72 
ECONOMIC GEOLOGY 
constant association of china-clay with tourmaline and topaz, 
and its absence from propylitic deposits due to steam com- 
bined with carbonic acid, indicates that its formation is 
pneumatolytic. Kaolinite is found in small quantities beside 
mineral veins in Anglesey. It has been formed in felspathic 
sandstones in Scotland ‘and the Midlands by superheated 
steam and carbonic acid from dykes having kaolinized the 
felspar grains; this kaolinite is therefore due to pneumato- 
hydrolysis. 
GerMAN DEPOSITS OF Various MoDpEs oF FormaTiOoN— 
The German china-clays are due to two processes. Some 
Loam., 
C.C. 
o.P 
Fie. 51.—CHINA-CLAY. 
China-clay (C.C.) resting upon and passing down into quartz-porphyry 
(Q.P.) at Baselitz (after Stahl). 
of the most important deposits, such as those at Meissen 
that are used for Dresden china, are due to descending 
waters charged with sulphuric acid produced by the decom- 
position of pyrites from brown coal. These china-clays occur 
as superficial sheets, which pass into the country rock, which 
at Meissen and Baselitz (Fig. 51) is quartz-porphyry and 
Brown Coal 
Clay & 
3 nd’ 
Quartz 
Porphym.: 
Frc. 52.—CHina-Cray. 
China-clay produced by the action of acids from brown coal in Bavaria 
on guartz-porphyry (after Stahl). 
volcanic tuff, and elsewhere the china-clay occurs where 
brown coal beds rest against quartz-porphyry (Fig. 52). 
These deposits do not contain tourmaline, except where, 
according to Stahl, it existed in. the original granite, and 
biotite is rarely preserved. Roesler attributes the formation 
of this china-clay to solutions rising through fissures: but