58 GENERAL DISCUSSION OF CLAYS. 



two great divisons i. e. residual and sedimentary, and 

 to these might perhaps be added a third class of clays, 

 namely, those formed by chemical precipitation. Un- 

 der each of the first two classes, it would be possible 

 again to find every gradation from pure to impure. 



It is not possible to make any classification based 

 upon the practical applications of the materials, for 

 some clays are used for as many as four to five dif- 

 ferent purposes, and it is probable that some classi- 

 fication which simply recognizes four or five important 

 groups is probably the most satisfactory and the least 

 confusing. Hill makes the following divisions :* 



China clays. 



Plastic, ball, pottery clays. 



Brick clays. 



Refractory or fire clays. 



He furthermore makes another table based on the 

 origin of the clay as found in the United States : 



I WHITE BURNING CLAYS. 



1. Rock or residual kaolin. 



2. Indianite or Indiana kaolin. 



3. Florida or sedimentary kaolins. 



4. White burnine* plastic clays. 



II COLOR BURNING CLAYS. 



Mixed clavs 



1. Brick clays, (Siliceous). 



2. Marly clays, (Calcareous). 



3. Pink clays, (Ferruginous). 



2. Cement clayp, (Silico-calc^reous). 



5. Alum clays. 



Altered clays (shale and slate). 



*U. S. Geol. Survey, Mineral Resources, 1893. 



