1877.] 249 [Prime. 
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. 
The Potsdam sandstone often, as elsewhere, contains Scolithus. 
Next above the Potsdam sandstone occur hydromica slates, which Rogers 
has called the Upper Primal Slates, but which really form a portion of the 
No. IT limestone, and gradually pass into this. They overlie the Potsdam 
conformably and are 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 lie along the north flank of the 
South Mountain and overlie the Potsdam conformably wherever this is 
visible. They are of great economic importance as carrying the lowest 
range of brown hematite iron ores, to be mentioned later. 
These slates are composed in great part of the mineral damourite and 
occur of a pink, gray, white, and yellow color. When exposed to the 
weather they very rapidly decompose to soft unctuous plastic clays in a 
few days, and some of these will in time probably become valuable in the 
manufacture of coarse kinds of pottery. Generally they contain more or 
less of the carbonates of lime and magnesia and silica mixed with the 
damourite.* Hydromica slate also occurs the greater portion of the dis- 
tance from the western boundary of Lehigh county to the Delaware River, 
at the junction of the No. II limestones with the No. III slates, here also 
carrying brown hematite ores in extensive deposits. 
It also occurs intercalated in the limestone, forming layers from the 
thickness of a sheet of paper to several feet, and these layers are innumer- 
able. Their existence has been seen both in rock outcrops as well as in 
wells which have been stink. 
The clay to which the hydromica slate decomposes is generally of a 
white color, although sometimes brown from the presence of hydrated 
ferric oxide. Analyses would seem to show that the clay contains rather 
less potash than the undecomposed rock. 
Overlying the hydromica slates, and conformable with these and the 
Potsdam sandstone, is the No. II or Magnesian limestone (Auroral of 
Rogers), which extends as a great mass varying from six to seven and a 
quarter miles in width. At four points gneiss crops out through the lime- 
stone. These are at Chestnut Hill north of Easton, at a hill two miles 
north of Bethlehem, the gneiss ridge north of the Lehigh, between Allen- 
town and Bethlehem, and at Jerusalem Church, two miles north of Emaus. 
Otherwise its continuity is unbroken. 
In its lower beds the limestone contains large quantities of chert, form- 
ing nodular masses of very various sizes and usually having their longest 
axes conformable to the bedding of the enclosing rock. This chert occurs 
in the manner described by Safford} as characteristic of the Knox dolo- 
mite of Tennessee. It disappears, however, in the upper strata. 
* Report of Progress for 1874 of Lehigh Dist. Geol. Survey of Pennsylvania, 
p. 12. 
+ See Geology of Tennessee, by Safford, p. 215, 218. 
PROC. AMER. PHILOS. Soc. xvir. 100. 2F. PRINTED JAN. 26, 1878. 
