1880.] NATURAL SCIENCES OP PHILADELPHIA. 305 



often rounded by attrition, have been found in this gravel. 

 Although the Trenton gravel has none of the features of a moraine, 

 it is true that the cliff at the base of Riverview Cemetery, holding 

 immense boulders, has the appearance of having been deposited 

 b}' glacial waters. At other places, the boulders resting upon the 

 sand overlying the gravel suggests the grounding of large ice- 

 cakes derived from some mass of ice large enough to be called a 

 glacier. 



It is difficult to imagine an origin for such a flood as we have 

 described other than the melting of a glacier. We have shown 

 that the flood was not an inroad from the sea, but that it 

 came down the valle}'. No rain-storms of modern experience 

 could have supplied such an amount of water. To call the time 

 of this flood a '■ Pluvial Epoch," will be of little assistance, since 

 no origin for such extraordinary rains is suggested, except under 

 a very different climate, or by evaporation from a melting glacier. 



Yet such a glacier cannot be the great glacier of the Glacial epoch. 

 That was the glacier which in its melting deposited the brick-clay 

 and red gravel which Tve have shown to be much older than the 

 Trenton gravel. It must have been, if a glacier at all, another 

 and more recent one whose melting caused the flood which formed 

 this gravel. This last glacial flood flowed in a channel excavated 

 through the deposits of the first glacial period. 



It appears, then, that there is evidence of a Second Glacial 

 Period — a period in which was deposited the last of the gravels, 

 and which has but lately passed away. From the limited extent 

 of its deposits it is inferred that the second glacier was much 

 smaller than the fti-st, and that its southern extremity was con- 

 fined to the valley. A second glacial period is recognized in 

 Europe under the name of the Reindeer Period. 



It is thought that the hypothesis of a second and more local 

 glacier, long subsequent in age to the first great glacier, will 

 explain all the facts observed. The Trenton gravel cannot be 

 assigned to the first glacial period except by assuming that there 

 have been no river gravels deposited since that time ; — an assump- 

 tion which can hardly be maintained. Some European archje- 

 ologists have held that the Palseolithic Era, the era of the river 

 gravels, is antecedent to the Reindeer Period, the period of the 

 cave-men. No such distinction has been observed on the Delaware. 

 Should future researches show that a separate and second glacial 



