Kaolin is almost always found in the neighbourhood of tin deposits; but, with tin ore, there always appear either quartz and granite with white mica, or pegmatite invariably accompanied by fluorine compounds.
This coincidence has led people to think that tin ore is formed by the action of a powerful mineralising agent which might be fluorine. As this metalloid violently attacks the silicates, it has been concluded that its appearance has reacted upon the rock containing them, that is to say, the pegmatite, so as to transform it into kaolin. This theory, suggested by Buch in 1824, with reference to the kaolin at Halle in Prussia, has been taken up by Daubr*še, and definitely adopted by de Lapparent in his Geology.
Fig. 1. represents a section of the tin deposit of Weisse Andreas in Saxony. The containing rock is formed by a mica schist (quartz and mica more or less mixed with felspar) which, in the neighbourhood of the tin ore, has become transformed into granite, the felspar being kaolinised and the crystals of quartz imbedded at right angles in the schist, reaching a length of several decimetres. This granite has received the name of Stockscheider, and is worked for the manufacture of porcelain.
Fig. 1. Stanniferous Deposit at Weisse Andreas (Saxony). 1, miscaschiste ; 2, tin ore ; 3, stockscheider.
Refractory clays other than kaolin, such as those of Bray and in the neighbourhood of Vierzon (France) and Andenne (Belgium) are usually connected with deposits of fresh-water sand, in the midst of which the clay forms not seams properly so called, but nests, accumulations, and veins. Opinion is divided as to their origin; some see in them the action of meteoric waters on volcanic rocks; then the transportation or mechanical displacement of the kaolin so formed, which, at that period, was more or less adulterated with foreign substances. Other authors, and these not less numerous, attribute the formation of these clays to the influences of temperature acting either on granitic masses at a considerable depth, or on sediments of the Primary Epoch.
The other clay deposits are the result of the agglutination of substances which have been in suspension for a long time, in the form of a more or less impalpable slime, and which came from the destruction of pre-existing rocks by the action of sea or river water or atmospheric agencies. The deposits were made during the period of the. formation of sedimentary soil, therefore clays are found in all stratified ground.