^98 PROCEEDINGS OF THE ACADEMY OF [1880. 



weather-worn and eaten hy age, and have thus a much more ancient 

 appearance than the smooth, fresh-looking pebbles of later gravels. 

 It contains no boulders of consequence and is believed to be of 

 Pliocene age. 



L^-ing at a lower level, within the Grlassboro gravel, and formed 

 of a mixture of its pebbles with others brought down the Dela- 

 ware valle}^, is a third gravel — the " Philadelphia red gravel." 

 This, like its overlying bi'ick-cla}', is confined to the river valley. 

 It is distinctly stratified ; it contains numerous fragments of 

 Triassic red shale and of gneiss, and smooth boulders of Silurian 

 rocks ; it shows flow and plunge structure and wave action on a 

 large scale ; and like the older gravels, it rests upon a decomposed 

 gneiss, which is sometimes interstratified with its lower laj^ers. 

 There are numerous exposures near the Universitj^ of Pennsjd- 

 vania. The writer has identified it on the Potomac and other 

 rivers, and it appears to belong to the age of the melting glacier — 

 the Champlain epoch. 



The last and newest of all the gravels is one which, at Philadel- 

 phia, seemed to be of little importance. It lies close along 

 the river, and rising a few feet above it, extends but a short dis- 

 tance back from the river bank. It covers the flat ground of 

 Camden and the lower part of Philadelphia, and forms islands in 

 the river. It was called The River gravel and sand. It is this 

 alluvial gravel, the latest, except the recent mud-flats, of all the 

 surface formations, which is the subject of the present paper, and 

 which, from its great development farther up the river, is now 

 named The Trenton Gravel. It is in this gravel, and in this 

 gravel only, that traces of man have been found. 



The Trenton Gravel at Philadelphia is composed principally of 

 a sharp micaceous sand, which, when below water-level, becomes 

 a "quicksand." Gravel lies below the sand. Unlike all the other 

 gravels, it contains but few pebbles of white quartz, and is of a 

 dark gray color. Its pebbles are made exclusively of the rocks 

 forming the upper valley of the river. Their shape is also very 

 characteristic. The pebbles of the older gravels are oval or egg- 

 shaped, but these are for the most part flat. This flat shape is 

 characteristic of all true river gravels. At several places along 

 the Delaware, gold has been obtained from this gravel. The 

 absence of clay in anj^ of its laj-ers indicates the action of swiftly- 

 running water. Data obtained from artesian wells have shown 



