MAN AND THE GLACIAL PERIOD. 671 



accompanying illustration, reproduced from a photograph. 

 The lower part of this cut consists of decomposed schist 

 rocks in place and of deposits which are preglacial. These 

 extend in the illustration to the top of the light band run- 

 ning through the picture. The portion above this light band 

 belongs to what was described in the preceding chapter as 

 pertaining to the formation denominated by Professor Lewis 

 the Philadelphia red gravel and brick-clay, being identical 

 with that at Philadelphia both in its composition and in its 

 stratigraphical relations, and extending continuously down 

 the river from that city (nineteen miles). By Mr. McGee this 

 would be denominated the Columbia formation, since he cor- 

 relates the deposits in the Delaware Valley with those in the 

 District of Columbia in the valley of the Potomac. The age 

 of this deposit we have already discussed,* and we need here 

 only repeat that it is without doubt a glacial formation of a 

 much earlier period than that farther up the valley at Tren- 

 ton, X. J., where Dr. Abbott made his important discoveries. 

 While that at Trenton belongs to the later stages of the Ice 

 age, this at Claymont is to be connected with the ice when 

 at its maximum extension, and when the level of the region 

 was depressed one hundred feet or more. In a preceding 

 chapter I have given my reasons for questioning the theory 

 of Mr. McGee, who would connect this deposit with a glacial 

 age previous to, and entirely distinct from, that which was 

 concerned with the deposits at Trenton, and which he would 

 make from three to ten times as remote. But, whichever view 

 upon this point prevails, whether that of two distinct glacial 

 epochs, or of one prolonged epoch with various halts in the 

 retreat of the ice, the Philadelphia red gravel and brick-clay 

 must be regarded on the least calculation as some thousands 

 of years older than the deposits at Trenton, N. J., Loveland 

 and Madisonville, O., Medora, Ind., and Little Falls, Minn. 



The circumstances of the discovery are thus reported by 

 Mr. Cressou : 



* See pp. 613, 635 et seq. 



