CLAY 
173 
the evidence cited by Barnitzke (Z. praki. G., xvii, 1909, 
p. 471) and A. Stahl (Arch. Lag., xii, 1912) favours the descend- 
ing movement of the acid water. Stahl has called such 
china-clay exogenous as due to alien waters, in contrast 
to the endogenous, which is due to waters acting during 
the original formation of the associated deposits. Some 
of the latter group, e.g. those of the Naab Valley in Bavaria, 
are due to pneumatohydrolysis, being due to ascending 
carbonic acid solutions which are there connected with basalt 
{Stahl, #bid., p. 128). 
The German china-clay is formed by both plutonic and 
meteoric waters. Those china-clay deposits in the United 
States that are kaolinized acid dykes may be due to meteoric 
water. The china-clay stocks of Victoria are associated with 
tourmaline, and are of pneumatolytic origin. 
China-clay is used for many purposes. Its price usually 
varies between £I and £2 per ton. Its most important use 
is for porcelain owing to its purity in colour and resistance to 
high temperatures. Impure varieties are used for refractory 
bricks. As china-clay is easily moulded, it is used for the 
manufacture of wall-paper with a raised surface; as it is 
inert and innocuous it has been used as an adulterant for 
confectionery and ice creams. 
FuLrer’s Earta—Fuller's earth differs from clay by 
being usually non-plastic. It is an amorphous hydro- 
silicate of alumina similar to halloysite; it is very fine in 
grain and has the property of absorbing grease and colouring 
matters. It was formerly used to remove grease from wool 
during the process known as fulling; its main present use 
is for the filtration of oils, and as a constituent of paper, 
soap, and drugs. Fuller's earth is generally interstratified 
in shallow water marine deposits which often contain glauco- 
nite. It was apparently laid down under conditions in which 
silicate of alumina was deposited as a fine clay mixed with a 
little sand composed of quartz or felspar, and often some 
carbonate of lime. It generally contains about 70 to 80 
per cent. of silicate of alumina, from 4 to 10 per cent. of 
ferric oxide, from 1 to 5 per cent. of carbonate of lime, from 
I to 2 per cent. of alkalis, and 8 to 25 per cent. of water. 
The material owes its absorptive properties to the minute 
size of its particles, which offer a large area for surface