weathering. Soil water trickling over a granite
cliff has little effect on the quartz particles; but it dissolves out
some of the silicon. The bits of feldspar are even more resistant to
water than quartz is, but the air causes them to decay rapidly, and
finally to fall away in a sort of mealy clay. Mica, like feldspar,
decays easily. Its substance is dissolved by water and carried away to
become a kind of clay. The hornblende rusts away chiefly under the
influence of moist air and trickling water.

We think of granite as a firm, imperishable kind of rock, and use it in
great buildings like churches and cathedrals that are to stand for
centuries. But the faces that are exposed to the air suffer, especially
in regions having a moist climate. The signs of decay are plainly
visible on the outer surfaces of these stones. Fortunate it is that the
weathering process cannot go very deep.

The glassy polish on a smooth granite shaft is the silicon which acts as
a cement to bind all the particles together. It is resistant to the
weather. A polished shaft will last longer than an unpolished one.

Granites differ in colouring because the minerals that compose them, the
feldspars, quartzes, micas, and hornblendes, have each so wide a range
of colour. Again, the proportions of the different mineral elements vary
greatly in different granites. A banded granite the colours of which
give it a stratified appearance is called a gneiss.

We have spoken before of the seventy elements found in the earth's
crust. A mineral is a union of two or more of these different elements;
and we have found four minerals composing our granite rock. It may be
interesting to go back and inquire what elements compose these four
minerals. Quartz is made of silicon and oxygen. Feldspar is made of
silicon, oxygen, and aluminum. Mica is made of silicon, oxygen, and
carbon, with some mingling of potassium and iron and other elements in
differing proportions. Hornblende is made of silicon, oxygen, carbon,
and iron.

The crumbling of a granite rock separates the minerals that compose it,
reducing some to the condition of clay, others to grains of sand. Some
of the elements let go their union and become free to form new unions.
Water and wind gather up the fragments of crumbling granite and carry
them away. The feldspar and mica fragments form clay; the quartz
fragments, sand. All of the sandstones and slates, the sand-banks and
sand beaches, are made out of