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owing to the fact that the silicious aluminates are the most resistant constituents of the raw materials. The more basic a rock, the more rapid its decomposition. An increase of soda and potash accelerates weathering, the soda being more effective than the potash in this respect. Similarly lime decreases resistance more than magnesia. Increase of iron in a rock lessens resistance. Increase of alumina checks decomposition and silica lessens it. The character of the residual clay is governed by the extent to which merely physical forces have aided rock decay. The chief difficulty in any theory of clay formation is to explain the presence of combined water in the product. Beside kaolinite, the monoclinic A1 2 3 2Si0 2 2H 2 0, there are numerous other hydrated aluminium silicates. Some, like lithomarge, nacrite, pholerite, and bentonite, have the same formula as kaolinite. Others are as follows : Pyrophyllite (pencil stone) A1 2 O 3 . 4SiO 2 . H 2 O Agalmatolite A1 2 O 3 . 4SiO 2 . H 2 O Allophane A1 2 O 3 . SiO 2 . 5H 2 O Amorphous Halloysite A1 2 O 3 . 2SiO 2 , nH 2 O Lenzenite A1 2 O 3 . 2SiO 2 . nH 2 O ,, Newtonite A1 2 O 3 . 2SiO 2 . 4H 2 O + aq. Rhombohedral Cimolite 2A1 2 O 3 . 9SiO 2 . 3H 2 O + 3aq. Amorphous Montmorillonite A1 2 O 3 . 4SiO 2 . H 2 O + n aq. ,, Collyrite 2A1 2 O 3 . SiO 2 . 9H 2 O Schotterite 8A1 2 O 3 . 3SiO 2 . 30H 2 O 8 CERAMIC CHEMISTRY.