340 



Subsurface Geologic Methods 



be distinguished by shale-density determinations. Densities will usually 

 be less immediately below the unconformity, especially if the uncon- 

 formable surface was exposed to weathering and resulting leaching for 

 an appreciable length of time. In some cases, it has been suspected that 



bEPTH OF BURIAL IN FEET 



Figure 145. Average shale-density curves. (After Skeels.) 



variations in shale densities along fault zones may be associated with 

 weathering. 



Some evidence has been observed that generalized correlation by 

 shale density may be accomplished in local areas. However, such data 

 must be used with care. 



Hedberg ^^ has shown the relationship of decrease of porosity with 



" Hedberg, HoUis D., op. cit., p. 263. 



