408  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
Hall  (Ohas.  E.),156  in  1881,  describes  Philadelphia  county  and  the 
southern  parts  of  Montgomery  and  Bucks.  The  schistose  rocks  are 
placed  in  the  three  belts  as  divided  by  Rogers,  but  there  is  an  inter- 
mediate belt  between  the  first  and  second  belts  of  Rogers.  The  first 
belt  is  made  up  of  gray  schistose  gneiss,  composed  of  quartz,  feldspar, 
and  brown  or  black  mica,  with  occasional  garnets,  interlaminated  with 
occasional  beds  of  black  hornblendic  slate  and  fine  grained  sandy 
gneiss.  The  second  belt  is  characterized  by  serpentine,  soapstone, 
silvery  micaceous  garnetiferous  schists,  light  colored  thin  bedded  sandy 
gneisses,  with  disseminated  light  colored  mica  in  minute  flakes.  The 
third  belt  is  composed  .chiefly  of  quartz,  feldspar,  and  hornblende. 
The  beds  are  often  massive,  but  usually  have  thin  bands  of  mica  or 
hornblende  through  them.  They  are  syenitic  and  gneissic  granites 
or  granitic  gneisses  in  which  is  found  a  peculiar  variety  of  blue 
quartz.  The  prevailing  northward  dips  of  the  schists  and  gneisses 
of  the  first  and  second  belts  do  not  hold  for  the  third.  The  Primal 
sandstone  (Potsdam),  wherever  it  occurs,  invariably  rests  upon  the 
rocks  of  the  third  belt,  and  its  sandstones  and  conglomerates  are  in- 
variably composed  of  debris  from  this  belt,  and  in  it  is  found  not  a 
single  flake  of  mica,  quartz,  or  other  material  which  can  belong  to  the 
first  or  second  belts.  For  considerable  distances  the  Primal  rocks  are 
found  between  the  third  belt  and  the  schists  of  the  second  belt.  At 
the  Schuylkill  the  rocks  of  the  first  and  second  belt  rest  upon  and 
against  the  rocks  forming  the  third  belt.  The  third  belt  is  regarded 
as  Laurentian  and  the  first  and  second  belt  are  assigned  a  position 
above  the  Primal  Potsdam  sandstone  and  the  Auroral  limestone.  In 
the  midst  of  the  roofing-slates  of  the  Susquehanna  river  occur  Hudson 
river  fossils,  and  the  first  and  second  belts  are  referred  to  or  above  the 
Hudson  river  group,  while  the  third  belt  is  referred  to  the  Laurentian. 
Lesley,157  in  1883,  describes  in  the  southern  part  of  Northampton* 
county  the  continuation  of  the  Highlands  of  New  Jersey.  There  are 
in  this  region  four  ranges.  In  the  valleys  are  limestones,  the  strat- 
ification of  which  is  visible  everywhere  but  is  much  broken  and 
crumpled.  The  stratification  of  the  gneiss  or  syenite  beds  of  the  moun- 
tains is,  on  the  contraiy,  rarely  to  be  seen  and  can  only  be  judged  from 
topographical  features.  Dips  are  hard  to  find,  owing  to  the  general 
decomposition  of  the  rock  surfaces  of  the  country,  to  the  amount  of 
debris  on  the  surface,  to  the  vegetation,  and  to  the  massive  and  ho- 
mogeneous character  of  the  beds  where  the  true  bedding  plane  has 
sometimes  been  made  out  by  observing  the  parallel  arrangement  of  the 
minerals.  The  South  mountain  gneisses  evidently  belong  to  a  different 
system  from  the  Philadelphia  belt  and  they  are  comparable  with  the 
Laurentian  system.  Why  they  are  not  covered  by  Huronian  or  Cam- 
brian rocks  is  not  known.  If  the  views  of  Hall  are  accepted  that  the 
Philadelphia  belt  underlies  the  Potsdam  and  overlies  the  Philadelphia 
syenites,  it  is  hard  to  see  why  they  do  not  appear  between  the  Potsdam 
