Relation, of Streams to Brtjii Maiicr Gravel. — Dasconi. 53 
west of Grassland, Delaware county.) The pebbles of the 
gravel are identical with those of the ironstone conglomerate 
and like theiu are associated with sand. A section shows 
coarse gravel, grading below into sand and fine pebbles, which 
in turn passes into a reddish clayey layer. The middle layer 
contains the conglomerate. So far as is known to the writer 
the gravel is further characterized by the absence of fossilif- 
erous pebbles, of fragments (tf the Newark shale, of gneiss or 
garnetiferous mica-schist, or of any disintegrated igneous 
material. This gives it a lithological character, as well as a 
topography, distinct from the Jamesburg or Pensauken of 
New Jersey. 
This gravel has been referred to the Tertiary (Upper Mio- 
cene) by the Second Geological Survey of Pennsylvania* and 
by Carvill Lewisf on the basis of lithological correlation with 
New Jersey gravels. On thi-s basis it has also been referred 
to tiie Columbia,;!; with the Jamesburg of New Jersey. McGee 
claims that the gravel as examined by him, in exposures un- 
fortunately of a more or less evanescent character, shows all 
the characteristics of the Potomac and is undoubtedly iden- 
tical with exposures north and east of Conshohocken of a 
gravel anderlyiiig plastic clays, hence he makes it the ''lower 
member" of the Potomac. § 
The fact that the Bryn Mawr gravel is elsewhere found at 
so much higher altitudes than the clay, the presence of the 
ironstone conglomerate in the gravel overlying the cla3^s at 
Pott's quarry and other localities, the general lithological 
similarity of this gravel with the Bryn Mawr gravel, and the 
dissimilarity, though slight, of the underlying gravel, have 
led the writer to reverse McGee's conclusion as to position. 
If the Bryn Mawr gravel is a member of the Rappahannock 
series, it must be an upper member of that series. 
The Bryn Mawr gravel has thus been referred to Mesozoic, 
to Tertiary and to Quaternary time. Topographical and lith- 
ological dissimilarity with tlie Quaternary gravels of New 
*Second Geol. Survey Penn., Report C, 1885, pp. 10-13. 
tProc. Acad. Nat. Sci. Phila., 1880, pp. 209, 272. 488. Jour. Franklin 
Institute, volume 85. 1883, p. 373. Proc. Acad. Nat. Sci. Phila., 1884, 
p. 240. 
JSeventh Annual Report, U. S. Geol. Survey, p. CIO. 
§Amer. Jour. Sci., vol. xxxv, p. 130. 
