338 H. N. EATON 
It is a hard, compact quartzite, usually white in color, weather- 
ing to white sand, and varying in texture from a rather fine basal 
quartz conglomerate to a whitish or reddish rock resembling jasper. 
Data for calculating the thickness of this formation on South 
Mountain are wanting. D’Invilliers' estimates the thickness 
around Reading at about 300 feet. Dr. Peck? assumes a thickness 
of 300 or more feet in Lehigh and Northampton counties. 
The quartzite lies unconformably upon the gneisses and doubtless 
formerly covered the mountain, although south of the Eagle Peak 
ridge no vestiges of it now remain. The scattered outliers of this 
formation on the gneiss of the Reading hills attest its once universal 
extent throughout the region. 
The greatest width of quartzite is roughly two and one-half 
miles measured eastward from the bed of Mill Creek north of 
Kleinfeltersville. 
East of Newmanstown the northern boundary nearly coincides 
with the railroad track for a distance of three and three-quarter 
miles, the width of outcrop of the quartzite decreasing rapidly until 
at Robesonia barely one-half mile of the rock intervenes between 
limestone and gneiss. From Robesonia eastward the quartzite 
steadily narrows until directly west of the Insane Asylum its width 
for the distance of a mile cannot exceed one-quarter of a mile and 
at one point is scarcely 200 yards. Thence eastward the northern 
boundary maintains an easterly direction, passing one-half mile 
north of Cushion Peak and along the northern foot of the hill east 
of Fritztown. The width of outcrop west of Cushion Peak is one 
and one-half miles. A tongue of quartzite, one-half mile wide, 
extends southwest from Cushion Peak to Laurel Ridge where it is 
concealed by the Triassic cover. Potsdam is not seen along the 
southern border west of this locality. The width of outcrop east 
of Fritztown is about one mile, the southern boundary maintaining 
a general easterly direction. 
Great Valley Limestone 
The Potsdam quartzite is conformably overlain by limestone of 
the Great Valley series everywhere along the northern border, with 
DO PuGisy1O2: 210 PGi 520: 
