Granites and lavas are about one-half quartz. The beautiful stones,
amethyst, agate, chalcedony, and jasper, are all different kinds of
quartz.

Another chief material in rocks is a white metal called _aluminium_.
United to oxygen it becomes alumina, the chief substance in clay.
Rocks of this kind--such as clays, and also the lovely blue gem,
sapphire--are called Argillaceous Rocks, from the Latin word for clay,
and belong to the second class. Such rocks keep fossils well.

Another is _calcium_. United to oxygen and carbonic acid, it makes
carbonate of lime, the chief substance in limestone; so all limestones
belong to the third class of Calcareous or Lime Rocks.

Other important materials may be mentioned, such as _magnesium,
potassium, sodium, iron, carbon, sulphur, hydrogen, chlorine,
nitrogen_. These, with many more, not so common, make up the remaining
quarter of the earth-crust.

Carbon plays as important a part in animal and vegetable life as
silicon in rocks. Carbon is most commonly seen in three distinct
forms--as charcoal, as black-lead, and as the pure brilliant diamond.
Carbon united, in a particular proportion, to oxygen, forms carbonic
acid; and carbonic acid united, in a particular proportion, to lime,
forms limestone.

_Hydrogen_ united to oxygen forms water. Each of these two gases is
invisible alone, but when they meet and mingle they form a liquid.

_Nitrogen_ united to oxygen and to a small quantity of carbonic acid