PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.      661 
the  crystallines  in  the  central  portion  of  the  district,  leaving  them 
exposed  in  belts  on  the  southeast  and  northwest  borders. 
The  more  easterly  of  these  two  belts,  with  an  extension  on  the 
Maryland-Pennsylvania  line  of  72  miles,  contracts  to  the  northeast, 
and  north  of  Trenton  disappears  altogether  under  the  cover  of  later 
deposits.  This  belt  contains  the  full  series  of  formations  found  else- 
where in  the  Pennsylvania  Piedmont.  The  series  has  been  subjected 
to  greater  metamorphism  than  is  the  case  in  the  westerly  belt,  due 
both  to  intense  folding  and  to  the  injection  of  great  masses  of  acidic 
and  basic  igneous  material. 
SEDIMENTARY    ROCKS. 
The  sedimentary  series  is  as  f ollowTs : 
Ordovician  :  Octoraro  schist. 
Cambro-Ordovician :  Shenandoah  limestone. 
Cambrian :  Chickies  quartzite. 
Pre-Cambrian|Wissahickon  mica  gneiss' 
[Baltimore  gneiss. 
Baltimore  gneiss. — This  formation  includes  primarily  a  medium- 
grained  granite  gneiss  in  which  hornblende  is  the  chief  ferromagne- 
sian  constituent,  and  subordinately  a  medium-grained  arkose  gneiss 
in  which  biotite  is  the  chief  ferromagnesian  constituent.  The  granite 
gneiss  has  a  more  considerable  distribution  than  the  arkose  gneiss  and 
presumably  represents  the  parent  rock  from  which  the  arkose  gneiss 
was  derived.  The  mica  of  the  arkose  gneiss  occurs  in  minute  plates 
and  is  never  developed  in  such  dimensions  or  to  such  excess  as  to  give 
a  schistose  character  to  the  formation,  which  is  normally  characterized 
by  a  pronounced  gneissic  banding,  often  fine  and  closely  plicated. 
The  rock  may  further  show  a  pseudoporphyritic  texture,  due  to  an 
original  conglomeratic  character.  Graphite  is  sometimes  present,  dis- 
seminated in  minute  scales  or  occurring  in  such  abundance  as  to  be 
of  economic  importance.  The  presence  of  this  mineral,  the  absence 
of  such  microscopic  pressure  effects  as  dynamic  force  produces  in  an 
igneous  formation,  the  rounded  apatites,  and  the  sorting  of  mineral 
constituents  are  microscopic  evidences  of  a  sedimentary  origin,  which 
are  supported  by  field  evidence. 
Buck  Ridge,  a  highland  occupying  the  central  portion  of  the  belt, 
is  composed  of  a  complex  of  the  Baltimore  gneiss  and  a  gabbro  by 
which  it  is  thoroughly  injected  and  which  gives  the  gneiss  a  massive 
and  granitic  aspect  along  contacts.  Where  Schuylkill  River  cuts 
through  Buck  Ridge  the  Baltimore  gneiss  is  exposed  in  an  anticline. 
On  the  southeast  the  Wissahickon  mica  gneiss  has  been  brought  adja 
cent  to  it,  presumably  by  a  thrust  fault,  and  on  the  northwest  it  is 
flanked  in  normal  succession  by  a  Cambrian  quartzite. 
