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.”

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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....

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[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 a piece of coal, you will find, more or less clearly, markings like those which are seen in a piece of wood.  Sometimes they are very distinct indeed.  Coal abounds in impressions of leaves, ferns, and stems, and fossil remains of plants and tree-trunks are found in numbers in coal-seams.