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Show PHYSICAL PEOPERTIES OF VOLCANIC EOCKS. 89 quantities of alkali (soda and potash) may have a larger percentage of silica without excess, than rocks containing more of lime, magnesia, and iron and less of alkali. Thus trachytes, which have a comparatively large proportion of soda and potash, and very little lime and iron, seldom show any evidence of excess of silica unless the percentage exceeds 68 per cent., and then, as the silica increases, they graduate into rhyolites. On the other hand, such rocks as propylite and andesite, which contain an abundance of lime and iron, begin to show evidence of an excess of silica when the percentage of it exceeds 62 per cent, or sometimes even 60 per cent. The reason for this is not far to seek. The alkalies are capable of forming definite combinations with a much higher percentage of silica than are lime, magnesia, and iron. The alkalies give rise to the acid feldspars, albite, and orthoclase, while the lime gives rise to the basic feldspar, anorthite, and iron and magnesia to the equally basic minerals of the pyroxenic, hornblendic, and olivin groups. On the other hand, the alkalies sometimes form basic minerals, such as leucite and nephelin. This happens whenever these bases are present in quantities in excess of those required to form feldspar, or, what amounts to the same thing, when the ratio of silicate of alumina to soda or potash is less than that required to form albite or orthoclase. Hence, in basic rocks rich in potash, we find leucite, and when they are rich in soda, nephelin, either or both replacing feldspar. Turning now to the magnesian minerals, the same kind of correlation is seen. Where the quantity of magnesia relatively to the silica is very great olivin is formed abundantly. This is the most basic mineral occurring in eruptive rocks, and is found only in rocks which are least siliceous. Where the quantity of magnesia is less, augite and hornblende are formed. In the two latter minerals it appears that lime, magnesia, and iron protoxide largely replace each other, lime predominating in augite, and magnesia in hornblende. They are moderately basic, but less so than olivin. In the more acid rocks magnesia takes frequently the form of mica (biotite), in which the quantity of protoxide base is still less than in hornblende. With regard to alumina, it is somewhat remarkable that although the |