PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.      709 
soul  hern  Appalachian  is  a  province  in  which  the  unconformity  be- 
tween the  Algonkian  and  the  Cambrian  generally  found  in  the  United 
States  is  represented  by  continuous  deposition.  Keith,  who  has  done 
the  most  work  in  the  district,  believes  that  the  whole  group  should  be 
placed  in  the  Cambrian  because  the  rocks  are  all  of  the  same  kinds 
and  form  a  continuous  series,  and  thai  the  absence  of  the  usual  un- 
conformity is  in  favor  of  the  Cambrian  age.  If  this  conclusion  be 
correct  it  is  probable  that  the  greater  number  of  belts  of  ancient 
sediments  in  the  great  central  Piedmont  area  are  of  Paleozoic  age. 
The  metamorphism  of  the  rocks  which  have  been  denominated 
Ocoee  increases  southward  and  eastward  from  Tennessee.  In  North 
Carolina.  Georgia,  and  Alabama  these  rocks  -.re  extremely  meta- 
morphosed. In  one  area  of  schists  in  Alabama  v  hich  has  always  been 
called  Ocoee  E.  A.  Smith  has  recently  found  fossils  that  are  probably 
Carboniferous  and  certainly  not  lower  than  Devonian,  and  this  dis- 
covery suggests  that  so-called  Ocoee  belts  of  noncrystalline  rocks 
upon  the  Piedmont  Plateau  in  the  southern  part  of  the  region,  now 
completely  metamorphosed  and  devoid  of  fossils,  may  be  of  post- 
Algonkian  age.  The  Carboniferous  beds,  however,  probably  owe 
their  present  position  to  an  extensive  eastward  overlap  in  Carbon- 
iferous time,  and  indicate  nothing  as  to  age  of  surrounding  rocks, 
with  which  they  are  in  marked  unconformity. 
Keith  believes  that  this  dual  classification  of  the  pre-Cambrian 
in  the  Piedmont  area  into  Archean  and  Algonkian  extends  north- 
ward into  Pennsylvania;  that  the  Carolina  gneiss  is  essentially  con- 
tinuous with  the  gneisses  around  Washington  and  Baltimore  and 
possibly  with  the  Fordham  gneiss  and  the  gneiss-limestone  series  of 
New  Jersey,  while  the  Algonkian  volcanics  are  continuous  with  the 
volcanics  of  South  Mountain,  Pennsylvania.  Farther  to  the  north  the 
volcanics  are  cut  out  and  the  Cambrian  rests  directly  against  what  he 
regards  as  equivalent  to  his  oldest  pre-Cambrian.  The  correlation  of 
the  New  Jersey  rocks  is  so  involved  with  that  of  the  Adirondacks  and 
of  the  Laurentian  and  Hastings  districts  that  its  assignment  to  the 
Archean  would  not  be  wan-anted  on  the  basis  of  the  classification 
made  by  Keith  in  the  Piedmont  area,  which,  as  already  indicated, 
is  designed  primarily  to  emphasize  a  dual  division  rather  than  cor- 
relation with  the  Archean  and  Algonkian  elsewhere. 
The  work  of  Walcott,  Mathews,  Bascom,  and  Keith  has  led  to  the 
conclusion  that  large  areas  of  semicrystalline  rocks  which  were  in 
early  years  supposed  to  be  pre-Cambrian  are  Paleozoic.  Thus  the 
Chickies  quartzite  is  Cambrian  and  the  Shenandoah  ("Chester 
Valley")  limestone  is  Cambro-Ordovician.  The  only  pre-CambriaD 
rocks  in  the  Philadelphia  district  of  Pennsylvania,  according  to 
Bascom,  are  the  Baltimore  gneiss  and  the  Wissahickon  mica  gneiss 
with  associated  intrusives.     Bascom   and   Mathews  agree  that   the 
