26 GEOLOGY 



of silicious minerals in igneous rocks is large. Geology deals with 

 these minerals as constituents of the earth, but only a few of them 

 are so abundant as to require special notice here. It may be 

 remarked also that, as they occur in the rocks, only a few of them 

 can be identified by simple inspection, partly because some of them 

 look much alike, and partly because the crystals are often minute 

 and intricately intermixed. 



Minerals of igneous rocks. Fortunately for the simplicity of 

 geological study, a few minerals make up the great mass of the 

 igneous rocks. These few are quartz, the feldspathic minerals, 

 the ferro-magnesian minerals, and the iron oxides. 



Quartz is the free acid already mentioned. Some of its leading 

 characteristics are given on p. 80. Crystals of quartz are six-sided 

 prisms, normally, capped by six-sided pyramids (Fig. 51); but 

 in igneous rocks their forms are usually very imperfect. 



The feldspathic group of minerals embraces those formed by 

 silica in union with alumina, together with either potash, soda, 

 or lime, or two or more of these together. The feldspathic min- 

 erals are normally light in color, ranging from white to red, gray, 

 or greenish. The feldspars are minerals of the first importance 

 in igneous rocks. The varieties of feldspar are given on p. 77. 



The ferro-magnesian group embraces minerals formed by the 

 union of silica with iron, magnesia, and lime, together with more 

 or less of the other basic oxides. The ferro-magnesian minerals 

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

 the great coloring element of rocks; but the color distinctions do 

 not hold good in detail, and cannot be safely trusted as a means of 

 identification. 



Among the ferro-magnesian minerals the most important are 

 the pyroxenes (p. 80), the amphiboles (p. 75), and the micas (p. 79). 

 The pyroxenes and amphiboles have nearly the same chemical 

 composition, but differ in crystal form 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 among the most abundant of 

 the dark minerals in crystalline rocks. 



