72

ECONOMIC GEOLOGY

constant association of china-clay with tourmaline and topaz,
and its absence from propylitic deposits due to steam combined
 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 pneumatohydrolysis.

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 decomposition
 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