1880.] NATURAL SCIENCES OF PHILADELPHIA. 2tl 



glomerate and gravel, similar in appearance, and composed of 

 the same materials as the formation in Pennsylvania. Tlie con- 

 glomerate has the peculiar ferruginous glaze already noticed. 

 It here overlies Cretaceous marls aud sands. 



From its abundance at this place, and in order to show its con- 

 nection with Pennsylvania deposits, we shall call the conglomerate 

 of the Biyn Mawr gravel, " Ji?. Holly Conglomerate.'''' Prof. H. 

 D. Rogers' suggests that this rock at Mt. Holly may be of Mio- 

 cene age ; but Prof. Cook, not distinguishing it from the modern 

 iron crusts in the red Philadelphia graAel near the river, considers 

 it veiy recent. In the consideration of its age it is worth noting 

 that the sand of southern New Jersej^, apparently of late Plio- 

 cene age, frequently contains rounded pebbles of Mt. Holly con- 

 glomerate, thus showing that the latter is an older formation. 



From the identit}^ of their contained boulders, it is probable 

 that the Branchtown cla}' and the Br3'n Mawr gravel are nearly 

 coeval. Being oceanic, it is presumed that they will be recognized 

 all along the gneissic hills of the southern Atlantic States. 



We have given this detailed description of each of the surface 

 formations near Philadelphia in the hope that they may be recog- 

 nized elsewhere by other geologists. It has been found that a 

 careful examination of the materials comprising each gravel, taken 

 in connection with their elevation above tide, is the onl}- means of 

 - discriminating between them. Desultory observations in detached 

 localities are of little value. Should this work be extended in 

 Penns^dvania and New Jersey, and the distinctions between the 

 four gravels described be carried out, it is thought that, notwith- 

 standing the shifting character of the underlying strata in the 

 latter State, much may be done not only towards an exact deter- 

 mination of their age, but towards a settlement of some of the 

 vexed problems of surface geology in Eastern America. 



Recapitulation. — The results obtained may be briefly summa- 

 rized as follows: — 



Forming the N W. bound.iry of the Philadelphia gravel and 

 brick-cla}' is a hill of gneis?, rising 200 feet or more above the 

 river, which ma}^ be called the Upland Terrace. It has a N. E. 

 and S. W. trend, and in this vicinity is at an average distance of 

 five miles from the river. 



1 Report ou the Geology of N. J., 1839. 



