THE EARTH'S CRUST 



49 



has already been discussed. The hornblendes (or amphiboles) in- 

 clude certain white or light-colored silicates of calcium and mag- 

 nesium, often with fibrous structure, of which common asbestos is a 

 good example; also silicates of aluminum, magnesium, and iron, 

 of darker colors, green or black. The micas include light-colored 

 or transparent potassium aluminum silicates and black silicates of 

 aluminum, magnesium, and iron. While the hornblendes are often 

 fibrous, the micas, as a rule, are easily split into the well-known mica 

 sheets. 



Quartz. Quartz, when pure, is crystallized silicon dioxid (SiO 2 ), 

 but it is often colored by small amounts of metallic compounds. 

 Aside from being a common constituent of granite and gneiss and 

 of many other less abundant silicate rocks, quartz is often found in 

 rock masses or seams in a nearly pure state. Quartz sand is not 

 uncommon, but the opinion that sand and quartz are synonymous 

 terms is very incorrect, for sand usually includes very considerable 

 amounts of granite or gneiss and other mineral particles. 



The following statement shows the composition of common 

 samples of original granite, fresh gneiss, and decomposed gneiss; 

 also the percentage of each constituent saved from the fresh gneiss 

 and found in the decomposed gneiss, as computed by Merrill, 1 

 assuming no loss of aluminum, which indicates a total loss of 44.67 

 per cent of the original rock. They serve only as illustrations, and 

 other samples may vary greatly from these. 



TABLE 4. PERCENTAGE COMPOSITION OF ROCK 



1 " Rocks, Rock Weathering, and Soils," 1897, p. 215. 



