THE EARTH'S ROCK FOUNDATIONS 47 



prized by the Indians and Eskimos, for from such they laboriously 

 cut off bits that could be hammered into arrow- and spearheads 

 or even shaped to make crude knives. The copper ore of the 

 famous Calumet and Hecla mine in upper Michigan is a rock 

 in which the native copper occurs in grains or threads. Gold 

 ordinarily occurs as grains, flakes, or strings in the quartz veins, 

 from which it is separated by crushing and washing. 



Some of the non-metals occur similarly in the free state as 

 elements. Sulphur is a good example. In volcanic regions, 

 especially, great deposits of it are found. In some of the coastal 

 regions bordering the Gulf of Mexico it is very abundant as 

 grains or small masses in the deeper layers of the sand. Live 

 steam is forced down to melt it, and the melted sulphur comes to 

 the surface through pipes sunk for the purpose. Until these 

 deposits were discovered and a method of working them perfected, 

 the United States imported its sulphur largely from Italy and 

 Sicily. Now we export sulphur in quantity. 



But for the most part the minerals are compounds; that is, 

 they consist of two or more elements united in a chemical com- 

 pound. Thus, while silicon is, next to oxygen, the most abundant 

 element in the earth's crust (making, it is estimated, one-fourth 

 of it), yet the element occurs nowhere in the earth free, but it is 

 united with other elements. Combined with oxygen it makes 

 silica or quartz, SiO 2 , one of the most widely distributed of 

 minerals. It is found as an element in many of the other com- 

 mon minerals which are complex silicates, as will be seen below. 

 Calcite, the mineral from which vast deposits of limestone and 

 marble are formed, is a compound of calcium, carbon, and oxygen, 

 CaC0 3 . 



While the mineralogist knows hundreds of minerals, most of 

 them are rare, and, fortunately for the beginning student, those 

 that occur as essential constituents of the common rocks are not 

 many, and they are, moreover, distinguished with comparative 

 ease. Before describing these it will be necessary to review some 

 of the important characters that serve as distinguishing features 



