252 MATERIALS AND THEIR ARRANGEMENT 



Potassium (K) 2.32 



Titanium (Ti) 41 



Hydrogen (H) .17 



Carbon (C) 12 



Phosphorus (P) .09 



Manganese (Mn) .07 



Sulphur (S) .07 



It will be seen that only eight of the elements enter into the 

 earth's crust to the extent of one percent, and no other one reaches 

 half of one per cent. Many elements that are of great importance 

 in the affairs of men occur in quantities too small to be estimated in 

 percentages. The precious metals, such as platinum, gold, and 

 silver, and even some of the more common ones, as lead, zinc, and 

 copper, are of little importance quantitatively. 



Union of elements. For present purposes we may neglect all but the first 

 eight of the elements mentioned above. Out of these elements come various 

 chemical combinations when the lava solidifies; out of these combinations come the 

 various minerals; and from combinations of minerals come various kinds of rocks. 

 The union of oxygen with the other seven elements may be taken as a funda- 

 mental step in this series of combinations. The result is the following oxides: 

 Silica (SiO2), alumina (AloOs), the ferrous, ferric, and magnetic oxides (FeO, 

 Fe 2 Os, and FeaOs), magnesia (MgO), calcium oxide (lime) (CaO), soda (Na 2 O), 

 and potash (K 2 O). The oxygen sometimes unites in proportions different from 

 those here given, but exceptions may be neglected here. 



Of these nine oxides, silica acts as an acid, or more strictly as an acid anhydride. 

 All the rest except the magnetic oxide of iron, and sometimes the oxide of alumi- 

 num, act as basic oxides. The proportion of silica in igneous rocks is so significant 

 that all such rocks are sometimes grouped into three classes, as follows: those 

 with more than 65% of silica are acidic; those containing 55 to 65%, intermediate; 

 and those containing less than 55%, basic. 



The union of silica (SiO 2 ) and lime (CaO) forms calcium silicate, CaO,SiO2, 

 or CaSiOa.. The union of silica and magnesia forms magnesium silicate, MgO,SiO 2 , 

 or MgSiOs. Corresponding unions of silica and the other oxides named, give rise 

 to other silicates. 



Formation of minerals. Since but one of the leading oxides (silica) that 

 abound in the average lava plays the part of an acid, a very simple conception of the 

 general nature of igneous rocks may be reached by noting that they are com- 

 posed mostly of silicates of the eight leading basic oxide those of alumina, potash, 

 soda, lime, magnesia, and iron. This general idea represents a most important 

 truth; but in its use we must not forget that there are many exceptions. Sulphur, 

 phosphorus, chlorine, and other elements unite with the bases to form sulphates, 

 sulphides, phosphates, phosphides, chlorides, etc. So also there are many minor 

 bases that form silicates; and these minor bases unite with minor acids to form 

 many of the rarer minerals. Again, there are native metals in some igneous rocks; 

 but altogether these minor compounds hardly reach more than one or two per 

 cent of the whole. 



