QUARTZ 13 



In the quartz porphyries and liparites it is found as a porphy- 

 ritie constituent, with well-defined crystal outlines, which may 

 however have become more or less obliterated through the cor- 

 rosive action of a molten magma. (See Fig. 3, PL 5.) In 

 the secondary rocks, quartzite and sandstone, the quartz occurs 

 as more or less rounded or irregularly angular grains without 

 crystal outlines, except it may be through a secondary deposition 

 of silica, as explained on p. 136. Quartz is the hardest and most 

 indestructible of the common constituents, and hence when rocks 

 containing it decompose and their debris becomes exposed to 

 combined chemical and mechanical agencies, it remains unaltered 

 to the very last, forming the chief constituent of beds of sand and 

 gravel, which in turn may become transformed into sandstones, 

 quartzites, or conglomerates. 



Quartz is usually easily recognized, either under the micro- 

 scope or by the unaided eye, by its clear, colorless appearance, 

 irregular, glass-like fracture, hardness, and insolubility in any 

 acids but hydrofluoric. Under the microscope it appears in 

 clear, pellucid grains, often highly charged with minute cavities 

 filled with liquid and gaseous carbonic acid, the latter like the 

 bubble in a spirit level dancing about from side to side of its 

 minute chamber as though endowed with life. 



As a secondary constituent quartz occurs, filling veins and 

 cracks in other rocks, and in the impure crypto-crystalline and 

 amorphous forms known as chalcedony, chert, flint, opal, hya- 

 lite, and agate is found as an infiltration product in the cavities 

 of many trappean rocks, in lenticular and oval concretionary 

 masses in limestones, and replacing the organic matter of wood 

 and other organisms. The name tridymite is given to a quartz 

 occurring in minute, usually microscopic, tablets in cavities in 

 volcanic rocks, particularly the more acid varieties. (See fur- 

 ther on p. 67.) 



The Feldspars. — Hardness, 5 to 7; specific gravity, 2.5 to 

 2.8. The feldspars are essentially anhydrous silicates of alu- 

 minum, with varying amounts of lime, potash, or soda, and 

 rarely barium. They have in common the characteristics of 

 two easy cleavages inclined to one another at an angle of 90"*, 

 or nearly 90'' ; close relationship in optical properties; similarity 

 in colors, which vary from clear and transparent through white, 

 yellowish pink, and red, more rarely greenish, and often opaque 



