xiv MAN AND THE GLACIAL PERIOD. 



possibly the gravel in which the implements were found 

 had been disturbed. In some cases, as in Little Falls 

 and at Madison ville, he thinks the implements may have 

 worked down to a depth of several feet by the overturning 

 of trees or by the decay of the tap-root of trees. A suffi- 

 cient answer to these suggestions is, that Mr. Holmes is 

 able to find no instance in which the overturning of trees 

 has disturbed the soil to a depth of more than three or 

 four feet, while some of the implements in these places 

 had been found buried from eight to sixteen feet. Even 

 if, as Mr. Chamberlin suggests,* fifty generations of trees 

 have decayed on the spot since the retreat of the ice, it is 

 difficult to see how that would help the matter, since the 

 effect could not be cumulative, and fifty upturnings of 

 three or four feet would not produce the results of one up- 

 turning of eight feet. Moreover, at Trenton, where the 

 upturning of trees and the decaying of tap-roots would 

 have been as likely as anywhere to bury implements, 

 none of those of flint or jasper (which occur upon the sur- 

 face by tens of thousands) are buried more than a foot in 

 depth; while the argillite implements occur as low down 

 as fifteen or twenty feet. This limitation of flint and jas- 

 per implements to the surface is conclusively shown not 

 only by Dr. Abbott's discoveries, but also by the extensive 

 excavations at Trenton of Mr. Ernest Volk, whose collec- 

 tions formed so prominent a part of Professor Putnam's 

 Palaeolithic exhibit at the Columbian Exposition at Chi- 

 cago. In the village sites explored by Mr. Volk, argillite 

 was the exclusive material of the implements found in the 

 lower strata of gravel. Similar results are indicated by 

 the excavations of Mr. H. C. Mercer at Point Pleasant, 

 Pa., about twenty miles above Trenton, where, in the 

 lower strata, the argillite specimens are sixty-one times 

 more numerous than the jasper .are. 



* American Geologist, vol. xi, p. 188. 



