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Rocks are formed of various materials called minerals. If we take a piece of sandstone rock, or a piece of granite, we shall probably be able to notice that the rock is made up of different substances.

On looking at a piece of sandstone, for example, especially if we use a magnifying glass, we see that it is composed of little rounded grains of a glassy-looking substance cemented together. In some specimens these grains are larger than in others. This cementing material is not the same in all sandstones, but in our specimen it is formed of calcium carbonate, for when we drop a little diluted hydrochloric acid on the rock there is an effervescence. The cementing material is dissolved, but the little rounded grains, which consist of quartz, are not affected by the acid. The sandstone, then, consists of quartz grains cemented together by calcium carbonate. It is called a calcareous sandstone.

Now take a piece of granite, and break it with a hammer to get a clean-cut face. On looking at this face we see that the rock is made up of three different substances.

One of these has a glassy appearance like the grains in the sandstone, and is so hard that we cannot scratch it with a knife. This is quartz. Another of the substances is of a dull white or pinkish color. It lies in long, smooth-faced crystalline patches, which easily break along a number of smooth parallel surfaces having a pearly lustre. It can be scratched with difficulty by the point of a knife. This substance is called felspar. The third substance consists of bright glistening plates, sometimes of a dark color, which can be easily scratched, and which readily split into transparent leaves. This is mica. Notice that these substances do not occur in any definite order, but are scattered about through the stone irregularly, the felspar occurring in some specimens in larger crystals than in others.