2i6 The Realm of Nature CHAP. 



substances, sometimes elements, although usually com- 

 pounds, and they are known as minerals. While the term 

 mineral is restricted to the pure constituents of rocks, the 

 word mineral is often used to include everything useful 

 found in the Earth's crust. 



286. Rock-forming Minerals. The crystalline mine- 

 rals which make up many rocks must have formed slowly 

 by the combination of their elements or the decomposition 

 of other compounds. Some were evidently deposited from 

 solution in water, as, for example, rocksalt and gypsum 

 (calcium sulphate) ; in both these and in some other unim- 

 portant instances rocks may be composed of only one 

 mineral. Other rocks have evidently crystallised from 

 a state of fusion ; in basalt, for example, the small 

 crowded and imperfect crystals bear evidence to rapid 

 cooling and solidification. Rocks, like obsidian, which show 

 a vitreous or glassy texture, quite smooth, and it may be 

 free from any appearance of crystals, have evidently been 

 cooled still more quickly, so that crystallisation could not 

 take place. After a rock has been formed its minerals may 

 undergo chemical changes. The process of weathering, or 

 slow alteration of rocks in air ( 3 1 o), affects some minerals 

 more than others. Many new kinds of mineral result from 

 chemical change brought about by the absorption of oxygen 

 (oxidation), by the absorption of water (hydration) pro- 

 ducing zeolites, etc., or by the formation and removal of 

 some product (decomposition). Mineralogists recognise 

 about 800 different minerals, most of which, however, occur 

 in very small quantities. Sixty or seventy only can be 

 considered important as rock formers. Indeed the bulk of 

 the rocky crust may be said to be composed of the following 

 minerals, and those resulting from their alteration felspar, 

 quartz, mica, amphibole, pyroxene, and iron oxides. 



287. Igneous Rocks, as a class, include all that have 

 solidified from a state of fusion or have been formed by the 

 accumulation of fragments thrown out by volcanoes. Most 

 of them are dense and hard ; they have a glassy or crystal- 

 line texture, and the minerals of which they are composed 

 are almost invariably silicates or silica. Silica as flint, 



