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 wide reaches of white chalk cliffs have been made.  Chalk cliffs and chalk layers extend from Ireland, through England and France, as far as to the Crimea.  In the south of Russia they are said to be six hundred feet thick.  Yet one cubic inch of chalk is calculated to hold the remains of more than one million rhizopods.  How many countless millions upon millions must have gone to the whole structure!  How long must the work of building up have lasted!

[Illustration:  THREE POLYPS OF CORAL.]

These little shells do not always drop softly and evenly to the 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.

[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.