THE COMPOSITION OF THE EARTH 21 



Quartz ' (silica, SiOa), familiar as the chief constituent of sand, is 

 generally light-colored and glassy in appearance. When pure it is 

 transparent, but various impurities give it different colors and 

 special names. Quartz sometimes forms crystals, usually six-sided 

 prisms capped with pyramids. In most rocks, it occurs as grains 

 without definite shape. It is a very stable compound and is the 

 hardest of the common minerals. Quartz will scratch glass and 

 cannot be scratched with a knife. The cleavage of quartz is very 

 poor; indeed, for all practical purposes, it may be regarded as with- 

 out cleavage. It has a glasslike fracture, which is often a great 

 help in distinguishing it in igneous rocks. Igneous rocks decay 

 when exposed to the weather, and the loose products make mantle 

 rock. In this process the more complex minerals are broken up, 

 their elements entering into new and simpler combinations ; but 

 the quartz remains unaltered. This loose material may then be 

 washed or blown away, the hard quartz particles becoming efficient 

 agents in wearing the rock surfaces with which they come in 

 contact. 



Feldspars. There are several kinds of feldspar, composed of 

 silica and alumina, together with potassium, calcium, or sodium. 

 The most common variety is orthodase (KAlSiaOs), or potash feld- 

 spar, in which potassium is the distinguishing constituent. Feld- 

 spar is not quite so hard as quartz, but too hard to be scratched 

 readily with a knife. The color of feldspar is variable, pale yellow, 

 pink, and especially white and red varieties being common. The 

 general color of many igneous rocks is determined by that of their 

 feldspars. The excellent cleavage of feldspar leaves flat, glisten- 

 ing faces, which often afford the readiest means of distinguishing 

 feldspar from quartz in rock. Feldspars are important constituents 

 'of most igneous rocks. Certain clays result from their rather ready 

 decomposition. 



Augite is a silicate of lime, magnesia, iron, and alumina. It is 

 dark green or black in color, and crystallizes in oblique rhombic 

 prisms. Augite crystals are short and stubby. 



Hornblende is very similar to augite in chemical composition. 

 Since the two minerals are also much alike in color and hardness, 

 they are easily confused, and when they occur in small grains are 

 often not distinguishable. Hornblende has two perfect cleavages, 

 the surfaces meeting at angles of 125 and 55, while in augite the 

 cleavage planes meet nearly at right angles. This difference helps 



1 It is hardly necessary to say that the study and identification of actual 

 specimens of minerals and rocks are absolutely essential to an understanding 

 of them. 



