Pennsylvania Geology Final Report Volume 1 1981

108 GEOLOGICAL SURVEY OF PENNSYLVANIA,
in Pennsylvania; and about this it is necessary to say some-
thing.
The eastern range of the Canadian mountains, through
Labrador to the ocean shore, is a country of massive and
schistose gneisses, many of which are made up of quartz
and labradorite feldspar/- This feldspar is essentially a
silicate of alumina, lime and soda (53 : 30 : 12 : 5), and might
be formed in a metamorphosed sediment just as well as any
other feldspar, provided the drainage of that sediment was
from a limestone region. It is a frequent constituent of
modern volcanic lavas, and is found also in some ancient
trap-dykes and porphyries; but so are several other feldspars.
It is in itself no proof of the plutonic origin of the
East Canada gneisses.
It is still a question what relation in time the great Lab-
radorian (Norwegian or Norian) system of gneisses holds
to the West Canada Laurentian (Ottawa or Granville) system
of gneisses.
Now, although the Norian rocks are commonly called
Upper Laurentian^ it is possible to consider them as two
geographical areas of one and the same system; in which
case the abundance of labradorite feldspar\ in the eastern
area would be an accident of drainage. On any other
the absence of labrado-
hypothesis it is difficult to explain
rite gneiss from the range of the New York, New Jersey and
Pennsylvania highlands ; for it looks as if we had in this
range the upper part of any Laurentian system which
could be established on anything like a sedimentary basis
of argument.
T?te argumentfrom marble.
White crystalline limestone beds are interleaved with
the gneiss rocks of the New Jersey highlands all along
their northwest border, and some are pure marbles (96.50 :
1.13 :]. 30 : 0.30) ; others nearly pure dolomites (53.00 :
42.26. 3.50 alum, and ox. iron : 0.50 silica and insoluble).:}:
*With hornblende, hypersthene and magnetic iron.
fOr rather of the whole group of plagioclase (soda and lime) feldspars.
jCook'sAn. Rt., 1864, page 15.