24 GEOLOGY 



as lead, zinc, and copper, are of little importance quantitatively. 

 We do not know just how the various elements of igneous rock are 

 united in the liquid lava, but their combinations after the lava has 

 become solid may be determined. 



Union of elements. In a general study of the igneous rocks wo 

 may for the present neglect all but the first eight of the elements. 

 Out of these elements come various chemical combinations when 

 the lava solidifies; out of the combinations come the various min- 

 erals; and from the combinations of the minerals come various 

 kinds of rocks. The union of oxygen with the other seven elements 

 may be taken as a fundamental step in this series of combinations. 

 The result is the following oxides: Silica (SiO 2 ), alumina (A1 2 O 3 ), 

 the ferrous, ferric, and magnetic oxides (FeO, Fe 2 O 3 , and Fe 3 O 4 ), 

 magnesia (MgO), calcium oxide (lime) (CaO), soda (Na 2 0), and 

 potash (K 2 O). The oxygen sometimes unites in proportions differ- 

 ent 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 aluminum, act as basic oxides. The 

 proportion of silica in igneous rocks is so significant that all such 

 rocks are sometimes divided into three groups, 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,Si0 2 , or CaSiO 3 . The union of silica and magnesia forms 

 magnesium silicate, MgO,SiO 2 , or MgSiO 3 . 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 mostly silicates of the 

 seven leading basic oxides; that is, the oxides of alumina, potash, 

 soda, lime, magnesia, and iron. This general idea is a very useful 

 one, and represents a most important truth; but in its use \\-< 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 



