CLAY 
[71 
present in the granite; and fluor-spar is conspicuous where 
the granite contained a lime felspar. Cassiterite is generally 
present and many of the china-clay masses were discovered 
by the Pheenicians and worked for tin. 
The conversion of granite into china-clay was formerly 
attributed to the percolation downward of water containing 
carbon dioxide, the removal of the potash of the felspar, and 
the recombination of the silica and alumina as kaolinite. 
G. Hickling (Tr. I.M.E., xxxvi, 190g, p. 2I) regards the 
kaolinite as formed from altered mica. The formation of 
the kaolinite by weathering, though adopted by W. Lindgren 
(Min. Dep., 1913, p. 305) is invalid for English china-clay. 
If it were due to the water containing carbonic acid the acces- 
sory minerals should include carbonates; they are however 
compounds of boric and fluoric acids, and include tourmaline 
(a variable boro-silicate), axinite (a boro-silicate of aluminium 
and calcium), and topaz (fluosilicate of alumina, AlF,, SiO). 
That china-clay was formed by hot deep-seated acids is 
also indicated by minerals which are clues to weathering; 
for biotite, which is readily thus destroyed, remains in china- 
clay formed from biotite-granite, while zircon, which resists 
weathering, has disappeared... No zircons could be found 
at the Carpella Mine in Cornwall, whereas they are abundant 
in the soil on the surrounding granite. Further, the clay- 
slate or killas beside a china-clay mass, as at the Carpella 
Mine, often contains much tourmaline due to the entrance 
of boric acid. Moreover, cassiterite, the metallic mineral 
characteristic of deep-seated acids, is a typical associate of 
china-clay. 
The distribution of the china-clay blocks is inconsistent 
with their formation by weathering. They are absent from 
many granites as in Scotland, and are confined in the British 
Isles to the roots of the Hercynian Mountains. There is no 
china-clay in many extensive areas of granite and gneiss 
in Scandinavia, although it occurs in Baltic islands that have 
been disturbed by the Hercynian movements; but as the 
kaolinization has there acted upon basic rocks such as dia- 
base, it has produced a material commercially of little value. 
R. H. Rastall (Tr. R.G. Soc. Cornwall, xv, 1925, pp. 415-38) 
suggests the formation of the Cornish china-clay by the 
action of steam on felspar without fluorine or boron ; but the