168 



QUARTZ. 



This well known mineral is by far the most 

 important in the composition of rocks ; entering 

 into a greater number than any other, and form- 

 ing a considerable proportion of most of those in 

 which it is an ingredient. 



FELSPAR. 



Two varieties of felspar, the common and 

 the glassy, are found in many of the compound 

 rocks ; but the former in much greater abun- 

 dance than the latter. An opaque kind, which 

 sometimes also occurs, appears to have been thus 

 changed by an incipient decomposition. 



CARBONAT OF LIME. 



This substance, sometimes pure, but more 

 commonly in a state of intimate mixture with 

 other earths, is so well known as the basis of 

 limestone that it requires no particular notice. 

 It is very rarely found entering, as an ingredient, 

 into mixed rocks. 



MICA 



Forms an important and conspicuous ingre- 



