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Show 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 composition. In truth, differences of chemical constitution apparently lie at the foundation of most of the other varying characters. It is the primary determinant 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 recognize 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.â€"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 intermediate 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 anticipated that free quartz would be segregated and crystallized in a rock containing a very large percentage of silica. Indeed, the law of definite proportions 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 substances present. But, in a general way, those rocks which contain large |