6 CLAYS AND CLAY INDUSTRY. 



this is that the change from limestone into clay does not take 

 place in the same manner as granite. Limestone consists com- 

 monly of carbonate of lime, with a variable quantity of clay 

 impurities, so that when the weathering agents attack the rock, 

 the carbonate of lime is dissolved out by the surface waters, and 

 the insoluble clay impurities are left behind as a mantle on the 

 undissolved rock, the change from rock to clay being, therefore, 

 a sudden one, and not due to a gradual breaking down of the 

 minerals in the rock, as in the case of granite. 



Fig. 1. 



Section showing the passage of the fully formed residual clay on the surface into the 



solid bed rock below. 



A. Clay. B. Clay and partly decomposed rock. C. Bed rock below, passing upward into 



rock fragments with a little clay. 



RESIDUAL CLAY. 



Where the clay is thus found overlying the rock from which 

 it was formed, it is termed a residual clay, because it represents 

 the residue of rock decay, and its grains are more or less insoluble. 



A residual clay formed from a rock containing little or no iron 

 oxide is usually white, and is termed a kaolin, and deposits of 

 this type generally contain a high percentage of the mineral 

 kaolinite. On the other hand, a residual clay derived from a 

 rock containing much iron oxide will be yellow, red, or brown, 

 depending on the iron compounds present. Between the pure 

 white clays and the brilliantly colored ones, others are found 



