and China-clay of Cornwall. 51 
lastly, the amount of mica scales, which give to them their 
tenacity or strength of body, considerably influence the cha- 
racter 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 ob- 
tain an accurate knowledge of the value and purity of our 
clays. ian 
The kaolin of both Devon and Derbyshire is of good work- 
ing quality, but can by no means compare with that of our 
county either for whiteness 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 pot- 
ash, has become converted into the amorphous powder I have 
just described; a singular instance of the effect of slight na- 
tural 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 disintegrat- 
ing process have but just overstepped this limit, we find, on 
fusion, a brick-like mass, white, opaque, adhering to the 
tongue, tenacious of moisture, and earthy on fracture. There 
are, however, as I before stated, many and varied interme- 
diate productions, from the pasty pipe-clay or tile, to porce- 
lain or glass, which is but another form of a fusible silicate. 
The clay stopes are oftentimes rendered useless by the pre- 
sence of some iron lode, which causes them to become 
loosened in texture, and reddened ; the stope is then termed 
‘‘brawny,” and this has to be thrown aside as useless. 
D2 
