PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.       663 
ably  of  pre-Cambrian  age  and  that  it  carries  lens-shaped  deposits 
of  Cambrian  mica  schist  and  Cambrian  quartzite;  the  Cambro- 
Ordovician  limestone  resting,  by  means  of  overlap,  in  some  places 
directly  upon  the  pre-Cambrian  mica  gneiss  surface  and  in  others 
upon  an  inconsiderable  thickness  of  Cambrian  material. 
The  whole  series,  pre-Cambrian  mica  gneiss.  Cambrian  mica 
schist  and  quartzite,  and  Cambro-Ordovician  limestone,  has  been 
folded  into  fan-shaped  anticlines  and  synclines,  in  the  troughs  of 
which  the  limestone  has  been  preserved  from  erosion  and  has  been 
thrust,  by  means  of  a  fault  hading  gently  southeast,  upon  the  Bal- 
timore gneiss  and  the  Paleozoic  series.  In  the  southern  portion 
of  the  belt  Ordovician  mica  schist  is  thus  brought  adjacent  to  the 
Wissahickon  mica  gneiss  and  is  with  very  great  difficulty  separated 
from  it.  To  the  north,  first  Baltimore  gneiss  and  then  Cambrian 
quartzite  are  brought  in  contact  with  the  AYissahickon  mica  gneiss. 
The  Wissahickon  mica  gneiss  is  to  be  correlated  with  the  mica 
gneiss  of  Maryland.  With  this  formation  the  mica  gneiss  of  Penn- 
sylvania is  stratigraphically  continuous.  The  Wissahickon  mica 
gneiss,  together  with  the  intrusive  granite  and  gabbro,  comprises 
Rogers's  first  and  second  gneiss  belts  ("  Hypozoic  ").  It  embraces 
the  "  Chestnut  Hill,"  "  Manayunk,"  and  k*  Philadelphia  "  mica  schists 
and  gneisses,  mapped  as  pre-Cambrian  b}^  the  Second  Geological 
Survey  of  Pennsylvania. 
IGNEOUS    ROCKS. 
The  igneous  rocks  of  the  Piedmont  belt  are  exclusively  intrusive 
in  character  and  embrace  both  acidic  and  basic  types.  The  oldest 
intrusive  is  a  granite  batholith,  characteristically  porphyritic  and 
gneissoid.  The  basic  intrusives  are  chonoliths  and  dikes  of  a  gab- 
broitic  type — gabbro,  hypersthene  gabbro,  norite,  pyroxenite,  perido- 
tite,  metapyroxenite  and  metaperidotite  or  serpentine  and  steatite, 
chlorite  schists,  and  associated  decomposition  products. 
These  igneous  bodies  occur  abundantly  in  the  Wissahickon  mica 
gneiss  and  the  Baltimore  gneiss.  While  isolated  islands  of  horn- 
blende gabbro  and  of  serpentine  occur  in  Ordovician  material,  the 
igneous  material  is  otherwise  confined  to  the  gneisses,  and  is  pre- 
sumptively pre-Cambrian  in  age.  Similar  intrusives  are  found  in 
the  Atlantic  belt  of  crystallines  from  Virginia  to  New  York. 
Dikes  of  Triassic  diabase  of  a  uniform  petrographic  character  are 
found  in  the  Piedmont  district.  Two  species  arc  present  -an  ophito- 
augite  bandose  and  an  ophiti-augite  auvergnose. 
