In the technical language of chemists and
physicists it begins to get supersaturated. Then the salts are thrown
down as a sediment at the bottom of the sea or lake, exactly as crust
formed on the bottom of a kettle. Gypsum is the first material to be so
thrown down, because it is less soluble than common salt, and therefore
sooner got rid of. It forms a thick bottom layer in the bed of all
evaporating inland seas; and as plaster of Paris it not only gives rise
finally to artistic monstrosities hawked about the streets for the
degradation of national taste, but also plays an important part in the
manufacture of bonbons, the destruction of the human digestion, and the
ultimate ruin of the dominant white European race. Only about a third of
the water in a salt lake need be evaporated before the gypsum begins to
be deposited in a solid layer over its whole bed; it is not till 93 per
cent. of the water has gone, and only 7 per cent. is left, that common
salt begins to be thrown down. When that point of intensity is reached,
the salt, too, falls as a sediment to the bottom, and there overlies the
gypsum deposit. Hence all the world over, wherever we come upon a bed
of rock salt, it almost invariably lies upon a floor of solid gypsum.
The Caspian, being still a very respectable modern sea, constantly
supplied with fresh water from the surrounding rivers, has not yet begun
by any means to deposit salt on its bottom from its whole mass; but the
shallow pools and long bays around its edge have crusts of beautiful
rose-coloured salt-crystals forming upon their sides; and as these
lesser basins gradually dry up, the sand, blown before the wind, slowly
drifts over them, so as to form miniature rock-salt beds on a very small
scale.
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