MINERALS OF IGNEOUS ROCKS 253 



There are 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, 

 i portion remains free and takes the form of quartz (SiO 2 ). If there is an excess 

 of the basic oxides, the weakest one is usually left out of the combination. This is 

 commonly an iron oxide (FesO.i), called magnetite. It is a singular fact that quartz 

 forms in some cases where there is no excess of silica, and magnetite where there 

 is no excess of base. Quartz (free acid anhydride) and magnetite (free basic oxide) 

 may occur in the same rock. The explanation of this is yet to be found. The 

 oxides of silicon and iron form rather important exceptions to the general state- 

 ment that igneous rocks are made up mostly of silicates; but, thus qualified, the 

 statement expresses the essential truth. 



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-silicales or ortho-silicates (ratio of oxygen of base to oxygen of silica, 1:1), 

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

 poly-silicates (ratio 1 13 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 one by one, the results would still 

 remain comparatively simple; but instead, it may unite 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, so that 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 deter- 

 minate limit to the number of combinations that may arise. 



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

 minerals in igneous rocks is large. Geology deals with these minerals as constit- 

 uents of the earth, but only a few of them are so abundant as to require special 

 nntu v here. It may be remarked also that, as they occur in the rocks, only a few 

 of Uu-m can be identified by simple inspection, partly because some of them look 

 much alike, and partly because many of the crystals are minute. 



Summary of salient facts. The salient facts are, (i) that out of 

 the yo-odd chemical elements now known in the earth, eight form 

 the chief part of it; (2) that one of these elements uniting with the 

 rest forms nine leading oxides; (3) that one of these oxides acts as 

 an acid and the rest as bases; (4) that by their combination they 

 form a series of silicates of which a few are easily chief; (5) that 

 these silicates crystallize into a multitude of minerals of which again 

 a few are chief; and (6) that these minerals are aggregated in various 

 ways to form rocks. Possessed of these leading ideas, we are pre- 

 pared to turn to the consideration of some of the conditions under 

 which these combinations take place in the formation of rocks from 

 liquid magmas. 



Principal minerals of igneous rocks. A few minerals make up 

 the mass of igneous rocks. These few are quartz, the feldspars, the 

 ferro-magnesian minerals (amphiboles, pyroxenes, micas), and the 



