22 GENERAL DISCUSSION OF CLAYS. 



ORGANIC MATTER IN CLAYS. 



Organic matter affects not only the color of clay but 

 also " its plasticity, absorptive power and tensile 

 strength. 



It is present in clays either in the form of finely 

 divided pieces of plant tissue or larger fragments of 

 stems or leaves, which settled in the clay during its 

 deposition, and have since become wholly or partly 

 converted into lignite. All surface clays contain 

 plant roots, but these exert little effect other than to 

 aid the percolation of surface waters. 



Clays colored by organic matter and containing no 

 iron, burn white, as the plant tissue burns off at 

 bright redness ; if sucli a clay, however be heated too 

 quickly, the surface of it becomes dense befo^'e all of 

 the organic matter has had opportunity to escape from 

 the interior, and the latter remains dark colored. 



Organic matter may also mask the presence of iron 

 so that the clay, instead of burning white, will burn 

 red at a temperature of above that at which the or- 

 ganic matter passes off, below that temperature the 

 vegetable matter will tend to keep the iron reduced. 

 The clay from Fernbank, Lamar County, Alabama, 

 contains 6.40 per cent of ferric oxide, and 2 to 2^ 

 per cent of organic matter, but in the raw material, the 

 latter hides the former. Organic matter exercises an 

 important influence on the plasticity, often increasing 

 it to an enornious degree, it also tends to elevate the 

 itensile strength, the clay just mentioned showing 185 

 pounds per square inch, but high plasticity does not 

 always indicate the presence of much organic mate- 

 rial. 



In the weathering of clays organic matter by its 

 slow oxidation, aidsi in breaking them up by the es- 

 cape of the carbonic acid gas. 



WATER IN CLAYS. 



All clays contain two kinds of v/ater : — 

 1st. Hygroscopic water or moisture ( mechanically 

 absorbed). 

 2nd. Chemically combined water. 



