STRUCTURE AND DISTRIBUTION OF CLAY DEPOSITS. 1 



dred feet, and their vertical extent depends in most 

 cases on the depth to which the weathering action has 

 progressed. 



Veins of kaolin seldom show great length, and when 

 followed along the surface not uncommonly pinch 

 out in both directions. They are often separated 

 more or less sharply from the country rock, and this 

 distinct line of demarkation is preserved even when 

 the wall rock itself is decomposed. They- further- 

 more frequently branch and at times contain lenses 

 of quartz, which resist the weathering agencies and 

 stand out in bold relief on the surface. It rarely pays 

 to work a vein under six feet in width. 



Deposits of kaolin of the type just described should 

 not be confused with sedimentary deposits of white 

 clay, which are usually of a much greater extent 

 than the vein formation. 



SEDIMENTARY CLAYS. 



These occur in the form of beds, which are either 

 close to the* surface or inter stratified with other de- 

 posHs which have been accumulated in water, such as 

 sandstone or limestone. They are not unfrequently 

 interbedded with coal deposits and many a coal seam 

 has a fire clay floor. Sedimentary clays are, as a rule 

 more homeogeneous than residual ones, and contain 

 probably a greater portion of fine particles. They are 

 also more plastic, and frequently contain much dis- 

 seminated organic matter. Furthermore, they do not 

 pass gradually into the underlying rock as residual 

 clays do, and indeed bear no relation, in a genetic 

 sense, to the rocks upon which they rest. 



When sedimentary clays become compressed by the 

 weight of overlying sediments, they assume the 

 character of hard or consolidated rock, and are known 

 as shale. Shales therefore simply represent the finest 

 clay sediment which has bcome consolidated. 



On grinding to a powder and mixing with water, 

 shales become just as plastic as other clays. By 

 mentamorphism, (that is heat and pressure developed 



