TRANSACTIONS OF WAGNER 
MINERALOGY OF THE NEWARK GROUP IN PENNSYLVANIA 
The trap near the base of the sheet is distinctly porphyritic, and its sections 
show large equidimensional feldspar crystals embedded in the groundmass 
containing the usual blades of the same mineral. (See Figure 2.) Two 
stages of crystallization are thus represented, the large crystals having perhaps 
.been formed before the extrusion of the magma. A section of a specimen from 
a height of тоо feet shows по trace of this porphyritic texture, but in it the 
cavities begin to appear, most of them being approximately circular in outline, 
and filled with calcite, occasionally enclosing flakes and concentric groupings 
of a chloritic mineral, showing a strong pleiochroism from dark bluish green to 
pale yellow. (Figure 3.) 
As noted in the description of the geology of the region, the top of the sheet 
has been so deeply weathered that there are no good exposures, but the frag- 
ments mingled with those of the overlying shale consist of a highly vesicular 
slag-like material. A thin section made of this showed it to consist chiefly 
of dark gray glass, containing feldspar in decidedly smaller amount than the 
lower portions of the sheet, and quite free from olivine and augite. (See 
Figure 4.) This no doubt represents the surface of the flow, and it is very 
similar to material found in corresponding positions in the Watchung Mountains 
in New Jersey. Sections were also made of the shale immediately below this 
extrusive sheet, and they showed but little difference from the ordinary unaltered 
sediment, with the exception of the presence of considerable crystalline calcite. 
There is certainly no development of secondary minerals, such as observed in 
the rocks underlying the southern sheet. 
The results of this microscopic examination are thus in complete agreement 
with those of the structural studies of the trap sheets, and add a finishing touch 
to the proof of their difference in origin. 
