I. _Flint Rocks_; II. _Clay Rocks_; III. _Lime Rocks_.

This is not a book in which it would be wise to go closely into the
mineral nature of rocks. Two or three leading thoughts may, however,
be given.

Does it not seem strange that the hard and solid rocks should be in
great measure formed of the same substances which form the thin
invisible air floating around us?

Yet so it is. There is a certain gas called Oxygen Gas. Without that
gas you could not live many minutes. Banish it from the room in which
you are sitting, and in a few minutes you will die.

This gas makes up nearly one-quarter by weight of the atmosphere round
the whole earth.

The same gas plays an important part in the ocean; for more than
three-quarters of water is _oxygen_.

It plays also an important part in rocks; for about half the material
of the entire earth's crust is oxygen.

Another chief material in rocks is _silicon_. This makes up
one-quarter of the crust, leaving only one-quarter to be accounted
for. Silicon mixed with oxygen makes silica or quartz. There are few
rocks which have not a large amount of quartz in them. Common flint,
sandstones, and the sand of our shores, are made of quartz, and
therefore belong to the first class of Silicious or Flint Rocks.