Sentence:  The hardness or softness of the hammerstone controls the interval of contact between the <WSD>percussor</WSD> and the flint-like material, for the time of contact is proportionate to the yield and density of the <WSD>percussor</WSD>. 
Meaning of the word marked with <WSD> tags:  A hammerstone or similar tool used for smashing, or chipping.