FREE INSTITUTE OF SCIENCE 
MINERALOGY OF THE NEWARK GROUP IN PENNSYLVANIA 
PETROGRAPHY OF THE IGNEOUS ROCKS 
ICROSCOPIC examination of thin sections of the southern mass, which, 
as shown above, is to be regarded as intrusive, shows it to be a typical 
diabase, quite similar in character to the many other occurrences in the 
Newark of Pennsylvania, and to the Palisade-Rocky Hill sheet extending across 
New Jersey. The minerals present are plagioclase feldspar and augite, with 
occasional grains of olivine and of magnetite. The plagioclase is in long slender 
blades, twinned according to the albite law, so that in polarized light they show 
dark and light bands running lengthwise. The maximum angle of extinction 
observed on crystals cut perpendicular to this twinning plane, when the parts 
become dark at equal distances from the plane, is about 40°, so that the feldspar 
represents a basic labradorite. These feldspars interlace to form a close net- 
work, the other minerals filling up the interspaces, presenting the characteristic 
diabase texture. The principal mineral in this position is augite, greenish gray 
irregular grains, often showing slight alteration to chlorite. The olivine and 
magnetite are usually in smaller grains, more or less surrounded or enclosed by 
the other minerals. 
At both the upper and lower contacts the character of the rock changes 
slightly, becoming much finer grained, and in the sections an increase in the 
amount of olivine is noticeable, the grains of this mineral becoming as large as 
the augites: Even at the very contact, however, there is no appearance of glass, 
and in this the greatest contrast with the northern sheet is found. (See Figure 
1.) The sediments at the contact show dull brownish yellow epidote taking 
the place of the red mud of the unaltered shales, and occasional minute 
dots of magnetite. 
The most striking feature of the sections of the northern trap sheet is the 
presence of considerable amounts of nearly black glass, in which the other 
minerals, feldspar, augite, olivine and magnetite are embedded. The feldspars 
are similar in shape to those of the other rock, but have slightly less extinction 
angles, usually about 30°, indicating their greater acidity. ‘The augite and 
olivine are in small grains, sometimes showing crystal outline, and occasionally 
enclosing groups of magnetite crystals. Examination of the glass under high 
power shows that its dark color is due in part to its containing abundant magnetite, 
and it is also full of feldspar, either in minute needles or in spherulites. 
