206 ON THE CHINA-STONE AND CHINA-CLAYS OP CORNWALL. 



tion strata, by which this is surrounded ; where, being exposed to the 

 action of lodes anl co-existing springs, on the occurrence of the slightest 

 convulsion, it has slid to the adjacent valleys, where its presence is 

 indicated by the generally smooth and flattened appearance of the sur- 

 face ; by the vegetation on it, which is often luxuriant, especially if the 

 clay contain an excess of potash ; and by the number of springs to which 

 it gives rise in the immediate vicinity, their height above the level of the 

 sea being necessarily limited by that of the valleys in which the clay is 

 deposited. 



The character of the clay very much assimilates to that of the 

 granites from which it has been formed by the disintegrating process to 

 which I referred while speaking of the formation of China-stone, not 

 only as to the quantity obtainable from a given amount of clay stope, 

 but also as to the purity of the article and its whiteness, the whitest 

 clay being formed from that granite which has the whitest felspar, and 

 is most free from iron, the presence of this giving the manufactured 

 wares an appearance termed " foxey " ; while, lastly, the amount of mica 

 scales, which give to them their tenacity or strength of body, consider- 

 ably influence the character and value of the clay, so that, as a general 

 rule, we can form a very good diagnosis of the character of the clay by 

 an examination of the granite from which it has been formed ; and in 

 doing this, I would advise the use of a good microscope, by which only 

 the clay producer can hope to obtain an accurate knowledge of the value 

 and purity of our clays. 



The kaolin of both Devon and Derbyshire is of good working quality 

 but can by no means compare with that of our county, either for white- 

 ness or strength ; it contains 60 of alumina, 20 of silica, and 20 of potash 

 (Wedgwood) ; and to this peculiarity of constitution (excess of silica) 

 is due its property of being infusible, and unchanged, at the highest 

 temperature ; it is extremely tenacious of moisture, and hence one great 

 difficulty in its preparation : to be hereafter discussed. 



The clay beds, or stopes, are formed by small irregular crystals of 

 quartz, the edges of which are by no means so well marked as in the 

 granite, nor is their opacity so great ; the mica is apparently unchanged, 

 consisting of silicic acid, potash, and alumina, in the form of double 

 silicate ; while the felspar of the granite or China-stone, by the loss of 

 its potash, has become converted into the amorphous powder I have just 

 described ; a singular instance of the effect of slight natural chemical 

 changes giving rise to the formation of two such dissimilar bodies, when 

 fused, as biscuit China, white, glassy, sonorous, and translucent ; when, 

 if the disintegrating process have but just overstepped this limit, we 

 find, on fusion, a brick-like mass, white, opaque, adherent to the tongue, 

 tenacious of moisture, and earthy on fracture. There are, however, as 

 I before stated, many and varied intermediate productions from the 

 pasty pipe-clay or tile, to porcelain or glass, which is but another form 

 of a fusible silicate. The clay stopes are oftentimes rendered useless by 



