22 CLAYS AND CLAY INDUSTRY. 



Illustrations of this were found at several clay pits, but the 

 operators, instead of recognizing the true reason for the presence 

 of the yellow clay, presumed that it was a different bed, entirely 

 separate from the unweathered blue. 



Leaching. — More or less surface water seeps into' all clays and 

 in some cases drains off at lower levels. Such waters contain 

 small quantities of carbonic acid which readily dissolves some 

 minerals, most prominent among them carbonate of lime. In 

 some areas, therefore, where calcareous clays occur, it is not un- 

 common to find that the upper layers of the deposit contain less 

 lime carbonate than the lower ones, due to this solvent action of 

 the percolating waters. 



Softening. — Most weathering processes break up the clay de- 

 posits, either by disintegration or by leaching out some soluble 



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clay 

 clay 



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Fig. 16. 



Section showing weathered (yellow) clay where the overburden is least. 



constituents that served as a bonding or cementing material, thus 

 mellowing the outcrop, and many manufacturers recognize the 

 beneficial effect which weathering has on their clay. They con- 

 sequently sometimes spread it on the ground after it is mined and 

 allow it to slake for several months or in some cases several years. 

 The effect of this is to disintegrate thoroughly the clay, render 

 it more plastic, and break up many injurious minerals such as 

 pyrite (see p. 46). Although mentioned under chemical changes 

 it will be seen that the process of softening is partly a physical 

 one. 



Consolidation. — This change is found to have taken place in 

 a few deposits, notably in some of the Clay Marls, and is due to 

 the formation of limonite crusts in the clay. At times these may 



