soil is underlaid, more or less near the surface, by layers of rock,
marl deposits, and similar impervious or hurtful substances. Such
deposits are not to be classed with the hardpans that occur normally
wherever the rainfall is small.

_Leaching.--_Fully as important as any of the differences above
outlined are those which depend definitely upon the leaching power
of a heavy rainfall. In countries where the rainfall is 30 inches or
over, and in many places where the rainfall is considerably less,
the water drains through the soil into the standing ground water.
There is, therefore, in humid countries, a continuous drainage
through the soil after every rain, and in general there is a steady
downward movement of soil-water throughout the year. As is clearly
shown by the appearance, taste, and chemical composition of drainage
waters, this process leaches out considerable quantities of the
soluble constituents of the soil.

When the soil contains decomposing organic matter, such as roots,
leaves, stalks, the gas carbon dioxid is formed, which, when
dissolved in water, forms a solution of great solvent power. Water
passing through well-cultivated soils containing much humus leaches
out very much more material than pure water could do. A study of the
composition of the drainage waters from soils and the waters of the
great rivers shows that immense quantities of soluble soil
constituents are taken out of the soil in countries of abundant
rainfall. These materials ultimately reach the ocean, where they are
and have been concentrated throughout the ages. In short, the
saltiness of the ocean is due to the substances that have been
washed from the soils in countries of abundant rainfall.