20 CLAYS AND CLAY INDUSTRY. 



Chemical Changes. 



Nearly all clay deposits are affected superficially, at least, by 

 the weather. The changes are chiefly chemical, and can be 

 grouped under the following heads : 



Change of color. 



Leaching. 



Softening. 



Consolidation. 



Change of Color. — Most clay outcrops which have been exposed 

 to the weather for some time show various tints of yellow or 

 brown. This coloration or rather discoloration is due to the 

 oxidation or rusting of the iron oxide which the clay contains. 

 This iron compound is usually found in the clay as an original 

 constituent of some mineral, and rusts out as the result of weather- 

 ing, so that the depth to- which the weathering has penetrated the 

 material can often be told by the color. The lower limit of this 

 is commonly not only irregular, but the distance to which it 

 extends from the surface depends on the character of the de- 

 posits, sandy open clays being affected to a greater depth than 

 dense ones. The discoloration of a clay due to' weathering does 

 not always originate within the material itself, for in many in- 

 stances, especially where the clay is open and porous, the water 

 seeping into the clay may bring in the iron oxide from another 

 layer (PI. Ill, Fig. 2), and distribute it irregularly through the 

 lower clay. 



The changes of color noticed in clay are not in every case to be 

 taken as evidence of weathering, for in many instances the differ- 

 ence in color is due to differences in mineral composition. Many 

 clays are colored black at one point by carbonaceous matter, 

 whereas a short distance off the same bed may be white or light 

 gray due to a smaller quantity of carbonaceous material. In 

 many of the Woodbridge pits there is often a change from blue 

 to red and white mottled and from this into red clay. This is 

 not the result of weathering, but is due to local variations in the 

 iron oxide contents of the different layers. 



