618 THE ICE AGE IN NORTH AMERICA. 



the so-called "bulb of percussion." When a sharp, well- 

 directed blow falls upon a flint nodule, the force distributes 

 itself in such a way that, in the immediate vicinity of the 

 blow, a slight hollow is made in the nodule, and the corre- 

 sponding bulb in the flake is shaped somewhat like the ball of 

 one's thumb, while the rest of the flake is straight and regu- 

 lar in form. It is possible that this bulb of percussion may 

 sometimes be made by the accidental falling of one stone 

 upon another ; but such an occurrence must, in the nature of 

 the case, be very rare, since the blow must be delivered at 

 exactly the right point and at the proper angle, in order to 

 produce the right result. The chances are exceedingly small 

 that such a blow should be delivered except by design. 



As to the arrangement of the fractures, the evidence is 

 even more conclusive. A simple cutting edge may readily 

 be formed by natural forces ; but, in the implements that 

 are regarded as of human origin, the arrangement of the fract- 

 ures producing the cutting edge is so complicated as to pre- 

 clude the supposition that they are undesigned. Nor does it 

 require many secondary chippings to establish the artificial 

 origin of an implement. A half-dozen subsidiary chippings 

 on a natural flint pebble, serving to bring it into a symmetry 

 such as would serve the purpose of a human being, is evi- 

 dence enough. A trained eye has no difficulty in distin- 

 guishing, at a glance, between natural forms and artificial 

 forms. The loose statements asserting that there is occasion 

 for grave doubt as to whether the mass of so called palaeo- 

 lithic implements are really implements can only be made, 

 and be believed, by those who have given little personal at- 

 tention to the subject. 



That I may not seem to place too much confidence in 

 my own judgment in this all-important matter, I have 

 thought it best to secure the opinion, concerning the imple- 

 ments of which this chapter treats, of one who has had ample 

 opportunity to examine them and compare them with those 

 from other parts of the world, and whose authority would 

 be second to that of none. I therefore addressed a letter 



