continent,--these inland lakes in time
become saturated with this and other mineral substances.

Salt is constantly being carried into the lake by the water of the
stream that feeds it, and the water is continually being evaporated,
leaving the salt behind. This process has been going on in the valley of
Utah for so long a period that 17 per cent. of the contents of the lake
is salt. The Humboldt River in Nevada, which empties into a small lake
of the same name, and lies at the foot of the Humboldt Mountains, is
said to have an underground outlet. This must be the case, because the
area of the lake is very small as compared with Salt Lake, while the
river that feeds the latter is very small compared with the one that
flows into the former. That is to say, in the one case a very small
stream empties into a large lake, while in the other case a much larger
stream feeds a very small lake. Besides, Humboldt Lake, unlike the Great
Salt Lake, is said to be a fresh-water lake; if it had no outlet it
would become in time saturated with salt. The largest body of water in
the world having no outlet to the ocean is the Caspian Sea, on the
border between Asia and Russia in Europe, it being 180,000 square miles
in extent.

Where rivers empty into large bodies of water, such as the great chain
of lakes on the northern border of the United States (and these lakes
have an outlet connecting one with the other, and finally by a river to
the ocean) a constant circulation is being kept up, and the water
remains fresh. Owing to the fact, however, of the great evaporating
surface that these lakes afford, there is a greater disproportion
between the rainfall upon the drainage area tributary to these lakes,
and the amount of discharge through the St. Lawrence River, than would
be the case with a river that was not connected with a system of lakes.
The amount of rainfall upon the area drained by the Mississippi River
during one year amounts to about 614 cubic miles of water, while the
discharge at the mouth of the Mississippi River is only about 154 cubic
miles. The difference between the two figures has been carried up by the
process of evaporation or stored in vegetation. These figures vary
considerably, however, with different years.

The proportion of rainfall to discharge will vary greatly in different
rivers from other causes than having a large evaporating surface. This
variation is due to the difference in the ability of the soil to r