88 GEOLOGY OF THE HIGH PLATEAUS. 



follows: 1. Chemical composition. 2. Mineral ingredients. 3. Texture. 

 4. Density. 5. Fusibility. 



Of these characters the most important surely is the chemical composi- 

 tion. In truth, differences of chemical constitution apparently lie at the foun- 

 dation of most of the other varying characters. It is the primary determi- 

 nant of the minerals which are formed in the lavas and certainly also of the 

 specific gravity and fusibility. The texture, also, is to a considerable extent 

 dependent upon it, though in this respect the rock is influenced more by 

 other conditions. But on the whole there is a well-marked correlation 

 among the physical properties of volcanic rocks, and we may easily recog- 

 nize the important fact that variations in the chemical composition carry 

 with them tolerably definite and dependent variations in the other physical 

 properties. 



Correlation between chemical composition and mineral ingredients. — The 

 minerals which are formed in volcanic rocks are to a very important extent 

 determined by the chemical composition of the magma. The most abundant 

 constituent of volcanic rocks is silica; its quantity ranging from 45 to 80 

 per cent. Those rocks which possess the higher percentages of silica have 

 on the whole more acid minerals than those which possess lower percentages 

 of silica. The minerals of the more acid rocks are quartz and potash-soda 

 feldspars, while those of the more basic rocks are lime-soda feldspars, augite, 

 and olivin. Rocks of intermediate constitution contain both kinds or inter- 

 mediate kinds of feldspar, with abundant hornblende or equivalent augite. 

 We may discern the principle of selection, which determines the minerals 

 by studying each chemical constituent in detail. It might be readily antici- 

 pated that free quartz would be segregated and crystallized in a rock con- 

 taining a very large percentage of silica. Indeed, the law of definite pro- 

 portions regulating the combinations of all substances requires us to believe 

 that in all ordinary volcanic rocks holding more than 65 to 68 per cent, of 

 silica this excess of silica must be present uncombined, whether as free 

 quartz conspicuous to the eye or as an intimate mixture of the groundmass. 

 There is no fixed percentage at which silica becomes excessive, since that 

 will depend largely upon the atomic weights and affinities of the other sub- 

 stances present. But, in a general way, those rocks which contain large 



