ORIGIN OF CLAYS. 5 



None of the Alabama clays thus far analyzed indic- 

 ate this exceptional composition. 



Knowing the mode of origin of kaolinite it will at 

 once be seen that the purity of the kaolin depends on 

 the nature of the parent rock. Feldspar often forms 

 large veins of considerable purity, and nearly free 

 from other associated minerals, and its decomposition 

 in such cases would give rise to deposits of pure or 

 nearly pure kaolin. In point of fact the purest clays 

 known have with few exceptions been formed in this 

 manner. More frequently quartz and mica are com- 

 mon accessory minerals, and remain intermixed with 

 the kaolinite, both of them being more resistent to 

 weathering than the feldspar. When these or other 

 minerals occtir in the kaolin they have to be separated 

 from it as much as possible by washing. 



Clays, which occur at or close to the locality in 

 which they have been formed, are called "residual 

 clays". They represent some of the purest types of 

 clay known as well as the most impure. The upland 

 region of the Southern States is underlain by a great 

 area of feldspathic, granitic and gneissic rocks which 

 have decomposed to a ferruginous clay of residual 

 nature, and one that is used extensively in the South 

 for the manufacture of common brick. 



In the general wearing down of .the land surface 

 which is continually taking place the particles of 

 residual clay are washed down into the lakes and 

 oceans and deposited there as sediments, thus giving 

 rise to what are known as sedimentary clays. They 

 are usually far more plastic than the residual clays,, 

 especially the purer ones. 



From the nature of their formation, we should sel- 

 dom look for kaolins of sedimentary origin, and when 

 they do occur they have probably been derived from 

 large areas of very feldspathic rock or possibly from 

 limestones which had an appreciable percentage of 

 silicate of alumina in their composition, in which case 

 the lime carborate would be carried off in solution, 

 and the clay components of the rock be left behind as 

 an insoluble residue. It! is seldom that sedimentary 



