THE COMPOSITION OF THE EARTH 19 



Canada, among the western mountains, and elsewhere. They 

 are found to differ among themselves in many ways. Their 

 particles are of different kinds, sizes, and shapes; some of 

 them are held together weakly, others firmly. Some rocks 

 are arranged in distinct layers, while others are not. Since 

 these and other differences are largely the result of the different 

 ways in which the rocks were formed, they have been classi- 

 fied in the first instance on the basis of origin. Rocks formed 

 by the solidification of lavas are Igneous Rocks (Fig. 4). Rocks 

 formed by the consolidation of sediments are Sedimentary 

 Rocks. Because the latter are usually arranged in layers or 

 strata, they are often called Stratified Rocks (Fig. 5). If the 

 character of an igneous or a sedimentary rock is radically 

 altered, it becomes a Metamorphic Rock (Fig. 6). The more 

 common rocks, and the minerals of which they are composed, 

 are discussed below. 



MINERALS 



The igneous rock shown in Figure 7 is made up of many angu- 

 lar particles of several distinct kinds, each of which has its own 

 constant characteristics. These particles can be separated, 

 and, when treated in the proper manner, may be divided 

 chemically into simpler things. Some of them, for example, 

 may be divided into oxygen and silicon. Although chemists 

 have been working with oxygen and silicon since their dis- 

 covery, it has been impossible to get any still simpler things 

 from them. They are accordingly called chemical elements. 

 While some 70 elements are found in rocks , only 8 are impor- 

 tant quantitatively. These are, in the order of abundance : 

 oxygen (0), silicon (Si), aluminum (Al), iron (Fe), calcium 

 (Ca), magnesium (Mg), sodium (Na), and potassium (K). 

 The first two make up three fourths of the earth's crust ; the 

 eight, 98.95 per cent. The oxygen unites with the other seven 

 elements to form the following oxides: silica (SiO 2 ), alumina 

 (A1 2 O 3 ), the iron oxides (FeO, Fe 2 O 3 , and Fe 3 O 4 ), lime (CaO), 



