ocean-floor, to become quietly part of a mass of shells. Sometimes,
where the ocean is shallow enough for the waves to have power below,
or where land currents can reach, they are washed about, and thrown
one against another, and ground into fine powder; and the fine powder
becomes in time, through different causes, solid rock.

[Illustration: CORAL POLYP.]

Limestone is made in another way also. In the warm waters of the South
Pacific Ocean there are many islands, large and small, which have
been formed in a wonderful manner by tiny living workers. The workers
are soft jelly-like creatures, called polyps, who labor together in
building up great walls and masses of coral.

[Illustration: CORAL ISLAND.]

[Illustration: YOUNG CORAL POLYP ATTACHED TO A ROCK AND EXPANDED.]

They never carry on their work above the surface of the water, for in
the air they would die. But the waves break the coral, and heap it up
above high-water mark, and carry earth and seeds to drop there till at
length a small low-lying island is formed.

The waves not only heap up broken coral, but they grind the coral into
fine powder, and from this powder limestone rock is made, just as it
is from the powdered shells of rhizopods. The material used by the
polyps in building the coral is chiefly lime, which they have the
power of gathering out of the water, and the fine coral-powder,
sinking to the bottom, makes large quantities of hard limestone. Soft
chalk is rarely, if ever, found near the coral islands.