CLAY AND ITS MODE OF OCCURRENCE. 7 



representing all intermediate stages, so that residual clays vary 

 -widely in their purity. 



The form of a residual clay deposit, which is also variable, 

 depends on the shape of the parent rock. Where the residual clay 

 has been derived from a great mass of granite or other clay-yield- 

 ing rock, the deposit may form a mantle covering a considerable 

 area. On the other hand, some rocks, such as pegmatites (feld- 

 spar and quartz), occur in veins, that is, in masses having but 

 small width as compared with their length, and in this case the 

 outcrop of residual clay along the surface will form a narrow belt. 



The depth of a deposit of residual clay will depend on climatic 

 ■conditions, character of the parent rock, topography and location. 

 Rock decay proceeds very slowly, and in the case of most rocks 

 the rate of decay is not to' be measured in months or years, but 

 rather in centuries. Only a few rocks, such as some shales or 

 other soft rocks, change to clay in an easily measurable time. 

 With other things equal, rock decay proceeds more rapidly in 

 a moist climate, and consequently it is in such regions that the 

 .greatest thickness of residual materials is to be looked for. The 

 thickness might also be affected by the character of the parent 

 rock, whether composed of easily weathering minerals or not. 

 Where the slope is gentle or the surface flat, much of the residual 

 •clay will remain after being formed, but on steep slopes it will 

 soon wash away. 



In some cases the residual materials are washed but a short 

 distance and accumulate on a flat or very gentle slope at the foot 

 •of the steeper one, forming a deposit not greatly different from 

 the original ones, 1 although they are not, strictly speaking, resi- 

 dual clays. Shallow deposits of clay of this sort are not uncom- 

 mon in the Highlands of New Jersey. 



Residual clays, usually of low purity, are widely distributed 

 over the southern portion of the United States, and they probably 

 existed at one time to an equal depth in the northern states, but 

 were removed by the great continental ice sheet, which, in the 

 glacial period, spread over that part of the country. The northern 

 portion of New Jersey was involved in this change, the southern 



Clays accumulated in this manner are termed colluvial by G. P. Merrill. 



