11. Having learned that all the rocks met with at the surface of the earth's crust are capable of being arranged under a few classes, we have now to investigate the matter more in detail. It will be observed that the classification adopted above is based chiefly upon the external characters of the constituent ingredients of the rocks, and the mode in which these particles have been collected. In some rocks the component materials are crystalline, in others they are rounded and worn; in one case they have been brought together by precipitation from an aqueous solution, or they have crystallised out from a mass of once molten matter; in another case their collection and intimate association is due to the mechanical action of the atmosphere or of water, or to the agency of the organic forces. We have next to inquire what is the nature of those crystals and particles which are the ingredients of the rocks? The answer to this question properly belongs to the science of mineralogy, with which, however, the geologist must necessarily make some acquaintance. 12. _Granite--its composition._--It will tend to simplify matters if we begin our inquiry by selecting for examination some familiar rock, such as _granite_. This rock, as one sees at a glance, is crystalline, nor is it difficult to perceive that three separate kinds of ingredients go to compose it. One of these we shall observe is a gray, or it may be, clear glassy-looking substance, which is hard, and will not scratch with a knife; another is of a pink, red, gray, or sometimes even pale green colour, and scratches with difficulty; while the third shews a glistering metallic lustre, and is generally of a brownish or black colour. It scratches easily with the knife, and can be split up into flakes of extreme thinness. If the granite be one of the coarse-grained varieties, we shall notice that these three ingredients have each more or less definite crystalline forms; so that they are not distinguished by colour and hardness alone. The metallic-looking substance is _mica_; the hard gray, or glassy and unscratchable ingredient is _quartz_; and the remaining material is _felspar_. The mineralogist's analysis of granite ends here. But there is still much to be learned about quartz, felspar, and mica; for, as the chemist will tell us, these are not 'elementary substances.' Quartz is a compound, consisting of two elements, one of which is a non-metallic body (silicon), and the other an invisible gas (oxygen). Felspar[A] is a still more complex compound, being made up of two metals (potassium, aluminium) and one non-metallic body (silicon), each of which is united to an invisible gas (oxygen). Mica, again, contains no fewer than four metals (potassium, magnesium, iron, calcium) and one non-metallic body (silicon), each of which is in like manner chemically united to its share of oxygen. Thus the rock-forming substances, quartz, felspar, and mica, have each a definite chemical composition.