400 GEOLOGY. 



These statements are only true in a very general sense. Admixtures, 

 replacements, and impurities are so frequent as to break down all sharp, 

 simple definitions. The feldspathic minerals are normally light in 

 color, ranging from white to red or gray. The ferromagnesian minerals 

 are normally dark (commonly greenish) from the presence of iron, the 

 great coloring element of rocks. But these color distinctions do not 

 hold good in detail and cannot be much trusted as a means of identifica- 

 tion. 



The feldspathic minerals (p. 462) embrace the potash feldspars, ortho- 

 clase and microcline; the soda feldspar, alhite; the hme feldspar, anorthite; 

 and the mixed feldspars intermediate between albite and anorthite, viz., 

 the soda-lime feldspar, oligodase, the Hme-soda feldspar, andesine, in 

 which lime and soda are nearly equal, and the hme-soda feldspar, lahra- 

 dorite, in which the hme predominates; together with leucite, a potash 

 sihcate higher in alkah than orthoclase, and nephelite, a soda silicate 

 higher in soda than albite. Leucite and nephelite are usually classified 

 as feldspathoids, not as feldspars. It is to be understood that alimiina 

 is normally present in all these. Additional details respecting these 

 minerals may be found in the reference list, p. 460. 



Among the ferromagnesian minerals the most important are the 

 pyroxenes, the amphiboles, and the biotite type of mica. Olivine is 

 of subordinate importance. The pyroxenes (p. 465) and amphiboles 

 (p. 460) have nearly the same chemical composition, but differ in crys- 

 tallization and physical properties. Hornblende (an amphibole) has been 

 melted, and on cooling under proper conditions found to take on the form 

 of augite (a pyroxene). Pyroxene is sometimes altered into uralite, one 

 of the amphiboles. The pyroxenes and amphiboles are the most abundant 

 of the dark minerals in crystalline rocks. The leading members of the 

 pyroxene group are augite, diallage, hypersthene, enstatite, and soda 

 pyroxene. The chief members of the amphibole group are hornblende 

 and the soda amphiboles. AU are essentially sihcates of magnesia and 

 iron oxide, with or without the addition of hme, soda, and alumina. 

 Details respecting these may be found in the reference list. 



The two leading micas are the iron-magnesia mica, biotite, and the 

 potash mica, muscovite, the familiar 'isinglass" of the stove-door. 

 Chemically, muscovite should go with the potash feldspars, but it is 

 distinguished from them by its crystalhne habit and physical proper- 

 ties. The biotite should go chemically with the pyroxenes and amphi- 



