THE EARTH'S ROCK FOUNDATIONS 53 



So, too, when such a rock as granite disintegrates under constant 

 weathering, the angular quartz grains wear down much less 

 readily than the other minerals. But still in time they are 

 rounded by water action and reduced to sand. Sand, the 

 grains of which are still angular and sharp edged, is called torpedo 

 sand. 



Pure quartz, when crystallized, forms transparent crystals in 

 the form of six-sided prisms with a six-sided pyramid on each 

 end. Such crystals, because quartz is so nearly indestructible, 

 are much used for spectacle lenses and for lenses in optical instru- 

 ments such as microscopes. The crystals are very likely in 

 nature to form on a surface, the prisms standing up on end 

 capped with a pyramid at the free end but lacking the pyramid 

 at the base (Fig. 26, p. 48, right end). 



While quartz does not dissolve readily in ordinary water, it 

 does dissolve with comparative ease in water that is charged with 

 carbon dioxide, especially if the water is hot and under pressure. 

 Now carbon dioxide results from the decomposition of organic 

 material. Soils usually contain a great deal of it, especially in 

 marshes and forests where much decaying plant and animal 

 material lies on or in the ground. Rain falling on the ground 

 percolates through it and absorbs much carbon dioxide as it goes. 

 If this water then finds its way down into the rock layers, running 

 through their cracks and crevices, and so sinks into the rock of 

 the earth's crust, it may become hot. As it heats it expands and 

 in the confined spaces may develop a high pressure. Then it 

 dissolves quartz readily. Later it may be forced to the surface 

 again, appearing as a hot spring. About the mouths of such hot 

 springs quartz is deposited abundantly, for as the water comes 

 to the surface it is free to expand, the pressure decreases, the 

 water cools and loses its carbon dioxide to the air, and so it can 

 no longer hold the quartz in solution. 



Not infrequently such alterations in temperature, pressure, 

 and carbon dioxide content occur in part as the water flows into 

 a cavity in the rock, and then the cavity is lined with layer after 



