; however, the amount of chlorine in
ordinary igneous rocks is so extremely small that, in order to explain
the amount of chlorine present in the sea, it has been thought necessary
to appeal to volcanic emanations or to some similar agency. Ocean water
contains about 3.5 per cent by weight of dissolved matter, over
three-fourths of which consists of the constituents of common salt.
Chief among the other dissolved materials are magnesium, calcium,
potassium, and SO_4 (the sulphuric acid radical).

When sea water evaporates it becomes saturated with various salts,
according to the amounts of these salts present and their relative
solubilities. In a general way, after 37 per cent of the water has
evaporated gypsum begins to separate out, and after 93 per cent has
evaporated common salt begins to be deposited. After a large part of the
common salt has been precipitated, the residual liquid, called a
"bittern" or "mother liquid," contains chiefly a concentration of the
salts of magnesium and potassium. Still further evaporation will result
in their deposition, mainly as complex salts like those found in the
Stassfurt deposit (p. 113).

The actual processes of concentration and precipitation in sea water or
other salt waters are much more complex than is indicated by the above
simple outline. The solubility of each of the various salts present, and
consequently the rate at which each will crystallize out as evaporation
proceeds, depends upon the kinds and concentrations of all the other
salts in the solution. Temperature, pressure, mass-action, and the
crystallization of double salts are all factors which influence the
nature and rate of the processes and add to their complexity. During a
large part of the general process, several different salts may be
crystallizing out simultaneously. It is evident that gypsum may be
precipitated in some quantity, and that external conditions may then
change, so that evaporation ceases or so that the waters are freshened,
before any common salt is crystallized out. This fact may explain in
part why gypsum beds are more widely distributed than beds of common
salt. At the same time the much greater amount of sodium chloride than
of calcium sulphate in sea water may explain the greater thickness of
many individual salt beds.

The evaporation of salt waters, either from the ocean or from other
bodies of water, is believed to have been responsible for nearly all of
the important deposits of c