gas forms our atmosphere.

Rocks of pure flint, pure clay, or pure lime, are rarely or never met
with. Most rocks are made up of several different substances melted
together.

* * * * *

In the fire-built rocks no remains of animals are found, though in
water-built rocks they abound. Water-built rocks are sometimes divided
into two classes--those which only contain occasional animal remains,
and those which are more or less built up of the skeletons of animals.

[Illustration: AMIBA PRINCEPS, ONE OF THE MANY ORDERS OF THE RHIZOPODA
CLASS, MAGNIFIED ONE HUNDRED TIMES.]

There are some exceedingly tiny creatures inhabiting the ocean, called
Rhizopods. They live in minute shells, the largest of which may be
almost the size of a grain of wheat, but by far the greater number are
invisible as shells without a microscope, and merely show as fine
dust. The rhizopods are of different shapes, sometimes round,
sometimes spiral, sometimes having only one cell, sometimes having
several cells. In the latter case a separate animal lives in each
cell. The animal is of the very simplest as well as the smallest kind.
He has not even a mouth or a stomach but can take in food at any part
of his body.

[Illustration: RHIZOPODS (MAGNIFIED).]

These rhizopods live in the oceans in enormous numbers. Tens of