302 The American Geologist. 
November, 1903 
In the southeastern belt, the oldest rock is the Balti- 
more gneiss of pre-Cambrian age. It is composed of light 
bands of quartz and feldspar alternating with dark horn- 
blendic layers, closely folded and contorted. This rock to- 
gether with gabbro forms Buckridge and strikes northeast 
and southwest, separating the Cambro-Silurian series of 
quartzyte, limestone and mica-schist from the Wissahickon 
mica-gneiss to the southeast. Unconformably overlying the 
Baltimore gneiss is the Chickies quartzyte, a thinly bedded 
crystalline ruck full of sericite, which gives it a buff to 
green color and a schistose character. This resistant rock 
forms the ridge of the north Chester valley hills. To the 
south of these hills is the limestone of the Chester valley. 
It is a magnesian limestone of Cambro-Silurian age and it 
grades upward into the Hudson River mica-schist, which 
caps the south Chester valley hills. The mica-schist is a 
schistose rock composed of mica and quartz ; the quartz 
is present in lenses about which the mica is bent. 
The age of the Wissahickon mica-gneiss has not yet 
been fully worked out. The rock is silvery gray in color, 
with alternating schistose and gneissic bands. Abundant 
mica shows on the cleavage planes and quartz and feld- 
spar on planes at right angles to the cleavage. 
The Triassic cover of red sandstone and shale still re- 
mains over a portion of the Piedmont plateau. Contem- 
poraneous with the deposition of the Triassic beds was a 
flow of basalt and an intrusion of diabase extending from 
Connecticut to Virginia. In Pennsylvania this resulted in 
a series of diabase dykes which extend southwest through 
the plateau. 
The older igneous rocks are more abundant in the re- 
gion. There was a period of post-Ordovician and pre- 
Triassic activity which assisted the forces of regional me- 
tamorphism in altering the sedimentary Palaeozoic and pre- 
Palaeozoic rocks. The igneous material is granite and 
gabbroitic, the main mass is gabbro which, in the vicinity of 
Philadelphia is intruded into the Baltimore gneiss and mica- 
gneiss. The rock is either an augite, or hypersthene-gabbro 
penetrated by dykes of peridotytes or pyroxenytes. — It is 
