
Parr Il. Sucr. ii, §4.] SALINE DEPOSITS. 399 
or because the salt, if deposited, has been subsequently dissolved and 
removed. In every case where an alternation of layers of gypsum 
and rock-salt occurs, there must have been repeated renewals of the 
water supply, each gypsum zone marking the commencement of a 
new series of precipitates. 
But the composition of many existing saline lakes is strikingly 
unlike that of the sea in the proportions of the different constituents. 
Some of them contain carbonate of sodium ; in others the chloride of 
magnesium is enormously in excess of the less soluble chloride of 
sodium. ‘These variations modify the effects of the evaporation of 
additional supplies of water now poured into the lakes. The presence 
of the sodium carbonate causes the decomposition of lime salts and 
the consequent precipitation of calcium carbonate accompanied with 
a slight admixture of magnesium carbonate, while by further 
addition of the sodium carbonate a hydrated magnesium carbonate 
may be eventually precipitated. Hunt has shown that solutions of 
bicarbonate of lime decompose sulphate of magnesia with the con- 
sequent precipitation of gypsum, and eventually also of hydrated car- 
bonate of magnesia, which, mingling with carbonate of lime, may give 
rise to dolomite. By such processes the marls or clays deposited 
on the floors of inland seas and salt lakes may conceivably be impreg- — 
nated and interstratified with gypseous and dolomitic matter, though 
in the Trias and other ancient formations which have been formed in 
enclosed saline waters, the magnesian chloride has probably been the 
chief agent in the production of dolomite (ante p. 305). 
The Dead Sea, Elton Lake, and other very salt waters of the 
Aralo-Caspian depression are interesting examples of salt lakes 
far advanced in the process of concentration. The great excess of 
the magnesium chloride shows, as Bischof pointed out, that the waters 
of these basins are a kind of mother liquor, from which most of the 
sodium chloride has already been deposited. The greater the pro- 
portion of the magnesium chloride the less sodium chloride can be 
held in solution. Hence as soon as the waters of the Jordan and other 
streams enter the Dead Sea, their proportion of sodium chloride 
(which in the Jordan water amounts to from ‘0525 to ‘0603 per cent.) 
is at once precipitated. With it there goes down gypsum in crystals, 
also the carbonate of lime which, though present in the tributary 
streams, is not found in the waters of the Dead Sea. In spring 
the rains bring large quantities of muddy water into this sea. 
Owing to dilution and diminished evaporation, a check must be 
given to the deposition of common salt, and a layer of mud is 
formed over the bottom. As the summer advances, and the supply 
of water and mud decreases, while evaporation increases, the deposition 
of salt and gypsum again proceeds.” As the level of the Dead 
Sea is liable to variations, parts of the bottom are from time to 
time exposed, and show a surface of bluish gray clay or marl full 
1 Sterry Hunt, in “ Geology of Canada” (1863), p. 575. 
2 Bischof, “Chem. Geol.’’ i. p. 397. Roth, “Chcm. Geol.” i. p. 476. 
