ter thus impregnated, all animal life on the earth
obtains its bone-building and shell-building materials.

_Carbon_ forms a large part of the tissues of plants and animals, and in
the remains of these it is chiefly found in the earth's crust. When
these burn or decay, the carbon remains as charcoal or escapes to the
air in union with oxygen as the well known carbonic acid gas. This is
one of the most important foods of plants. Joined with calcium it forms
the mineral calcite, or carbonate of lime.

_Hydrogen_ is one of the two gases that unite to form water. Oxygen is
the other. Many kinds of rock contain a considerable amount of water.
Surface water sinks into porous soils and rocks, and accumulates in
pockets and veins which feed springs, and are the reserve water supply
that keeps our rivers flowing, even through dry weather. More water is
held by absorption in the earth's solid crust than in all the oceans and
seas and great lakes.

Hydrogen, combined with carbon, occurs in solid rocks where the remains
of plants and animals have slowly decayed. From such processes the
so-called hydrocarbons, rock oil and natural gas, have accumulated. When
such decay goes on above ground, these valuable products escape into the
air. Marsh gas, whose feeble flame above decaying vegetation is the
will-o'-the-wisp of swamps, is an example.

_Magnesium_, _potassium_, and _sodium_ are found in equal quantities in
the earth's crust, but never free. In union with chlorine, each forms a
soluble salt, and is thus found in water. Common salt, chloride of
sodium, is the most abundant of these. Water dissolves salt out of the
rocks, and carries it into the sea. Clouds that rise by the evaporation
of ocean water leave the salt behind, hence the seas are becoming more
and more salty, for the rivers carry salt to the oceans, which hold fast
all they get.

_Phosphorus_ is an element found united with oxygen in the tissues of
both plants and animals. It is most abundant in bones. Rocks containing
fossil bones are rich in lime phosphates, which are important commercial
fertilizers for enriching the soil. Beds of these rocks are found and
mined in South Carolina and elsewhere.

_Sulphur_ is well known as a yellow powder found most plentifully in
rocks that are near volcanoes. It is a needed element in plant and
animal bodies. It occurs in rocks, united with many different elements.
In union with oxygen and a metal it forms the group of mineral