ON «CHE CHINA-STONE AND CHINA-CLAYS OF COKNWAL&. 199 



Attention was first directed to the fact, that the disintegrated granite 

 and clays of Cornwall, as well as those of Devon, when fused or burnt, 

 could be rendered available to the potter, in 1768 by the late Mr. Cook- 

 worthy, of Plymouth, who extensively exported them, to the potteries 

 of Staffordshire, for that purpose, from Devon ; subsequently to which, 

 large beds of a like description of clays were found in the parish of St, 

 Stephens ; 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 pre- 

 sent 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- 

 elay or kaolin. This article of commerce 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 pur- 

 poses ; though, from its price and the efforts that have been made by 

 chemists both here and in the potteries, 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, horneblende, or diallage, the presence of either of which render 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 variation, too, may 

 be found in the amount of each of the ingredients which I have named, 

 but this affects neither the clay formed on the continuation of the disin^ 

 tegrating 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 instituted 

 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 be^s of which are 

 placed horizontally, and felspar (or feldspar) forming its predominating 

 ingredient. 



The bed from which it is obtained is about three-fourths of a mile in 

 extent on the contiguous borders of the parishes of St. Dennis and St. 

 Stephens, occupying almost the centre of the central granite district of 

 the county, and is surrounded by other primary rocks of igneous origin, 

 which, as they stretch towards the coast on either side, merge into beds 

 of killas or clay-slate ; on the eastern and northern boundaries the 

 granite is more irregular and abrupt in character than on the other 

 sides, is more porphyritic, and contains a much larger proportion of 



