J:.' MAltKS ol' I'KIM rssloN CN si l.I(.K( US UdCKS, 



archc-colilliic man in Europe, treated such a nucleus dif- 

 ferently : he turned the ])arent block round till the plane 

 cJ' fracture was fairly level, and then struck it with a suiarc, 

 sharp blow at an angle of about 45deg. If the blow was 

 effective, the flake became detached, and it is easv to see 

 that if the blow fell under an angle of 45deg.. the new 

 plane of fracture, that is to say, the Internal or Pollical 

 face of the new Hake must form an angle of 135deg. with 

 the old one. (See Fig, 2a.) Hence the significance of 

 the angle formed by the Percussion face and the Internal 

 (Pollioal) face, because we can gauge from it the angle 

 under which the hammer stone struck the parent block. 

 In the specimens mentioned above it would be. 



Tcro-watta from Pontvillo (Shene) 57deg. 



Kosedale 56deg. 



„ Mcrton Vale 53deg. 



„ Winton 51dcg. 



„ Mary vale 50deg. 



,, Hutton-park 47deg. 



Old Beach 45deg. 



These figures pi'ove conclusively the statement which, 

 if I am not mistaken, was ilrst promulgated by Sir John 

 Evans, that in order to be effective the critical angle under 

 which the hammer must strike the parent block is approxi- 

 mately 45deg. 



It is now also clear why this plane of fracture is called 

 the Percussion face 



Wc might call flakes thus detach?G oxt.ernal flakes of 

 the second order, and such a flake v/ould exhibit three 

 faces, viz. : — 



(2) 

 Percussion Face. 



^ 





The External face being the okbst. the Percuss-on 

 face the next, and the Tut M'nal face the voungcst in order 



