TRANSACTIONS OF WAGNER 
MINERALOGY OF THE NEWARK GROUP IN PENNSYLVANIA 
brian limestone, which lies conformably upon the quartzite of the same age 
forming the hills to the north, because of the abrupt disappearance of the trap 
masses at the boundary, and the fact that the close synclinal to which the’ trap 
hills owe their shape also ceases there, the structure of the Cambrian rocks 
being quite different. Like many of the faults limiting the Newark belt, 
however, it does not continue to determine the boundary for any great distance, 
but turns on the southwest into the Newark, soon becoming obscured, and to 
the northeast into the Paleozoics, so that from a point north of Stonersville 
eastward the boundary is a true overlap, up to the next fault below 
Boyertown. 
The trap masses are, as already mentioned, indicated by prominent 
elevations, although they can not be accurately mapped on this basis, for in 
places the sedimentary rocks are exposed far up the flanks of the hills, and again 
the trap descends to the level of the streams. The exact boundaries between 
the trap and shale can, however, be determined by the character of the soil, 
especially in the case of the northern mass, where the change from red-brown 
shale soil to the yellow of that derived from the trap is particularly marked. 
The present map, representing the results of some two weeks of field work, 
has been prepared as carefully as possible with this point in view. 
The section through these hills cut by Antietam Creek affords the best 
view of their character and structural relations, and may therefore be described 
in detail. Starting from Wamsher's Mill, on the east side of the creek, one 
mile north of Lorane Station, on the Philadelphia and Reading Railway (called 
?xeter on the older maps), and proceeding northwards along the mill-race, the 
first rock encountered is a bright red sandy shale, dipping 30° north 30° 
east. With this there are interbedded several layers of limestone-pebble- 
conglomerate, showing that the large area of this rock to the west does not end 
in a rounded point, as indicated on the previous maps, but thins out gradually 
and is replaced eastward by the shale. A lens of yellow sandstone is also ex- 
posed here, containing an abundance of plant remains, apparently pieces of 
leaves and stems, but in too fragmentary condition for determination. 
Three-eighths of a mile above the mill the first trap appears, in the form 
of a dike of rather fine grained diabase about 50 feet wide; this can be traced 
southwestward for some three miles, to connection with the large body of 
igneous rock forming Gibraltar Hill, as will be described below. It appears 
to stand practically vertical, and has altered the shale to a distance of ı 5 feet, 
