vanhise.]  EASTERN    UNITED    STATES.  405 
mount  the  true  dip  of  the  rocks  is  very  steep,  although  there  is  a  decep- 
tive appearance  of  a  nearly  horizontal  stratification  in  thick  and  almost 
parallel  beds ;  but  this  is  not  to  be  confounded  with  the  genuine  strati- 
fication or  grain  of  the  rock  as  marked  by  the  general  distribution  of 
its  mica  and  other  minerals.  In  this  belt  there  are  really  two  groups 
of  rocks,  which,  viewed  broadly,  constitute  one  synclinal  wave.  The 
lower  is  a  harder  feldspathic  and  hornblendic  gneiss  at  the  south  side, 
dipping  northward,  and  reappearing  in  steep  and  multitudinous  con- 
tortions on  the  other  side  of  the  trough ;  and  the  upper  is  a  more  mica- 
ceous group  filling  the  synclinal  center  of  the  trough  and  compressed 
into  lesser  folds. 
In  the  middle  division  the  rocks  are  mostly  of  the  granite-like  varie- 
ties of  feldspathic  gneiss,  with  hard  hornblendic  gneiss,  such  as  consti- 
tute the  central  ridges  of  the  South  mountain.  These  are  believed  to 
be  in  a  series  of  anticlinal  and  synclinal  waves,  and  in  addition  to  the 
folds  there  is  a  series  of  folds  along  which  the  iron -ore  deposits  are 
found  in  V-shaped  masses. 
The  northern  or  South  mountain  zone  is  composed  of  massive  or  thick- 
bedded  gneisses,  with  which  is  no  talcose  slate,  or  else  the  Primal 
white  sandstone,  the  lowest  member  of  the  Paleozoic.  The  limestone 
associated  with  the  gneisses  is  generally  found  in  the  synclinal  valley. 
The  gneisses  are  regarded  as  stratified,  dip  to  the  southeast,  and,  as  the 
breadth  of  the  chain  is  so  great,  the  structure  is  believed  to  be  due  to 
overturn  flexures. 
On  the  Delaware  section  is  found  the  best  evidence  of  unconformity 
discovered  between  the  semicrystalline  rocks  called  Primal  and  the 
gneiss.  In  one  case  here  the  Primal  siliceous  slates  and  quartzites  are 
a  porphyritic  and  crystalline  quartzose  conglomerate.  Below  this  is 
an  arch  or  wave  of  granitoid  gneiss  containing  injections  of  syenite; 
and  the  dip  of  the  gneiss  seems  also  to  be  steeper  than  that  of  the 
Primal  conglomerate.  The  relations  are,  however,  best  seen  at  Durham 
creek.  Here  at  one  place  the  sandstone,  slates,  and  conglomerates  rest 
with  their  beds  almost  perpendicular  to  the  lamination  of  the  gneiss. 
The  lower  part  of  the  Paleozoic  rocks  are  Primal  crystalline  schists, 
or  the  Azoic  group;  Primal  conglomerate;  Primal  older  slate;  Primal 
white  sandstone,  Potsdam  of  New  York.  The  Primal  series  contains 
but  few  eruptive  rocks,  even  trap  dikes  being  uncommon,  which  is 
regarded  as  proving  that  the  metamorphism  is  due  to  heated  gases 
through  fissures  rather  than  to  the  contact  of  igneous  material. 
The  Primal  southern  belt  is  first  considered.  At  Attleboro  there  is 
no  marked  discordance  between  this  and  the  gneissic  series  which 
is  supposed  to  be  older.  East  of  the  Schuylkill  and  in  Montgomery 
county,  the  observer  is  very  liable  to  confound  the  lowest  Primal  beds 
with  the  uppermost  hornblendic  feldspathic  layers  of  the  adjacent  genu- 
ine gneiss.  West  of  the  Schuylkill  the  Primal  slates  are  of  so  crystal- 
line a  character  that  it  is  sometimes  difficult  to  distinguish  the  strata 
