THE ORIGIN AND DESCENT OF BOCKS. 397 



Union of elements. — In a general study of the igneous rocks we 

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

 Out of these elements spring various chemical combinations, and out 

 of these combinations spring the various minerals, and out of the com- 

 binations of minerals come the various 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: Sihca 

 (SiOj), alumina (AI2O3), ferrous, ferric, and magnetic oxide (FeO, Yefi^, 

 and FegOJ, magnesia (MgO), calcium oxide or lime (CaO), soda (NaaO), 

 and potash (K2O) . The oxygen sometimes unites in proportions different 

 from those here given, but such exceptions may be neglected in a gen- 

 eral study. We thus have nine leading oxides. Of these, sihca acts 

 as an acid, or more strictly according to the newer chemical view, as 

 an acid anhydride. All the rest, except the magnetic oxide of iron, 

 and sometimes the oxide of aluminum, act as basic oxides. 



In the older chemical philosophy these oxides were supposed to com- 

 bine by the simple union of an acid oxide with a basic oxide, and to 

 remain as oxide joined to oxide; thus silica (Si02) and hme (CaO) formed 

 silicate of hme (CaOjSiOj). The symbols express the idea better than 

 the words. This method is used in the older geological works and in 

 some of the later. But in the newer chemical doctrine, the oxides are 

 not believed to remain so distinct after their union, and the symbols are 

 written CaSiOg, and the compound is named calciimi silicate. Accord- 

 ing to the modern doctrine of solution, some of the calcium, siHcon, and 

 oxygen may exist as free ions in molten rock. The precise way in 

 which the elements are related to each other in these compounds can 

 scarcely be said to be known. For the general purposes of geology 

 it is most convenient to think of these oxides as uniting in the simple 

 fashion first named, and this involves no apparent geological error in 

 general studies, since they are oxides when they enter the compound,, 

 and if the compound is decomposed they usually come forth again as 

 oxides; but in closer studies more complex unions, attended by dissocia- 

 tions (ionization), must be recognized. 



Formation of minerals. — As but one of the leading oxides that 

 abound in an average magma plays the part of an acid, the sihca, 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 

 —alumina, potash, soda, lime, magnesia, and the iron oxides. This 



