1880.] NATURAL SCIENCES OF PHILADELPHIA. 291 



which, however, may be of later age, has been passed through by 

 artesian wells in the southern part of Philadelphia. The next 

 oldest clay appears to be the potters' clay of the Moutgomer}- Co. 

 limestone valley, which, containing sometimes lignite, and overlaid 

 by kaolin, decomposed hydromica slate, etc., belongs with its asso- 

 ciated limonite ores, to an inland Tertiary formation, the " Bran- 

 don Period,'' possibly of Oligocene age.^ A third clay, the 

 " Branchtown claj^" found at high elevations in a few places in 

 the gneissic region, containing occasional boulders, was made at a 

 period of general submergence and appears to be of a late Tertiary 

 age. The " Philadelphia brick-cla3' " of more recent formation, 

 of large extent, and with numerous boulders, is confined to the 

 river valley. This clay, deposited at the close of the Glacial period 

 by the waters resulting from the melting of the great Northern 

 Glacier, rests against the rocky " upland terrace " at a height of 

 about 150 feet above the present river. The fifth and newest 

 clay is the recent bog claj^ or mud in the flood-plain of the 

 river, still in process of formation. 



The gravels are distinguished from one another both by their 

 composition and b}' their relative hj'psometrical positions. The 

 " Bryn Mawr gravel" — the oldest gravel of consequence in this 

 region — is readil^^ distinguished from others by the peculiar mate- 

 rials composing it, and is also known b}' being found at high 

 elevations (400 feet), in often isolated patches, capping the gneissic 

 hills. It is characterized by absence of fossiliferous or Triassic 

 pebbles and by the presence of an iron conglomerate, and is of 

 oceanic origin, and probably upper Tertiary age.^ A similar gravel 

 occurs on the heights of Georgetown, D. C. The next oldest 

 gravel, also oceanic, and which here occurs at lower elevations 

 than the last, the writer called in a former paper " The Fossilifer- 

 ous Gravel." It frequently contains pebbles formed of Niagara 

 limestone and other fossiliferous rocks, and has been found abun- 

 dantly' in New Jersey as well as in Pennsylvania. It is well 

 exposed in the railroad cut at Ridley Park, Del. Co. It is the 

 yellow gravel Avhich caps the watershed between the Atlantic 

 and the Delaware at a height of nearly 200 feet, and is now named 

 for distinction " The Glassboro gravel." Its pebbles are frequently 



^ F. "The Iron Ores and Lignite of the Montgomery Co. Valley," by 

 the writer. Oct., 1879. 



= 7. "On the Bryn Mawr Gravel," by the writer, Mar., 1879. 

 20 



