China-clays of Cornwall. 93 



which, large beds of a like description of clays were found in 

 the parish of St Stephen's : and it having been ascertained 

 that the decomposing granite, from which such beds are 

 formed, was capable, when fused, of forming a suitable glaze 

 for the articles made of the clay, a large trade was at once 

 opened, which has continued progressively to increase till 

 the present time. 



The disintegrated granite, under the name of China-stone, 

 from the use to which it was applied, was exported at a later 

 period than the China-clay or kaolin, — this article of com- 

 merce not having been introduced till the year 1802, when 

 it was first raised from a bed of great purity, containing no 

 iron or manganese, but merely felspar, silica, and mica, in 

 varying proportions. And this is at present the only source 

 from which it can be obtained of a sufficient degree of purity 

 for ordinary purposes ; though, from its price, and the efforts 

 that have been made by chemists, both here and in the pot- 

 teries, to gain a substitute for it, it is very doubtful whether 

 it will long continue so ; more especially if the distance we 

 are placed from Stafford be taken into consideration. 



Most of the granites from which the China-stone was 

 formed, differ from ordinary granite only in the existence in 

 the latter of plates of talc, hornblende, or diallage, the pre- 

 sence of either of which renders the China-stone in which 

 they are found, though but in small proportions, of not even 

 the slightest use, from the black or brown-coloured slag of 

 silicate of iron or manganese found on fusion. Some varia- 

 tion, too, may be found in the amount of each of the ingre- 

 dients which I have named, but this affects neither the clay 

 formed on a continuation of the disintegrating process, nor 

 is it supposed to exert any influence on the glazing properties 

 of the stone. 



The places in which a search for this article would be in- 

 stituted with the greatest probability of success, is in the 

 proximity of fissured granite rocks, containing, or supposed 

 to have contained, softened stone ; or in hills with rounded 

 heavy summits, the beds of which are placed horizontally, 

 and felspar (or feldspar) forming its predominating ingre- 

 dient. 



