6 GENERAL DISCUSSION OF CLAY. 



clays exhibit such reniarkale purity as those from 

 Chalk Bluff, Alabama, or the plastic ball clays of 

 Florida. 



The clays of the Cretaceous and Tertiary forma- 

 tions, which underlie the Coastal Plain, as well as the 

 Palaeozoic shales found in Alabama, are all of sed ; - 

 nientary origin. 



GEOLOGICAL STRUCTURE AXD DISTRIBU- 

 TION OF CLAY DEPOSITS. 



BESIDUAL CLAYS. 



The mode of origin of these has already been ex- 

 plained. They may occur either in the form of a broad 

 mantle overlying the bed rock and showing a variab lt 

 thickness as well as extent, or they may occupy the 

 position of a vein cutting across the strike of the other 

 rocks, or extending at times with the bedding or lami- 

 nation of them. 



Residual clays are commonly made up of a mixture 

 of angular grains which are chiefly undecomposed 

 mineral matter, and clay particles which are mostly 

 of sufficient fineness to remain suspended in water for 

 an almost indefinite period. There is also generally 

 a gradual transition from the fully formed clay at the 

 surface to the f^esh rock beloAv, whose decomposition 

 has given rise to the plastic mass above. 



The depth below the surface at which the unaltered 

 rock is encountered may be as little as three to four 

 feet, while in some regions where the surface 'has been 

 little eroded, and decomposition has been active, the 

 thickness of the residual clay may exceed one hundred 

 feet. 



The structure of the parent rock such as stratifica- 

 tion or lamination is at times often noticeable in the 

 lower portion of the residual deposits, and in some 

 cases it may even be preserved right up to the surface. 



Residual deposits of the vein type result commonly 

 from the decomposition of veins of granite or feldspar. 

 They vary in width, from a few inches to several hun- 



