52 OUR PHYSICAL WORLD 



by heat and pressure so that the individual quartz grains are 

 fused together. Quartz veins occur in many rocks. When the 

 rock cracks under the terrific strains of crust movements, wide 

 fissures open that run for many miles in length and extend deep 

 into the earth. Such fissures are often later rilled with quartz 

 deposited from water. Such seams of quartz are known as 

 veins. Then again quartz is a very common constituent of 

 many rocks like granite, diorite, etc. 



Pure quartz in the amorphous or uncrystallized state is a 

 milky-white rock that is so hard it cannot be scratched with a 

 knife blade. When you attempt to scratch it, the steel rubs off 

 on to the quartz, leaving a metallic streak. Quartz scratches 

 glass easily. Quartz breaks with a conchoidal fracture, and the 

 freshly broken surface has a glassy sheen, or, as the mineralogist 

 says, a vitreous luster. Quartz is so hard it is little subject to 

 the wear and tear of the elements. Heat and cold, rain and frost, 

 have little effect upon it, so that quartz veins usually stand out 

 of the rock in which they occur since the rock containing them is 

 likely to weather more readily than the quartz. Quartzite hills 

 are likely to be rugged for the same reason, the contours being 

 angular, the slopes precipitous. 



Ultimately, of course, even resistent quartz is broken up under 

 the incessant attacks of the elements. It will crack as it is 

 alternately heated intensely by the mid-day sun and suddenly 

 cooled by the rain or the low temperature of night. Water 

 accumulating in the tiny cracks changes to ice in winter and in 

 changing expands, heaving the quartz apart and widening the 

 crevices. Thus even quartz breaks in time into angular frag- 

 ments. The pelting rain, acting through countless centuries, 

 will wear away the angular edges, rounding off the fragments. 

 The smaller pieces may be washed down the slopes into the 

 streams, rolled along by the spring freshets, and ground against 

 each other until they are worn down to rounded pebbles. In 

 time they may be carried to the lake or sea and further pulverized 

 by wave action until the quartz block is transformed into sand. 



