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.

Stratified rocks, on the contrary, which make up a very large part of