CHAPTER XIV 

 MINERALS AND ORES; THEIR VALUE AND SOURCE 



187. The Crust of the Earth. The solid outside part 

 of the earth is composed of rock. This is covered in most 

 places by a layer of mantle rock, a fine material consisting of 



1. Oxygen 50$ 5. Calcium 6# 



2. Silicon 25$ 6. Magnesium 1.3j 



3. Aluminum 8# 7 Sodium H 



4. Iron 7$ 8*. Potassium 1* 



9. Ail others less than \<fr 



FIG. 82. PERCENTAGE OF ELEMENTS IN ROCKS 



1. What element abundant in living matter is not included in this 

 diagram? 2. About how many elements are known? 3. How many 

 elements must then be included in the "less than 1 %" ? 



soil together with sand, gravel, or clay, which has generally 

 been made from the rocks which it now covers. 



188. Minerals. Rocks consist of minerals. A mineral 

 is sometimes an element, but more often it is a definite com- 



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