4 GENERAL DISCUSSION OF CLAYS. 



with orthoclase, the potash feldspar, serving as the 

 type of the group, as well as being the commonest 

 species. 



Under the influence of chemical action, which may 

 be the result of weathering or in some cases probably 

 of acid vapors ascendning from the interior of the 

 earth, the feldspar becomes decomposed, and the result 

 of this is that the potash of the feldspar is removed 

 partly in the form of solube carbonate, or perhaps 

 silicate, or even fluoride, while the alumina and silica 

 remain and unite with water to form the hydrated 

 silicate of alumina, kaolinite, whose composition is 

 expressed by the formula A1 2 3 , 2S10 2 , 2H 2 0., or 

 in the proportion of silica, 47.30 per cent.; alumina, 

 39.80 per cent. ; water 13.90 per cent. 



The change can be illustrated still better by the fol- 

 lowing in which the first column indicates the com- 

 position of the feldspar, the second the amount of 

 water taken up in the process of decomposition, the 

 third, the amount, of matter removed in solution, and 

 the fourth the relative amounts of the three ingredi- 

 ents of kaolinite. 



Feldspar. Added. Dissolved out. Kaolinite. 



Alumina 18.3 0.0 18.3 



Silica 64.8 .... 41.8 23.0 



Potash 16.9 .... 1.9 



Water 6.4 .... 6.4 



Many clays approach quite closely to kaolinite in 

 their composition, and in some the percentage of 

 alumina even exceeds the theoretic amount, by one or 

 two per cent., and is evidently not due to errors of an- 

 alysis. 



It has been suggested by some that this may be due 

 to the presence of a certain amount of pholdrite, the 

 amorphous variety of kaolin,* and while this is pos- 

 sible the same composition might be shown by a cer- 

 tain amount of bauxite or alumina hydrate mixed in 

 wit'h the clay. 



Wheeler, Clays of Missouri, Missouri Geological Survey, XI. 



