52 Tne American Geologist. Jamuu-.v, i897 
of this clay are found northeast of Conshohocken in the neigh- 
borhood of Harmanville (two miles from the Schuylkill on 
the county line road). At Keys' clay pits and at Pott's mar- 
ble quarry, at an elevation of one hundred and eighty feet, 
these mottled clays are well seen. At the former locality the 
clays have a thickness of lifty-five feet. The succession of 
colors is yellow, red, white, blue (lignitic), yellow and mot- 
tled, red. The clays are overlaid by gravel varying from zero 
to eighteen feet in thickness and at a depth of lifty-five feet 
gravel again appears. At Keys' clay pits there is a steep dip 
to the south. About thirty feet below the surface in the lig- 
nitic clay were found fragments of wood. 
These are undoubtedly the "mottled clays" which Ward 
has recently referred to the Rappahannock series or "Basal 
Potomac." 
Disassociated from and also in association with these clays 
is a gravel formation which has been known as the " Bryn 
Mawr gravel." This gravel is found in isolated patches at 
elevations of from three hundred and twenty-five (Media) to 
four hundred and fifty feet (one-third mile northwest of Gra- 
dyville) and is also (in the writer's opinion) the gravel which 
overlies the mottled clays. It also occurs in Delaware, where 
it covers the summits of some of the higher hills. (It has been 
observed by the writer one and one-half miles northwest of 
Wilmington, Delaware, at the crossing of the Lancaster turn- 
pike by the Wilmington and Northern railroad.) The pres- 
ence of an ironstone conglomerate serves as a ready means of 
recognition of the gravel. This conglomerate is made up of 
rounded 'waterworn pebbles of pellucid quartz, of milky white 
quartz, of quartz stained wnth the red oxide of iron and of 
friable quartzite, both white and amethystine, imbedded in a 
tough sandy and ferruginous matrix. 
There are also present fragments of kaolinized feldspar and 
of what appears to be a siliceous limestone, from which the 
lime carbonate has been removed by solution. This detrital 
material ranges in size from a fine sand to pebbles which may 
be nearly six inches in diameter. (Pebbles of this size have 
been found by the writer in a conglomerate one-quarter mile 
*Ward, Lester F. The Potomac Formation. Fifteenth Annual Re- 
port U. S. Geol. Survey, pp. .324-336. 
