EAETHY MINERALS. 57 



chapter, the silica is in chemical combination with other 

 substances, forming silicates. Silica is a very hard sub- 

 stance, scratching glass readily, but inferior in hardness 

 to the diamond. It is insoluble, and can not be melted, 

 even in the strongest heat that can be obtained by the 

 blow-pipe. It appears in greater variety of form and 

 color than any other mineral, but its qualities are so 

 marked that it is easily recognized. As it is so abun- 

 dant in the rocks, and yet hard and insoluble, its frag- 

 ments abound, most of the pebbles in gravel and in the 

 common soil, most of the sand, and much of the hard 

 grains even in what we call fertile earth, being silica. 

 Of course, in every case where there is a decided color 

 there is something besides the silica that is, there is 

 some coloring substance. But where the mineral, when 

 colored, is clear and transparent, there is so little of a 

 foreign substance, and it is so intimately mixed or com- 

 bined with the silica, that it can hardly be regarded as 

 an impurity. But sometimes, especially when the min- 

 eral is opaque, the impurity is palpable oxyd of iron, 

 clay, etc. Flint is one of those opaque forms. This, 

 which was formerly in such wide use in muskets and in 

 the common tinder-boxes, but is now superseded by per- 

 cussion caps and lucifer matches, is quite an abundant 

 mineral, and is extensively used in pottery. 



113. Quartz. There are said to be three varieties of 

 v quartz the vitreous, the only one which appears in crys- 

 talline forms, its name coming from the fact that its frac- 

 ture is glassy ; the chalcedonic, generally translucent, 

 with a waxy lustre, often exhibiting several colors, gen- 

 erally fantastically arranged ; and the jaspery, of a dull 

 red, sometimes yellow color. But in common language, 

 even among mineralogists, it is only specimens of the 

 first variety that are usually spoken of as quartz ; and, 

 on the other hand, the two terms silica and quartz are 

 often used as being synonymous. The crystals of quartz 

 are usually six-sided prisms, terminated with six-sided 



C 2 



