654  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
which  it  contains.  The  upper  or  northern  belt  of  gneiss  is  regarded 
as  a  part  of  the  lower  Primal  rocks  and  as  resting  unconformably 
upon  the  upper  gneissic  group,  the  belief  being  based  upon  the  man- 
ner of  the  flexure  of  the  two  formations  rather  than  upon  actual 
unconformable  contacts. 
Tn  the  Philadelphia  belt  there  is  a  general  prevalence  of  the  north- 
ward dip  of  the  strata,  varying  generally  from  30°  to  50°.  At  Fair- 
mount  the  true  dip  of  the  rocks  is  very  steep,  although  there  is  a 
deceptive  appearance  of  a  nearly  horizontal  stratification  in  thick  and 
almost  parallel  beds ;  but  this  is  not  to  be  confounded  with  the  genu- 
ine stratification  or  grain  of  the  rock  as  marked  by  the  general  dis- 
tribution 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  contortions  on  the  other  side  of  the  trough;  and  the 
upper  is  a  more  micaceous  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  va~ 
rieties  of  feldspathic  gneiss,  with  hard  hornblendic  gneiss,  such  as 
constitute  the  central  ridges  of  the  South  Mountain.  These  are  be- 
lieved to  be  in  a  series  of  anticlinal  and  synclinal  waves,  and  in  addi- 
tion 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  either  of  mas- 
sive or  thick-bedded  gneisses,  with  which  is  no  talcose  slate,  or  else  of 
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 ;  they  dip  to 
the  southeast,  and,  as  the  breadth  of  the  chain  is  so  great,  the  struc- 
ture is  believed  to  be  clue  to  overturned  flexures. 
In  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  Dur- 
ham Creek.  Here  at  one  place  the  sandstone,  slates,  and  conglom- 
erates rest  with  their  beds  almost  perpendicular  to  the  lamination  of 
the  gneiss. 
The  lower  part  of  the  Paleozoic  rocks  consists  of  Primal  crystalline 
schists,  or  the  Azoic  group ;  Primal  conglomerate ;  Primal  older  slate ; 
Primal  white  sandstone,  and  Potsdam  of  New  York.  The  Primal 
series  contains  but  few  eruptive  rocks,  even  trap  dikes  being  uncoin- 
