CLAY AND ITS MODE OF OCCURRENCE. 21 



It may be asked, therefore, how changes in color due to weather- 

 ing- can be distinguished from differences in color of a primary 

 character. Discoloration caused by weathering begins at the 

 surface and works its way into^ the clay, penetrating to a greater 

 distance along planes of stratification or fissures, and even follow- 

 ing plant roots as shown in Fig. 15. 



Where the clay deposit outcrops on the top and side of a hill, 

 it does not follow that because the whole cliff face is discolored, 

 the weather will have penetrated to this level from the surface, 

 but indicates simply that the weathering is working inward from 

 all exposed surfaces. The overburden often plays an important 

 role in the weathering of clay, for the greater its thickness, the 



Fig. 15. 



Shows how weathering penetrates a clay bed particularly along roots, cracks and joint 



planes. 



less will the clay under it be affected. This fact is one which the 

 clay worker probably often overlooks, and, therefore, does not 

 appreciate the important bearing which it may have on the be- 

 havior of his material. Some unweathered clays crack badly in 

 drying or burning, but weathering seems to' mellow and loosen 

 them, as well as to increase their plasticity, so that the tendency 

 to crack is sometimes either diminished or destroyed. If a clay 

 which is being worked shows this tendency it will be advisable to 

 search for some part of the deposit which is weathered, and if the 

 clay is covered by a variable thickness of overburden, the most 

 weathered part will be found usually under the thinnest stripping 

 as shown in Fig. 16. 



