258 MAN AND THE GLACIAL PERIOD. 



been previously worn out by the stream. Indeed, in every 

 place where opportunity offers for direct observation the 

 Trenton gravel is seen to be distinctly subsequent to the 

 other. It was not buried by the Philadelphia red gravel and 

 brick-clay, but to a limited degree overlies and buries it. 



The data for measuring the absolute length of time 

 between these two stages of the Glacial period are very 

 indefinite. Mr. McGee, however, supposes that since the 

 Columbia period a sufficient time has elapsed for the falls 

 of the Susquehanna to recede more than twenty miles 

 and for those of the Potomac eighteeu miles, and this 

 through a rock which is exceedingly obdurate. But, in 

 channels opening, as these do, freely outward, it is diffi- 

 cult to tell in what epochs the erosion has been principally 

 performed, since there are no buried channels, as in the 

 glaciated area, enabling us to determine whether or not 

 much of the eroding work of the river may have been ac- 

 complished in preglacial times. 



The lapse of time which, upon the least calculation, 

 separates the Columbia epoch from the Trenton, gives 

 unusual importance to any discovery of palaeolithic imple- 

 ments which may be made in the earlier deposits. We 

 are bound, therefore, to consider with special caution the 

 reported discovery of an implement in these deposits at 

 Claymont, Delaware. The discovery was made by Dr. 

 Hilborne T. Cresson, on July 13, 1887, during the prog- 

 ress of an extensive excavation in constructing the Balti- 

 more and Ohio Railroad, nineteen miles south of Phila- 

 delphia. The implement was from eight to nine feet 

 below the surface. As there is so much chance for error 

 of judgment respecting the undisturbed condition of the 

 strata, and as there was so little opportunity for Dr. Cres- 

 son to verify his conclusion, we may well wait for the cumu- 

 lative support of other discoveries before building a theory 

 upon it ; still, it will be profitable to consider the situation. 



Both Mr. McGee and myself have visited the locality 

 with Dr. Cresson, and there can be no doubt that the 



