FREE INSTITUTE (OF SCIENCE 
MINERALOGY OF THE NEWARK GROUP IN PENNSYLVANIA 
GEOLOGY OF THE DISTRIUT 
HE First Geological Survey of the State of Pennsylvania overlooked the 
presence of igneous rocks in the Newark to the southeast of Reading. 
In the course of the far more detailed work carried on by the Second 
Survey, however, the occurrence of such rocks was soon observed, and in the 
report on the Geology of the South Mountain Belt of Berks County, they were 
described by Mr. E. V. D'Invilliers in the following words: 
“The two concentric trap dykes here met with in Exeter Township form 
a prominent landmark in the topography of the country, rising from the west, 
where their summits are about 400 feet above ocean level, to the east, where 
they reach above 500 feet. 
“The level of the red shale country surrounding them and contained 
between their walls is about 300 feet, which unlike the Boyertown deposit, 
has not been changed in the least either in the position or character of its beds 
from the usual succession of shale and sandstone so familiar in the general 
formation.” 
The northern “dyke” is further described as “dark, fine-grained trap, 
containing a good deal of augite”; the southern one is said to be lighter in color, 
and the occurrence of a “variegated red shale and sandstone” at its lower 
boundary is mentioned, although elsewhere it is stated that the “sandrocks 
south of Jacksonwald have scarcely been changed by the two trap dykes there.” 
This district received no further attention until 1908, when two brief 
notes upon it were published simultaneously. Dr. A. C. Spencer, in describing 
the occurrence of iron ores on the Esterly farm, states “The diabase [asso- 
ciated with the ore] is the outer of two concentric curving sills which follow the 
bedding of the Mesozoic shales and sandstones, here thrown into a rather sharp 
synclinal fold.” 
In the paper on the Newark copper deposits, mentioned in the introduction, 
the present writer announced that the inner of these two trap masses is not a 
sill, but an overflow-sheet, presenting as evidence the vesicular character of 
the rock, and the complete absence of alteration in the enclosing shales. 
Further proof of the correctness of this interpretation is given herewith. 
The accompanying geological map, Plate 1, shows the areal distribution 
of the rocks. The Newark is considered to be faulted against the Cam- 
