MATERIALS OF THE EARTH 25 



are many minor bases that form silicates; 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 alto- 

 gether these minor compounds hardly reach more than one or two 

 per cent of the whole. 



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

 the liquid lava 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 (Si0 2 ). 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 (Fe 3 O 4 ). 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 ex- 

 planation of this is yet to be found. The oxides of silicon and iron 

 form rather important exceptions to the general statement that 

 igneous rocks are made up mostly of silicates, but, thus qualified, 

 the statement expresses the essential truth. 



Sources of complexity. But here simplicity ends, and the 

 sources of complexity are several. In the first place, silica unites 

 with the bases in different ratios, and thus gives rise to uni-silicates 

 or ortho-silicates (ratio of oxygen of base to o'xygen of silica, 1:1), 

 sub-silicates (the above ratio more than 1), bisilicates (ratio 1:2), 

 tri-silicates or poly-silicates (ratio 1:3 or higher), etc. All the bases 

 are not known to combine in all these ways, but many do in more 

 than one. 



If the silica united with each of the bases by itself alone, the 

 results would still remain comparatively simple; but instead, it 

 sometimes unites with two or more at the same time. Thus we 

 may have an aluminum-calcium silicate. Not only this, but the 

 different silicates may crystallize together in the same mineral. 

 Thus a crystal may be made up of alternating layers of different 

 silicates. As such alternations are not governed by any known 

 mathematical law, there is no determinate limit to the number 

 of combinations that may arise. 



As a result of all this fertility of combination, the total number 



