Triclinic System.—Three unequal axes, all crossing obliquely. Example, oblique rhomboidal prism. Hexagonal System.—Three equal axes lying in one plane and intersecting at angles of 60°, and a fourth axis crossing each of these at right angles and longer or shorter. Example, hexagonal prism. By the truncation and bevelment of the angles and edges of these fundamental forms a vast variety of secondary forms are produced. The limits of the guide will not permit us to follow this topic farther; but it may be added that for the proper elucidation of even the simpler crystalline forms the teacher should be provided with a set of wooden crystal models and Dana’s “Text-Book of Mineralogy.” The crystallization of a mineral may be manifested in two ways: first, by the regularity of its internal structure or molecular arrangement, as shown by cleavage and the polarization of transmitted light; and, second, by the regularity of external form which follows, _under favorable conditions_, as a necessary consequence of symmetry in the arrangement of the molecules. When a mineral is entirely devoid of crystalline structure, both externally and internally, it is said to be _amorphous_. Perfect and distinct crystals are the rare exception, most mineral specimens being simply aggregates of imperfect crystals. In such cases, and when the mineral is amorphous, the _structure_ of the _mass_ may be:— Columnar or fibrous. Lamellar, foliaceous, or micaceous. Granular.—When the grains or crystalline particles are invisible to the naked eye the mineral is called _impalpable_, _compact_, or _massive_. And the _external form_ of the mass may be:— Botryoidal, having grape-like surfaces. Stalactitic, forming stalactites or pendant columns. Amygdaloidal or Concretionary, forming separate globular masses in the enclosing rock. Dendritic, branching or arborescent. 3. _Hardness._—By the hardness of a mineral we mean the resistance which it offers to abrasion. But hardness is a purely relative term, calcite, for example, being hard compared with talc, but very soft compared with quartz. Hence mineralogists have found it necessary to select certain minerals to be used as a standard of comparison for all others, and known as the _scale of hardness_. These are arranged at nearly equal intervals all the way from the softest mineral to the hardest, as follows:— _Scale of Hardness._ (1) Talc. (2) Gypsum. (3) Calcite. (4) Fluorite. (5) Apatite. (6) Orthoclase. (7) Quartz. (8) Topaz or Beryl. (9) Corundum. (10) Diamond.