closely cemented by silica, the chemical substance of which quartz
is made, are known as quartzites.

We are now prepared to understand how sandstone is affected by the
action of the weather. On ledges where the rock is exposed to view
its surface is more or less discolored and the grains are loose
and may be rubbed off with the finger. On gentle slopes the rock
is covered with a soil composed of sand, which evidently is
crumbled sandstone, and dark carbonaceous matter derived from the
decay of vegetation. Clearly it is by the dissolving of the cement
that the rock thus breaks down to loose sand. A piece of sandstone
with calcareous cement, or a bit of old mortar, which is really an
artificial stone also made of sand cemented by lime, may be
treated in a test tube with hydrochloric acid to illustrate the
process.

A LIMESTONE QUARRY. Here also we find the rock stratified and
jointed (Fig. 2). On the quarry face the rock is distinctly seen
to be altered for some distance from its upper surface. Below the
altered zone the rock is sound and is quarried for building; but
the altered upper layers are too soft and broken to be used for
this purpose. If the limestone is laminated, the laminae here have
split apart, although below they hold fast together. Near the
surface the stone has become rotten and crumbles at the touch,
while on the top it has completely broken down to a thin layer of
limestone meal, on which rests a fine reddish clay.

Limestone is made of minute grains of carbonate of lime all firmly
held together by a calcareous cement. A piece of the stone placed
in a test tube with hydrochloric acid dissolves with brisk