34 GEOLOGY. 



transparent and perfectly crystalized, when it is im- 

 properly called diamond. Diamond rocks and hills are 

 known in many towns in almost every section of our 

 country. The diamond is found but in two or three 

 places upon the earth. 



Crystalized quartz is sometimes of a purple color, 

 when it is called amethyst. Jasper, carneleon, calcedony , 

 opal, and several other precious stones, are ranked in 

 the family of quartz. 



Felspar is generally white or of a light color, some- 

 times yellowish, light red, or green, seldom of a dark 

 color. Its fracture differs from that of quartz, as it 

 breaks in small even surfaces or plates, somewhat re- 

 sembling steps. A strong light thrown upon a recent 

 fracture, gives it a peculiar indescribable lustre, by 

 which it can always be distinguished from quartz. 



The two minerals are not only useful as constituting 

 the greater part of soils, rocks, and mountains, but for 

 an important purpose to which each is applied in the 

 arts. Quartz is the essential, and almost only ingredi- 

 ent used in the manufactory of glass, whether for win- 

 dows, decanters, tumblers, bottles, or any other purpose. 

 Felspar is always used in the manufactory of porcelain 

 or china ware. The substance known by the name of 

 kaolin, or porcelain clay, used both in China and this 

 country in the manufactory of porcelain, is decomposed 

 felspar. 



Mica, frequently but improperly called isinglass, is 

 extensively associated with the two simple minerals 

 already described in the structure of rocks. This min- 

 eral is sometimes found in plates two feet in diameter, 

 but much more commonly in fine scales but little larger 

 than the head of a pin. It is commonly white, but 

 sometimes black, and always more or less transparent. 



In some places, especially in Muscovy, mica is used 

 for the windows of houses, and is hence called Muscovy 

 glass. It is also used for lanterns, and some purposes 

 aboard of ships, where glass would be liable to break. 



STRATA OF ROCKS. 



Into the oldest and most common rocks upon the 

 earth, no other minerals enter in considerable quantities 



