CHAPTER XLII. 



MINERALS AND ROCKS. 



THE greater part of geological work pertains to rocks. In 

 geology every thing that helps to make up the crust of the earth 

 is rock, whether it be hard and enduring, like the limestone and 

 granite, or loose earthy materials like sand, clay or dust. Rocks 

 are sometimes simple chemical compounds, but generally they 

 are composed of a mixture of several distinct compounds called 

 minerals. There are a great many different kinds of minerals ; all 

 of them are interesting and many are valuable, but only a few 

 are of much importance as rock-forming materials. 



The most abundant and common of the minerals is silica, 

 composed of oxygen and silicon. It crystallizes in six-sided 

 prisms, appearing something like glass. It is hard enough to 

 scratch glass, and this quality distinguishes it from all other 

 minerals having a similar appearance. Silica is sometimes 

 called quartz. It is the chief ingredient of sand, gravel and 

 sandstone. 



The Feldspars, often associated with silica, are a group of in- 

 teresting and abundant minerals. Orthoclase, the most common 

 feldspar, is a silicate of aluminium and potassium. It occurs in 

 all colors from white to a fleshy-red. It sometimes has the ap- 

 pearance of glass, but is readily distinguished from quartz by 

 its inferior hardness, as it will not scratch glass, and may be 

 scratched with a good knife. It crystallizes in rhombic prisms, 

 and splits easily and smoothly in two directions, but it some- 

 times has a massive or granular structure showing no signs of 

 crystallization. Moonstone and sunstone, often used as jewels, 

 are varieties of this feldspar. Sanadine, a glassy variety, is 

 common in many lavas. 



Albite, another species of feldspar, is a silicate of aluminium 

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