Proceedings of the Polytechnic Association. 1041 



enhanced the agricultural prosperity of the lands of that State, and 

 which belongs to the cretaceous formation, is a sandstone containing 

 iron and potash. 



As already stated, sand and quartz are pure silica ; still there is no 

 mineral that assumes so many forms and colors as quartz, though 

 non© is more easily distinguished. Its characteristic features 

 are : 



1. Its hardness, which is from 6.5 to 7, enabling it to scratch glass 

 with facility. 



2. Its infusibllity ; when heated alone before the blow-pipe it does 

 not melt. 



3. Its insolubility, as it is not, like limestone, attacked by the 

 strong mineral acids. 



4. Its want of cleavage, which has been mentioned above. This 

 is one of the first characteristics of quartz. 



5. Its crystalline character, occurring mostly in six-sided prisms, 

 more or less modified and terminated. 



6. Its low specific gravity of 2.5 to 2.7, is an unfailing distinctive 

 character of quartz. 



Rock crystal is a pure pellucid quartz, and was known by the 

 ancients under the name of c;'2/5^aZ/o5, meaning ice. It is used for 

 optical instruments, spectacle glasses, and cut with facets, for jewelry. 

 The crystals are often called real California diamonds. In ancient 

 times it was cut into cups and vases, and it is said that on hearing- of 

 his final overthrow, I^ero dashed into pieces a cup which was worth 

 $3,000. To this class of quartz belongs the finest ornaments which 

 adorn the palaces of ancient arid modern times ; and some forms, such 

 as amethyst, rose quartz, false tojxis, smohy quartz, known as Scotch 

 pebbles, or cairngorm, the favorite ornaments of the sportsmen of the 

 Highlands, are used as jewels. 



Milky quartz, or greasy quartz. 



Prase is of leek green color. 



Avanturine, more commonly known as gold-stone, is 'a quartz 

 spangled throughout with scales of golden yellow mica, although the 

 artificial imitation looks more beautiful than natural stone. 



CJialcedony is a translucent variety of quartz, which often lines 

 the cavities of other rocks, and in the form of stalactites, which are 

 then called icicles of chalcedony, and forming grottos several feet in 

 diameter. We find such in the Faroe Islands, in Florida, and in 

 many volcanic rocks, probably o-wing to silicio«s waters filtering at 



[Inst^] QQ 



