216 ECONOMIC GEOLOGY 
sodium), langbeinite (double sulphate of magnesium and 
potassium); and with the triple sulphate polyhalite (sulphate 
of calcium, magnesium, and potassium with two molecules 
of water). After the sulphates follows the precipitation of 
the chlorides, such as sylvite (KCl) which may be combined 
with magnesium chloride, as carnallite (KCl, MgCl, 6H,0), 
or as hartsalz, a mixture of sylvite, rock-salt, and kieserite. 
Intermediate between the sulphates and the chlorides is 
kainite—sulphate of magnesia combined with chloride of 
potassium and water (MgSO,, KCl, 3H,0). These minerals 
may be formed by the complete evaporation of an inland sea, 
of which the waters have remained nearly saturated for a 
long time. 
SeconpaRY CoNCENTRATIONS—Potash salts, owing to 
their extreme solubility, are less abundant than rock-salt ; 
for the potash is less often deposited, and more likely to be 
removed in solution. The largest supply of known potash 
salts is in Central Germany, where, as explained on page 209, 
the bittern was occasionally evaporated and beds of primary 
potash laid down. Even there, however, owing to the solu- 
bility of the potash salts many of the beds of commercial 
importance are secondary, and have been redeposited. The 
upturned edges of a series of beds of rock-salt, carnallitite 
{i.e. impure carnallite), and kieserite is covered by a sheet of 
kainite which occasionally projects into the underlying beds. 
This kainite is of secondary origin for it is traversed by the 
upward continuation of layers of rock-salt which have re- 
sisted solution ; whereas the carnallite and kieserite, owing to 
their greater solubility, have been replaced by kainite. At 
the Hercynia Mine near Vienenburg, rock-salf, hauptsalz (a 
mixture of carnallite, rock-salt, and kieserite), anhydrite, 
carnallitite, and sylvinite (i.e. KCl NaCl), are capped by 
secondary deposits of sylvinite and kainite} which have not 
also replaced the rock-salt or anhydrite. The secondary 
origin of the sylvinite is further proved at the Ronenberg 
Mine, as a fault has displaced the rock-salt and anhydrite, 
but not the sylvinite, which occurs along and across the fault 
plane. 
These secondary potash deposits like the German salt 
domes were due to the percolation of water along fractures 
contemporary with the uplift of the Alps.