The solid constituents of sea-water amount to about 3½ per cent of
its weight, or nearly half an ounce to the pound. Its saltness is
caused as follows: Rivers which are constantly flowing into the
ocean contain salts varying from 10 to 50, and even 100, grains per
gallon. They are chiefly common salt, sulphate and carbonate of lime,
magnesia,[41] soda, potash, and iron; and these are found to constitute
the distinguishing characteristics of sea-water. The water which
evaporates from the sea is nearly pure, containing but very minute
traces of salts. Falling as rain upon the land, it washes the soil,
percolates through the rocky layers, and becomes charged with saline
substances, which are borne seaward by the returning currents. The
ocean, therefore, is the great depository of every thing that water
can dissolve and carry down from the surface of the continents; and
as there is no channel for their escape, they consequently accumulate
(_Youmans’ Chemistry_). They would constantly accumulate, as this very
shrewd author remarks, were it not for the shells and insects of the
sea and other agents.