658  PRE-CAMB-RIAN    GEOLOGY    OF    NORTH    AMERICA. 
Lesley,12  in  1883,  describes  the  continuation  in  the  southern  part 
of  Northampton  County  of  the  Highlands  of  New  Jersey.  There  are 
in  this  region  four  ranges.  In  the  valleys  are  limestones,  the  strati- 
fication of  which  is  visible  everywhere  but  is  much  broken  and 
crumpled.  The  stratification  of  the  gneiss  or  syenite  beds  of  the 
mountains  is,  on  the  contrary,  rarely  to  be  seen  and  can  be  judged  only 
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  homogeneous  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  are  comparable  with 
the  Laurentian  system.  Why  they  are  not  covered  by  Huronian  or 
Cambrian  rocks  is  not  known.  If,  as  supposed  by  those  who  do  not 
accept  the  views  of  Hall,  the  rocks  of  the  Philadelphia  belt  underlie 
the  Potsdam  and  overlie  the  Philadelphia  syenites,  it  is  hard  to  see 
why  these  rocks  do  not  appear  between  the  Potsdam  and  the  gneisses 
at  South  Mountain.  The  ridges,  instead  of  being  simple  anticlines, 
are  a  series  of  anticlines  andsynclines.  At  Morgan  Hill  there  is  dis- 
cordance between  the  dips  of  the  Potsdam  and  the  gneiss,  showing 
apparent  unconformity.  The  syenite  rocks  underlie  the  limestones, 
which  may  represent  residual  material  that  has  not  been  removed  by 
erosion.  The  crystalline  character  of  these  outlying  ridges  of  lime- 
stone may  be  explained  by  the  fact  that  the  material  has  been  buried 
30,000  or  40,000  feet  below  the  surface.  At  Chestnut  Hill  Gap,  on 
the  Delaware,  the  Potsdam  sandstone  is  sometimes  vitreous  and  over 
it  are  limestones  changed  into  crystalline  dolomites  holding  serpen- 
tines. At  a  contact  with  a  dike  of  coarse  granite  near  the  south  side 
of  the  gap  the  slates  are  changed  into  chlorite,  mica  slate,  and  horn- 
blende slate,  but  in  the  coarser  grits  the  original  pebbles  are  seen. 
Hall  (C.  E.),13  in  1883,  describes  many  localities  of  slates,  gneisses, 
and  granites  in  the  South  Mountain  area. 
D'Invilliers,14  in  1883,  states  that  the  existence  of  anticlinal  and 
synclinal  folds  in  the  South  Mountain  belt  of  Berks  County  is  sug- 
gested by  the  alternate  anticlinal  and  synclinal  belts  of  limestone  and 
slate,  but  it  is  not  conclusively  proved,  for  these  formations  belong 
to  different  systems  of  rocks,  and  no  doubt  lie  unconformably  upon 
the  older  mountain  rocks.  The  South  Mountain  rocks  are  gneisses 
and  granites,  which  are  of  two  kinds — a  distinctly  stratified,  thick- 
bedded,  massive  gneiss,  and  a  stratified  syenite  in  which  hornblende 
is  predominant.  The  eroded  edges  of  the  Potsdam  sandstone  run 
along  the  northern  slope  of  the  belt  overlying  the  gneissoid  rocks. 
Hall  (C.  E.),15  in  1885,  places  the  syenites  of  Delaware  County 
with  the  Laurentian.     Overlying  these  are  the  micaceous  and  garnet- 
