262 F. Prime, Jr.—Lower Silurian Fossils, 
occurs on the northwest flank of the hill and undoubtedly has 
a northwest dip. It next occurs in two small patches on the 
northern flank of the main range of the South Mountain near 
Macungie (formerly Millerstown). A small patch of it is also 
found associated with the gneiss, where the latter crops out 
through the limestone in the gorge of the Little Lehigh Creek 
at Jerusalem Church, two miles northwest of Emaus. But it 
is first seen to any great extent along the north flank of the 
main range just south of Emaus, where its occurrence is con- 
stant, but of varying thickness, and continues for a distance of 
four and a half miles, after which it can no longer be traced. 
It occurs again at the ridge of the South Mountain, close to 
Allentown, which forms the southern barrier of the Lehigh 
to the oxidation of the ferrous oxide it contains. The change 
‘om a pudding-stone to a compact quartzite in the sandstone 
shows that there has been a a sinking of the earth's crust 
and an increase in the depth of the sea, thus preparing the way 
for the subsequent deposition of the limestone. 
e Potsdam sandstone often, as elsewhere, contains Scolithus. 
Next above the Potsdam sandstone oceur hydromica slates, 
which Rogers has called the Upper Primal Slates, but which 
really form a portion of the No. LI limestone, and gradually pass 
into this. They lie along the north flank of the South Mountain 
and cverlie the Pots Jam conformably wherever this is visible, 
eing far more persistent in their occurrence, continuing with 
few intervals the entire distance from the western boundary of 
Lehigh county to the Delaware River. They are of great eco- 
nomic importance as carrying the lowest range of brown hem- 
atite iron ores, to be mentioned later. : 
se slates are composed in t part of the mineral 
damourite and occur of a pink, Sigil and yellow color. 
