4 8 SCIENCE PROGRESS 



by the floor of the press. The presence of a cone of per- 

 cussion at each end of these flakes differentiates them from 

 those detached by human blows, on which only one bulb of 

 percussion is visible. When a duster was placed upon the floor 

 of the press so that the flint rested upon a soft basis, it was 

 found that a flake could be removed, but only with the exertion 

 of considerably more pressure than was needed in the former 

 case. It was noticed also that such flakes exhibited only one 

 cone of percussion, at the point where the small upper stone 

 impinged upon the flint, as in the case of flakes removed by 

 human blows. The majority of these cones of percussion, 

 however, are flat and appear to be only partly developed, and 

 in this respect they differ from the majority of cones of per- 

 cussion produced by human blows. Pressure flakes, too, 

 seldom exhibit well-marked eraillures, while the fissures and 

 ripple-marks, if present, are of a different order from those 

 present upon humanly-struck flints. 



These differences are readily observable when a representative 

 series of each kind of flake is put out for examination, and 

 appear to be due to the different nature of the two fracturing 

 forces. The human flaker is able to guide the line of fracture, 

 within limits, in any direction he pleases, while in the case of 

 pressure the flint is squeezed until it can stand the strain no 

 longer, and finally fractures along the line of least resistance. 

 Future investigations may perhaps modify this explanation, 

 but at present the author regards it as affording a likely reason 

 for the difference, which undoubtedly exists, between flakes 

 removed by pressure and those detached by human blows. 

 Experiments were also conducted in which a hard resistant 

 stone was placed upon the floor of the press, and another with 

 a sharp edge placed against it in such a manner that the 

 descending ram would cause this sharp edge to move under 

 pressure over the surface of the other specimen. By this 

 means a flaked hollow was produced in the sharp edge of the 

 uppermost stone. 1 Several of these hollows were produced 

 and compared with others formed by human blows. This 

 comparison demonstrated that there was a marked difference 

 in the appearance of the two series of hollows. 



It appears that when the sharp-edged flint is being moved 



1 A similar result can be obtained by placing a sharp-edged flint upon another 

 stone lying on the ground and turning one's heel round on them, under pressure. 



