CLAYS OF KEW YOKE 501 



iind, if the deposit is found in tke locality where it was formed, 

 it is known as a residual day. 



Clay may also be derived from the decomposition of almninous 

 limestones. 



Deposits of kaolin are not very common, but residual clays are. 

 Indeed in many of the southern states the surface soil over many 

 square miles is nothing more than a residual clay. Such residual 

 deposits often bear a close resemblance in chemical composition, 

 to the rock from which they were formed. 



Under the influence of weathering the residual surface materials 

 are washed down into the rivers and carried to seas or lakes 

 where they are spread out over the bottom as sediments. "We 

 thus have another class of clay deposits known as sedimentary 

 clay, no longer resembling the parent rock, but composed of the 

 residuum of several different areas. 



These two types of clay deposits, the residual and the sedi- 

 mentary, present certain distinguishable features, bearing on their 

 origin. 



Residual clays are cojnposed of a mixture of angular grains rep- 

 resenting in part undecomposed rock, and fine rock flour of clay, 

 that is, particles sufficiently fine to float in water. There is- generally 

 a gradual transition from the fully formed clay at the surface to 

 the unaltered parent rock below. The depth below the surface 

 at which unaltered rock is reached varies from three or four feet 

 to 150 or 200 feet. The structure of the parent rock is sometimes 

 retained for a certain distance upward in the residual clay. 



Sedimentary clays are stratified and occur in beds. They are 

 as a rule more homogeneous than residual clays and contain a 

 greater proportion of fine particles. They are also more plastic, 

 and frequently have much disseminated organic matter, but tliey 

 bear little or no relation to the rocks on which they rest. 



Sedimentary clays occur either at the surface, or may lie deep 

 below it, interbedded with other rocks. 



When sedimentary clays suffer consolidation under pressure they 



