REFLEXION OF ELASTIC WAVES 153 
face outcrop for our purpose is clay, and we have found that where 
the surface consists of soft sands or hard limestones our chances of 
success are much reduced. Special technique devised to overcome local 
difficulties has resulted in a measure of success being attained in 
nearly every area so far investigated, and I feel that the two methods of 
attack outlined in this paper should be competent to furnish results 
in most gently folded areas which have failed to yield surface evidence 
of sub-surface structure. 
813 
