hard, glassy, or dull brick.
From aqueous rock materials, fire has produced a metamorphic rock.
Volcanic action is imitated in this common, simple process of
brickmaking.

Milwaukee brick is made of clay that has no iron in it. For this reason
the bricks are yellow after baking. Most bricks are red, on account of
the iron in the clay, which is converted into a red oxide, or rust, by
water and heat.

Common flower pots and the tiles used in draining wet land are not
glazed, as hard-burned bricks are. The baking of these clay things is
done with much less heat. They are left somewhat porous. But the tiles
of roofs are baked harder, and get a surface glaze by the melting of the
glassy particles of the sand.

As bricks vary in colour and quality according to the materials that
compose them, so the metamorphic rocks differ. The white sand one sees
on many beaches is largely quartz. This is the substance of pure, white
sandstone. Metamorphism melts the silica into a glassy liquid cement;
the particles are bound close together on cooling. The rock becomes a
white, granular quartzite, that looks like loaf sugar. If banded, it is
called gneiss. Such rocks take a fine polish.

Pure limestone is also white and granular. When metamorphosed by heat,
it becomes white marble. The glassy cement that holds the particles of
lime carbonate shows as the glaze of the polished surface. It is silica.
One sees the same mineral on the face of polished granite.

Clays are rarely pure. Kaolin is a white clay which, when baked, becomes
porcelain. China-ware is artificially metamorphosed kaolin. In the
early rocks the clay beds were transformed by heat into jasper and
slates. In beds where clay mingled with sand, in layers, gneiss was
formed. If mica is a prominent element, the metamorphic rock is easily
parted into overlapping, scaly layers. It is a mica schist. If
hornblende is the most abundant mineral, the same scaly structure shows
in a dark rock called hornblende schist, rich in iron. A schist
containing much magnesia is called serpentine.

The bricks of the wall, the tiles on the roof, the flower pots on the
window sill, and the dishes on the breakfast table, are examples of
metamorphic rocks made by man's skill, by the use of fire and water
acting on sand and clay. Pottery has preserved the record of
civilization, from the making of the first crude utensils by cave men to
the finest expression of decorative art in glass and porcelai