I 



GENERAL DISCUSSION OF CLAYS, 



By Hbinbich Ribs. 



ORIGIN OF CLAY. 



Clay is to be met almost every where, and while it 

 varies in form, color and other physicial properties, 

 nevertheless it always forms a pasty or plastic mass 

 when mixed with water, by virtufe of which it may be 

 molded into any shape, which it retains when dried; 

 furthermore when exposed to a high temperature it 

 hardens to a rock like mass. These two properties, 

 the plasticity and the hardening when burnt are what 

 make clay of such inestimable value tb man. 



Pure clay or kaolin is composed entirely of the min- 

 eral kaolinite, which is a hydrated silicate of alumina. 

 It rarely happens, however, that clay is perfectly pure, 

 for owing to the nature of its formation from another 

 rock as will be explained later, it is very apt to have 

 other minerals mixed in with it. These foreign min- 

 erals may sometimes be present in such quantities as 

 to completely mask the character of the kaolinite. 



We can therefore define clay as a mixtureof kaolin- 

 ite with more or less quartz and other mineral frag- 

 ments, especially feldspar and mica, the whole posses- 

 sing plasticity when mixed with water, and becoming 

 hard when burned. 



The so called flint clays form an exception to the 

 above, for while they often approach pure kaolin in 

 composition, still they are almost devoid of plasticity 

 when ground and mixed with water. 



Kaolinite is a secondary mineral resulting from the 

 decomposition of feldspar. The feldspars are a group 

 of silicate minerals of ".ather complex composition. 



