hese as they flow will drop layers of lime which
harden into rock. Or a lime-laden spring, making its way through the
roof of an underground cavern, will leave all kinds of fantastic
arrangements of limestone wherever its waters can trickle and drip.
Such a cavern is called a "stalactite cave."

* * * * *

So there are different kinds of fossil rock-making. There may be rocks
made of other materials, with fossil simply buried in them. There may
be rocks made entirely of fossils, which have gathered in masses as
they sank to the sea-bottom, and have there become simply and lightly
joined together. There may be rocks made of the ground-up powder of
fossils, pressed into a solid substance or united by some other
substance.

Rocks are also often formed of whole fossils, or stones, or shells,
bound into one by some natural soft sticky cement, which has gathered
round them and afterwards grown hard, like the cement which holds
together the stones in a wall.

The tiny rhizopods (meaning root foot) which have so large a share in
chalk and limestone making, are among the smallest and simplest known
kinds of animal life.

There are also some very minute forms of vegetable life, which exist
in equally vast numbers, called Diatoms. For a long while they were
believed to be living animals, like the rhizopods. Scientific men are
now, however, pretty well agreed that they really are only vegetables
or plants.

The diatoms have each one a tiny shell or shield, not made of lime
like the rhizopod-shells, but of flint. Some think that common flint
may be formed of these tiny shells.

Again, there is a kind of rock called Mountain Meal, which is entirely
made up of the remains of diatoms. Examined under the microscope,
thousands of minute flint shields of various shapes are seen. This
rock, or earth, is very abundant in many places, and is sometimes used
as a polishing powder. In Bohemia there is a layer of it no less than
fourteen feet thick. Yet so minute are the shells of which it is
composed, that one square inch of rock is said to contain about four
thousand millions of them. Each one of these millions is a separate
distinct fossil....

* * * * *

[Illustration: SUCCESSION OF BURIED COAL-GROWTHS AND ERECT
TREE-STUMPS. SYDNEY, CAPE BRETON.

_a._ Sandstone, _b._ Shales, _c._ Coal-seams, _d._ Bed containing
Roots and Stumps _in situ_.]

If you examine carefully