Their geological formation is metamorphic gneiss, veined with felspar and quartz, and interspersed with reddish porphyrite.
The felspar decomposes into kaolin and quartz; its alkalis are for the most part set free and removed in solution, but are partly retained in the white mica which is constantly found in crude china-clays.
In the mica-schists of this group biotite or muscovite may be the principal mineral and often both are present in varying proportions; the mica has developed from the argillaceous matter of the original rock; in addition there is always quartz and sometimes felspar (albite or oligoclase).
But it is questionable whether, taken altogether, the mass of sodium they represent is as much as that disseminated throughout the rocky crust in the form of soda felspar (i.e.
Throughout much of the Piedmont Plateau and Mountain regions the decomposition of felspar and of other aluminous minerals has resulted in a deep soil of clay with which more or less sand is mixed.
To these may be added emery, steatite, barytes, felspar and ochre, in considerable quantities; excellent lithographic stone is obtained at Solenhofen; and gold and silver are still worked, but to an insignificant extent.
Upwards, are washed on plain sieve plates, but for finer-grained duff the sieve is covered with a bed of broken felspar lumps about 3 in.
These sediments are fine and tenacious; their principal components, in addition to clay, being small grains of quartz, zircon, tourmaline, hornblende, felspar and iron compounds.
Kaolin and muscovite are formed principally after felspar (and the felspars are the commonest minerals of all crystalline rocks); also from nepheline, leucite, scapolite and a variety of other rock-forming minerals.
The industries include the manufacture of fine pottery, and of so-called porcelain buttons made of felspar and milk by a special process; its inventor, Bapterosses, has a bust in the town.