



229 



ill France and examined by M. Sauvage, a French chemist, four 

 or five years ago : this silicious bed is alluded to in Knapp's 

 Technology * in these terms: — 



" Another variety of Puzzolana, not of volcanic origin, has very recently 

 been discovered by Sauvage in the Dep. des Ardennes. It there covers the 

 clays of the gault, a fossiliferous formation below the chalk. It is of a pale 

 grey or greenish colour, very soft, and, according to Sauvage, is constituted in 

 the following manner : — 



Soluble silica (gelatinous) . . .56 



Clay 7 



Fine quartz sand . . . . .17 

 Fine grey limestone (chlorite) . . .12 

 Water , . . . . .8 



100" 



It is plain, therefore, that the deposit is of the same character and 

 in the same geological position as that which we are about to 

 describe. 



This circumstance is interesting chiefly in showing how 

 general must have been the conditions under which the deposit 

 was formed. 



We should, perhaps, here observe that the beds or strata in 

 question are easily recognized by their external as well as che- 

 mical characters. Those which contain a high per-centage of 

 silica are, when dry, remarkably light, of a fawn or reddish yellow 

 colour, and very soft, except in some instances w^here the silica is 

 associated with carbonate of lime, as in the building-stone which 

 we shall have presently to speak of. The soluble silica is dis- 

 solved out with perfect ease from these rocks (when reduced to 

 powder) by boiling them in solution of potash or soda ; and by 

 collecting and washing that portion which does not dissolve, the 

 per-centage they contain is readily ascertained.! 



We consider these beds to consist generally of clay, with a 

 greater or less admixture of soluble silica ; when treated as above 

 the silica is dissolved and the clay remains unacted on. 



We now proceed to describe geologically and agriculturally 

 the beds as they are formed in those localities which we have as 

 yet had the opportunity of examining. 



The stratum to which we have to direct attention is that which 

 geologists technically term the " fire-stone rock." It extends 

 ujncards from the gault to the chloritic bed of phosphatic marl, 

 described by us in vol. ix. But although this term applies to a 

 stratum which in some places is from 100 to 150 feet and 

 upwards in thickness, it varies greatly in depth, being in some 

 places not more than 4 feet : its subdivisions also exhibit con- 

 siderable diversity in their lithological character. Locally, too, 



* Extracted we believe from the ' Annales des Mines.' 



t For more accurate results the silica must be precipitated by acids from the 

 solution in the way usually practised. 



