Serpentines Near Philadelphia.— Jonas. 303 
these ultra basic rocks which furnish the original material 
from which the serpentine is derived. 
* The serpentines of the. Philadelphia belt occur as 
dykes, intrusive in the mica-gneiss and Baltimore gneiss 
and strike southwest and northeast. 
They may be grouped into four dykes, each composed 
of a series of non-continuous outcrops extending southwest 
from near the Schuylkill. They form prominent ridges 
which are covered but scantily with soil and which are 
characterised often by a growth of cedar trees. 
The southeast dyke is intrusive in the Wissahickon 
mica-gneiss and extends from Chestnut Hill southwest to 
one mile east of Bryn Mawr station. The dyke as exposed 
at Lafayette is typically a grayish green soapstone. At 
Black Rock quarry the serpentine is a grayish green rock 
mottled with large dark green olivine crystals ; specimens 
have been found in which are large cross twins of ser- 
pentinised olivine. Under the microscope the mass of the 
rock is steatite, appearing with crossed nicols as brilliantly 
polarising scales. The steatite is considered to be an alter- 
ation product both of an original pryoxene and of the ser- 
pentine which it penetrates. The olivine present is altered 
only along the periphery and cracks and cores of it still 
remain. In addition the rock contains very abundant cal- 
cite and magnetite. 
The rock is an altered peridotyte whose original con- 
stituents are olivine and pyroxene which have altered to ser- 
pentinised talc and the by-products which accompany ser- 
pentinisation. Along the contact of the dyke with the mica- 
gneiss there has been formed chlorite-schist, a green schis- 
tose rock composed of chlorite with needles of hornblende. 
The second dyke lies to the northwest of the first and is 
intrusive in the Wissahickon mica-gneiss close to the 
boundary between it and the Baltimore gneiss. The dyke 
shows an exposure east of the Schuylkill. From Lafayette 
on the west side of the river it extends southwest with al- 
most continuous outcrops to Delaware county where it 
widens out considerably. The rock is very similar to that 
