For, to begin with, the history itself is written in a strange language, a language which man is only just beginning to spell out and understand.  And this is only half the difficulty with which we have to struggle.

If a large and learned book were put before you and you were set to read it through, you would perhaps, have no insurmountable difficulty, with patience and perseverance, in mastering its meaning.

But how if the book were first chopped up into pieces, if part of it were flung away out of reach, if part of it were crushed into a pulp, if the numbering of the pages were in many places lost, if the whole were mixed up in confusion, and if then you were desired to sort, and arrange, and study the volume?

Picture to yourself what sort of a task this would be, and you will have some idea of the labors of the patient geologist.

Rocks may be divided into several kinds or classes.  For the present moment it will be enough to consider the two grand divisions—­Stratified rocks and Unstratified rocks.

Unstratified rocks are those which were once, at a time more or less distant, in a melted state from intense heat, and which have since cooled into a half crystallized state; much the same as water, when growing colder, cools and crystallizes into ice.  Strictly speaking, ice is rock, just as much as granite and sandstone are rock.  Water itself is of the nature of rock, only as we commonly know it in the liquid state we do not commonly call it so.

[Illustration:  UNSTRATIFIED ROCK.—­A VOLCANIC BLOCK.]

“Crystallization” means those particular forms or shapes in which the particles of a liquid arrange themselves, as that liquid hardens into a solid—­in other words, as it freezes.  Granite, iron, marble, are frozen substances, just as truly as ice is a frozen substance; for with greater heat they would all become liquid like water.  When a liquid freezes, there are always crystals formed, though these are not always visible without the help of a microscope.  Also the crystals are of different shapes with different substances.

If you examine the surface of a puddle or pond, when a thin covering of ice is beginning to form, you will be able to see plainly the delicate sharp needle-like forms of the ice crystals.  Break a piece of ice, and you will find that it will not easily break just in any way that you may choose, but it will only split along the lines of these needle-like crystals.  This particular mode of splitting in a crystallized rock is called the cleavage of that rock.

Crystallization may take place either slowly or rapidly, and either in the open air or far below ground.  The lava from a volcano is an example of rock which has crystallized rapidly in the open air; and granite is an example of rock which has crystallized slowly underground beneath great pressure.