i6o THE POT AS SIC FERTILISERS [chap. 



nection with the ocean so as to admit of the constant 

 inflow of fresh sea water. It has been shown that all 

 the various minerals — salts of sodium, potassium, 

 magnesium, and calcium — which occur in these deposits, 

 are formed at different stages in the concentration and 

 drying up of a solution originally of the composition of 

 sea water, and the sequence of their deposition has led 

 to the estimation that a period of about 13,000 years 

 was necessary for the formation of the bed. The 

 sequence of deposits varies somewhat from shaft to 

 shaft, especially where the influx of water in the past 

 has effected some rearrangement in the salts ; but in 

 the main, after passing through 600 to 800 feet of red 

 sandstone, limestone, etc., a bed of gypsum is first 

 reached, underneath which is a bed of very pure, 

 "younger" rock-salt, which in its turn gives place to a 

 bed of anhydrite (anhydrous sulphate of lime) with 

 some gypsum. Below the anhydrite comes a bed of 

 tough impervious salt clay, which has acted as a water- 

 proof layer and prevented the solution of the highly 

 soluble potash and magnesium salts immediately below ; 

 at the top is a layer 50 to 130 feet thick of carnallite, 

 a crude double chloride of potassium and magnesium 

 which is the main source of the manufactured salts. 

 Below the carnallite comes the " kieserite " region, where 

 this mineral, a crude sulphate of magnesia, predominates, 

 and below it again comes a " polyhalite " region, char- 

 acterised by the prevalence of this complex sulphate of 

 potash, lime, and magnesia. The polyhalite overlies 

 the " older " rock salt, 2000 feet or more in thickness, 

 interspersed with and underlaid by layers of anhydrite, 

 before the limiting bituminous sandstones are reached. 

 It will be noticed that the bottom layer of anhydrite 

 represents the least soluble salt in sea water ; above it 

 comes the sodium chloride in bulk ; while at the top are 



