[Illustration: 1. WHITE CORAL. 2. PORTION OF A BRANCH (MAGNIFIED).]

Limestones are formed in the same manner from the grinding up of other
sea-shells and fossils, various in kind; the powder becoming gradually
united into solid rock.

There is yet another way in which limestone is made, quite different
from all these. Sometimes streams of water have a large quantity of
lime in them; and these 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.