We see, then, that speaking generally, granite is the source of both the materials whose intimate mixture in the state of the finest comminution constitutes the paste of porcelain. It thus happens that it is only in regions of primitive rocks, far away as a rule from centres of industry and indeed from the usual sources of the clay used for fictile ware, that the materials essential for making porcelain are found. By the term granite we mean here a crystalline rock consisting of felspar, quartz, and mica, and we include in the term gneiss, which differs only in the arrangement of its constituents. The many varieties of rock that are named as sources of kaolin and petuntse, such as pegmatite, graphic granite, or growan-stone, are as a rule varieties of granite[4] distinguished by containing little or no mica, and above all by the absence of iron in appreciable quantity. As felspar is also the sole or at least the principal element in the glaze with which porcelain is covered, it will be seen that it is the mineral with which we are above all concerned. Now, of the three minerals that enter into the constitution of these granitic rocks (the others are quartz and mica), felspar is the one most easily acted on by air and water. The carbonic acid which is always present in the surface-water gradually removes the alkaline constituents in the form of soluble carbonates, the silicate of alumina which remains takes up and combines with a certain quantity of water, and in this form it is washed down into hollows to form the beds of white crumbly clay known as kaolin. This is, of course, a somewhat general and theoretical statement of what happens. If we were to examine the actual position and geological relation to the surrounding rocks of the beds of kaolin in Cornwall and in the south-west of France, there might be some exceptions to be made and difficulties to explain. Where, indeed, as in many places in Cornwall, the kaolinisation has extended to great depth, the decomposition may have been caused by deep-seated agencies; in such cases the kaolin is often associated with minerals containing fluorine and boron.[5]