EARTH'S CRUST — WASHINGTON. 275 



in the different rocks. Altogether, about 23 are to be found, and are 

 more or less commonly determined and recorded among the better- 

 class rock analyses. Indeed, as has been said by Dr. W. F. Hille- 

 brand, the foremost analyst of rocks, " a sufficiently careful exami- 

 nation of these [igneous] rocks would show them to contain all, or 

 nearly all, the known elements, not necessarily all in a giv r en rock, 

 but more than anyone has yet found." Proper study, therefore, of 

 the chemistry of igneous rocks, and their chemical analysis, if this 

 be complete as to the determination of all the constituents probably 

 present, is evidently a somewhat complicated matter, and one not 

 without difficulties of various kinds. 



From the many chemical analyses of rocks that have been made 

 since this was first attempted tery early in the nineteenth century 

 (the total number of published rock analyses is now about 12,000), 

 we have a good idea of what chemical constituents make up rocks, 

 their relative abundance, and their various ranges in percentage. 



By far the most important and generally the most abundant are 

 what are called the " major " constituents. These are nine in number 

 and, stated as oxides, are: Silica (Si0 2 ), alumina (A1 2 3 ), ferric 

 oxide (Fe 2 3 ), ferrous oxide (FeO), magnesia (MgO), lime (CaO), 

 soda (Na 2 0), potash (K 2 0), and water (H 2 0). 4 Together these nine 

 oxides make up about 98 per cent of igneous rocks, and all of them 

 are present in greater or less amount in practically every rock, so 

 that the amount of each must be determined in every chemical anal- 

 ysis of a rock that makes the slightest pretense to good quality. 



As the most abundant and essential rock minerals are either silica 

 or silicates, and as all igneous rocks, with the exception of some rare 

 and small iron ore bodies of magmatic origin, are consequently sili- 

 cate rocks, silica shows easily the highest maximum and the widest 

 range, both in extremes and in the usual run of occurrence. A few 

 igneous rocks are known that are composed almost entirely of quartz, 5 

 and the highest silica percentages recorded for igneous rocks are 

 98.77 and 97.65, in rocks from the Transvaal, while one from Cum- 

 berland (England), the border facies of a granitic mass, shows 96.16, 

 one from Massachusetts shows 93.38, and one from Arizona 92.59. 

 In general, however, the percentage of silica ranges from about 75 

 to about 34, and it drops to zero only in some " magmatic " iron-ore 

 bodies. In almost all rocks it is the most abundant constituent. 



Alumina, which is almost invariably the next most abundant 

 constituent, reaches a maximum of about 60 per cent in some corun- 

 dum-bearing syenites from Canada and the Urals, and has a general 



* This order is not quite that of relative abundance, but that which is commonly used in 

 the statement of rock analyses. 



5 Quartz veins are not considered here, as they are usually of nonigneous origin, at least 

 in the commonly accepted sense. 



