THE EARTH'S ROCK FOUNDATIONS 51 



niter, a nitrate of soda; gypsum used as a fertilizer and in making 

 plaster of Paris; sulphur; and corundum, which is so hard it is 

 used in making grinding disks. 



Mention should be made also of the very beautiful and rare 

 minerals that are used as gems. The diamond is crystallized 

 carbon. The sapphire and ruby are pellucid varieties of corun- 

 dum. Emerald and aquamarine are lustrous forms of beryl, a 

 silicate of berylum and aluminium. Topaz is a fluosilicate of 



FIG. 28. A zinc mine 



aluminium. Garnet is also a silicate and the different varieties 

 vary in the metals present: lime, aluminium, iron, soda, chro- 

 mium, etc. Turquoise is a phosphate of aluminium. 



Of all the minerals quartz is the most abundant in the rocks 

 at the earth's surface. Sand consists largely of grains of quartz 

 more or less rounded by water action. Sandstone, which is the 

 prevalent surface rock over wide areas and is extensively used 

 as a building stone, is simply sand cemented together to form 

 rock, and so is quartz in great measure. Quartzite is another 

 common rock made of quartz. It is really sandstone modified 



