MINERALOGY. 423 



FIRST CLASS. 



EARTHY MINERALS OR EARTHS. 



This class of minerals comprehends those structures 

 which are insoluble in water, and remain unaltered by 

 any common igneous action or force. 



FIRST ORDER. 



I. Quartz. Mostly white or vitreous, crystalline, 

 transparent, sometimes only diaphanous ; fracture con- 

 choidal, emits light when rubbed in the dark, II. = 7.0. 

 G. 2.5 to 2.7. Quartz is a constituent of many rocks, 

 and is found almost everywhere in the form of pebbles 

 or gravel. 



There are many varieties of which the most prominent 

 specimen is 



The Rock Crystal or Pure Pellucid Quartz, which 

 is very transparent ; found in high mountain regions 

 among granite, gneiss, etc., in vaulted openings, resem- 

 bling small caves, which are filled with these beautiful 

 crystals. These vaults, called Crystal-cellars, hidden 

 away among rocks, are commonly discovered only by 

 accident ; many of them are found to contain crystals 

 valued at ten thousand florins. Pure pellucid crystals 

 are often found in rivers, which, when polished, present 

 a brilliant appearance, as, for instance, -the White Stones 

 of the Rhine. The species of crystal known as Marowyn 

 Stone is found in the rivers which divide French and 

 Dutch Guiana. Although seldom of weight reaching 



