398 GEOLOGY. 



general idea is a very useful one and represents a most important truth; 

 but in its use we must not forget that there are many exceptions. Sul- 

 phur, 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 sihcates; and these 

 minor bases unite with the minor acids to form many more or less rare 

 minerals. Again, there are native metals in some igneous rocks. 

 But altogether these hardly reach more than one or two percent, oi 

 the whole. 



There are, however, two exceptions of more importance. In the 

 molten magma the acid and basic elements are not always evenly matched. 

 When there is an excess of silica, a portion remains free and takes the 

 form of quartz (SiO^). If there is an excess of the basic oxides, the 

 weakest one is usually left out of the combination. This is commonly 

 the iron oxide, which then usually takes the form of magnetite (FegOJ. 

 It is a singular fact that quartz often forms when there is no excess of 

 silica, and magnetite when there is no excess of base. Quartz (free acid 

 anhydride) and magnetite (free basic oxide) sometimes occur in the same 

 rock. The explanation for this is yet to be found. These form rather 

 important exceptions to the generalization that the igneous rocks are 

 mostly made up of sihcates, but, thus quaUfied, it expresses the essen- 

 tial truth, and has the merit of embodying the central chemical fact 

 relative to these rocks. 



Sources of complexity. — But here simphcity ends. As we pass on 

 to the specific silicates that are formed, we encounter several sources of 

 complexity. In the first place, the siHca unites with the bases in differ- 

 ent ratios and thus gives rise to unisilicates or orthosilicates (ratio of 

 oxygen of bases to oxygen of silica, 1: 1), subsilicates (ratio more than 1), 

 bisilicates (ratio 1:2), trisihcates or polysihcates (ratio 1:3 or higher), 

 and combinations of these. All the bases are not known to combine in 

 all these ways, but many do in more than one of them. Still, if the 

 siHca were content to unite with each of the bases by itself alone, the 

 results would remain comparatively simple ; but instead of this it unites 

 with two or more at the same time; and, more than that, it unites 

 with them in varying amounts. The case would still remain measur- 

 ably simple if these chemical compounds always crystallized out by 

 themselves, each compound forming one mineral, and but one; but 

 the different silicates have the confusing habit of crystallizing together 



