ion, 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
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
millions are ever coming into existence, living out their tiny lives,
dying, and sinking to the bottom.

There upon the ocean-floor gather their remains, a heaped-up multitude
of minute skeletons or shells, layer forming over layer.

It was long suspected that the white chalk cliffs of England were
built up in some such manner as this through past ages. And now at
length proof has been found, in the shape of mud dredged up from the
ocean-bottom--mud entirely composed of countless multitudes of these
little shells, dropping there by myriads, and becoming slowly joined
together in one mass.

Just so, it is believed, were the white chalk cliffs built--gradually
prepared on the ocean-floor, and then slowly or suddenly upheaved, so
as to become a part of the dry land.

Think what the enormous numbers must have been of tiny living
creatures, out of whose shells the