Kaolins are seldom found in a sufficiently pure state to be used direct, but must be freed from large amounts of undecomposed rock, quartz, mica, etc., by a process of washing and sedimentation. When purified in this manner, the best qualities of china clay yield, on analysis, alumina, silica and water in the proportions indicated by the formula H4Al2Si2O9 together with about 5 per cent. of mica and other impurities. Some high class commercial kaolins contain over 30 per cent. of mica and 10 per cent. of quartz. The chief constituents of rocks which take part in the production of kaolins appear to be the felspars, but the natural processes by which these felspars are decomposed are by no means perfectly understood. Some kaolins appear to have been formed by weathering and others by subaerial action. Thus Collins (19) has stated very emphatically that the kaolinization of Cornish felspar has been chiefly effected by fluorine and other substances rising from below and not by carbonic acid and water acting from above. Ries (6) and other American observers are equally convinced that certain kaolins they have examined are the result of 'weathering.' German and French investigators are divided in their opinions, and Fuchs has found that the Passau (Saxony) kaolin is derived from a special mineral, not unlike a soda-lime felspar deficient in silica, to which he has given the name 'porcelain spar.' The _felspars_ form a class of minerals whose chief characteristic is the combination of an alkaline or alkaline-earth base with silica and alumina. Orthoclase (K2OAl2O3·6SiO2)--the chief potassium felspar--is typical of the whole class. When treated with water under suitable conditions, the felspar appears to become hydrolysed and some of the water enters into combination, the potash being removed by solution. Attempts to effect this decomposition artificially have proved abortive though several investigators appear to have effected it to a limited extent by electrolysis or by heating under great pressure (3). The effect on felspars of waters containing carbon dioxide in solution has been studied by Forschammer, Vogt, and others, and they have concluded that kaolinization may occur with this agent though it does not appear to be the chief cause in the formation of Cornish china clays. [Illustration: Fig. 13. Orthoclase Felspar, natural size. (_From Miers_' Mineralogy _by permission of Macmillan & Co._)]