8 GENERAL DISCUSSION OF CLAYS. 



by mountain making processes) taking place in the 

 crust of the earth, a shale may lose its chemically com- 

 bined water, develop a cleavage, and become converted 

 into slate. It is then no longer possible to develop 

 any plasticity in the material. 



It is not to be understood that all sedimentary clays 

 are of a homogeneous structure throughout. Some 

 beds may exhibit a wonderful similarity of composi- 

 tion throughout extended areas, while again theru may 

 be a wide variation in the character of any bed within 

 narrow limits. Apart from this variation laterally, 

 there may also be a vertical one ; n cases where the de- 

 posit is made up of a number of beds, one over the oth- 

 er, each showing distinctive characters. With such oc- 

 currences it is possible to obtain several different 

 grades of clay from the same pit. Such conditions are 

 apt to be the rule rather than the exception. 



A not uncommon phenomon in many of the coastal 

 plain formations is the occurrence of large lenses 'of 

 clay, free from grit surrounded by beds of sandy clay 

 or even sand. 



DISTRIBUTION. 



Clays and shales occur in practically every geologi- 

 cal formation with the exception of the oldest. Most 

 of those which are older t'han the Creataceous are 

 'hard and shale like in their nature, while those 

 of the Cretaceous and Tertiary on the other hand are 

 usually soft and plastic, but deposits of Creataceous 

 and also Tertiary shales are known. 



The geological age of a clay or shale is no indication 

 of its quality, and it is only of use at times for a means 

 of comparison between two beds situated near each 

 other, but even here it is not altogether a safe guide. 



The geological relations of the clays of Alabama 

 are treated somewhat more in detail below in a separ- 

 ate chapter. 



PROPERTIES OF CLAYS. 



These fall into two classes-, i. e. (1) Chemical and 

 (2) Physical. Two clays may correspond in their 



