PALEOLITHIC MAN IN AMERICA. 



31 



bowlders, etc., representing the later deposit, and the homogene- 

 ous loam passing downward into coarse gravels representing the 

 older formation. The thickness of the deposit ranges from a 

 trifling veneer to forty feet or more ; and its surface, where it has 

 escaped erosion, forms a 

 plain inclined gently south- 

 ward from an altitude of 

 about forty - five feet in 

 north Trenton to tide-level 

 midway between Bristol 

 and Philadelphia ; this in- 

 clination of the deposit be- 

 ing the measure of the 

 northward tilting of the 

 land during the later ice 

 epoch. 



Within the Trenton 

 gravels two types of im- 

 plement are found — viz., 

 " turtle - backs " and the 

 rude " leaf -shaped " imple- 

 ments regarded by Abbott 

 as of Esquimau pattern. 

 Both types are chipped 

 from a peculiar argillite 

 which is found in the de- 

 posit only as (presumptive- 

 ly) finished implements or 

 as large bowlders. The im- 

 plements, which occur in such numbers that over twenty -five 

 thousand have been collected by Abbott, are seldom water-worn 

 though frequently weathered, while the bowlders are somewhat 

 worn by water and similarly weathered. It is significant that the 

 " turtle-back " type is found throughout the deposit from top to bot- 

 tom, but most abundantly in the lower half and in progressively 

 diminishing abundance from bottom to top of the upper half, while 

 the " leaf -shaped " type is found only in the upper half and in pro- 

 gressively increasing abundance upward ; it is also noteworthy that 

 both types of implement are occasionally found over contiguous 

 surfaces of the Columbia formation (which were above water-level 

 when the Trenton gravels were deposited), commonly associated 

 with chipped implements of higher type ; and it is equally note- 

 worthy that the implements of higher type occur over the surface 

 of the Trenton gravels but never within them, while the ruder im- 

 plements found within the gravels do not occur upon the gravel 

 surface. 



