A rock, on the other hand, is a mere aggregate of mineral particles, or of molecules that, under proper conditions, would group themselves to form mineral species. It may consist entirely of granules or crystals of one species; but the structures in these have no common orientation, as they would have in a single large continuous crystal. The rock itself has no crystalline form, and any structures that simulate such forms will be found on measurement to have none of the regularity that characterises genuine crystals. A rock, moreover, formed of several mineral species in association will by no means possess a constant chemical composition, and the variations from point to point form a feature of especial interest in the study of igneous masses, of sediments deposited on a shore, or of alluvium in a valley stretching far between the hills.
In the pages that follow we hope, then, to bear in mind the relations of rocks to the earth and to ourselves. Like the ancient Romans, we build our cities with huge blocks and slabs brought from crystalline masses oversea. We now tunnel, for our commercial highways, through the complex cores of mountain-chains. Everywhere rocks are our foundations, throughout our travels or in our settled homes. They rise as obstacles against us, or they spread before us fields of fertile soil. Some knowledge of them is part of the general body of culture that makes us, in the best sense, citizens of the world.
LIST OF THE COMMON MINERALS THAT FORM ROCKS
Actinolite. See [Amphiboles].
Albite. See [Felspars].
Amphiboles. A series of silicates with the general formula RSiO3, where R is magnesium, iron or calcium; in many, such as the common species Hornblende, molecules occur in addition in which aluminium and triad iron are introduced. Hornblende thus consists of m (Mg, Fe″, Ca) SiO3. n (Mg, Fe″) (Al, Fe‴)2 SiO6. Actinolite is a non-aluminous amphibole occurring in needle-like prisms. The amphiboles crystallise in prisms having angles of about 56° and 124°. See [Pyroxenes].
Anatase. See [Rutile].
Andalusite. Aluminium silicate, Al_{2}SiO_{5}, crystallising in the rhombic system. Sillimanite consists also of Al2SiO5 and is rhombic, but crystallises with different fundamental angles.
Anorthite. See [Felspars].
Apatite. Calcium phosphate, with fluorine, or sometimes chlorine, (CaF)Ca4(PO4)3 = 3Ca3(PO4)2. CaF2.