40 CLAYS AND CLAY INDUSTRY. 



Organic matter. 

 Soluble salts. 



Origin. 



Quantity present in clays. 



Prevention of coating. 



Method of use. 



Introductory. — A discussion of the physical properties of clay- 

 should, perhaps, have preceded a description of the chemical 

 properties, because they are considered more important, but for 

 the sake of clearness it is probably better to discuss the chemical 

 ones first. 



Before taking up the main theme of discussion of this chapter, 

 it may be well to give a few explanatory statements regarding 

 the occurrence of mineral compounds in the rocks of the earth's 

 crust, of which the clay bearing formations are a part. 



Many chemical elements are found in the rocks of the earth's 

 crust, 1 yet only a few of them are widespread and important. 

 But, by averaging up the analyses of several hundred rocks 

 from all parts of the world, a fairly accurate estimate can be 

 made of the average quantity of each element present. This has 

 been done by F. W. Clarke, chief chemist of the United States 

 Geological Survey, and the results obtained by him are given 

 in the following table. The name of the element is given first, 

 followed by its symbol in parenthesis, and then the average per 

 cent. 



Table showing percentage of elements found in the earth's crust. 



Oxygen (O), 47.02 Hydrogen (H), 0.17 



Silicon (Si), 28.06 Carbon (C), 0.12 



Aluminum (Al), 8.16 Phosphorus (P), 0.09 



Iron (Fe), 4.64 Manganese (Mn), 0.07 



Calcium (Ca) 3.50 Sulphur (S), 0.07 



Magnesium (Mg), 2.62 Barium (Ba), 0.05 



Sodium (Na), : 2.63 Strontium (Sr), 0.02 



Potassium (K), 2.32 Chromium (Cr), 0.01 



Titanium (Ti), 0.41 



1 For the benefit of those who have not studied geology it may be stated 

 that the earth was probably originally a nebula. As this cooled many of the 

 gaseous elements became molten solids and finally cooled to solid rock. The 

 watery vapor surrounding this early globe condensed eventually to form 



