Potassium is one of the eight most abundant elements in the earth. It occurs as a primary constituent of most igneous rocks, some of which carry percentages as high as those in commercial potash salts used for fertilizers. It is present in some sediments and likewise occurs in many schists and gneisses. Various potassium silicates--leucite, feldspar, sericite, and glauconite--and the potassium sulphate, alunite, have received attention and certain of them have been utilized to a small extent, but none of them are normally able to compete on the market. Potential supplies are thus practically unlimited in amount and distribution. Deposits from which the potash can be extracted at a reasonable cost, however, are known in only a few places, where they have been formed as saline sediments. In the decomposition of rocks the potash, like the soda, is readily soluble, but in large part it is absorbed and held by clayey materials and is not carried off. Potash is therefore more sparingly present in river and ocean waters than is soda, and deposits of potash salts are much rarer than those of rock salt and other sodium compounds. The large deposits in the Permian beds of Stassfurt, as well as those in the Tertiary of Alsace and Spain, have been formed by the evaporation of very large quantities of salt water, presumably sea water. They consist of potassium salts, principally the chloride, mixed and intercrystallized with chlorides and sulphates of magnesium, sodium, and calcium. In the Stassfurt deposits the potassium-magnesium salts occupy a relatively thin horizon at the top of about 500 feet of rock salt beds, the whole underlying an area about 200 miles long and 140 miles wide. The principal minerals in the potash horizon are carnallite (hydrous potassium-magnesium chloride), kieserite (hydrous magnesium sulphate), sylvite (potassium chloride), kainite (a hydrous double salt of potassium chloride and magnesium sulphate), and common salt (sodium chloride). The potash beds represent the last stage in the evaporation of the waters of a great closed basin, and the peculiar climatic and topographic conditions which caused their formation have been the subject of much speculation. This subject is further treated in the discussion of common salt beds (pp. 295-298).