13. _Minerals._--Now, any inorganic substance which has a definite chemical composition, and crystallises in a definite crystalline or geometric form, is termed a _mineral_. Having once discovered that quartz is composed of silicon and oxygen--that is, silica--and that the faces of its crystals are arranged in a certain definite order, we may be quite sure that any mineral which has not this composition and form cannot be quartz. And so on with mica and felspar, and every other mineral. The study of the geometric forms assumed by minerals (crystallography) forms a department of the science of mineralogy. But, in the great majority of cases, the mineral ingredients of the rocks are either so small individually, or so broken, and rounded, and altered, that crystallography gives comparatively little aid to the practical geologist in the field. He has, therefore, recourse to other tests for the determination of the mineral constituents of rocks. Many of these tests, however, can only be applied by those who have had long experience. The simplest and easiest way for the student to begin is to examine the forms and appearance of the more common minerals in some collection, and thereafter to accustom his eye to the aspect presented by the same minerals when they are associated together in rocks, of which illustrative specimens are now to be met with in most museums. The microscope is largely employed by geologists for determining the mineralogical composition of certain rocks; and, indeed, many rocks can hardly be said to be thoroughly known until they have been sliced and examined under the microscope, and analysed by the chemist. But with a vast number such minute examination is not required, the eye after some practice being able to detect all that is needful to be known. [A] There are various kinds of felspar; the one referred to above is _orthoclase_, or potash-felspar. ROCK-FORMING MINERALS.