PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.       707 
The  rocks  of  this  belt  comprise  a  great  variety  of  gneisses,  of  gran- 
ites, and  of  intermediate  and  basic  igneous  rocks,  including  both  plu- 
tonic  and  volcanic.  Also  there  are  associated  with  these  noncrystal- 
line rocks  extensive  formations  of  sedimentary  rocks  which  vary  in 
their  metamorphism  from  induration  to  complete  recrystallization. 
In  the  earlier  studies  of  this  region  practically  all  of  the  rocks  of  the 
region  Avere  placed  with  the  pre-Cambrian,  and  were  distributed  be- 
tween the  Hnronian  and  the  Lanrentian.  A  closer  study  has  shown, 
just  as  in  the  New  England  region,  that  this  complex  mass  of  rocks 
includes  those  varying  in  age  from  Archean  to  Paleozoic. 
When  the  first  edition  of  this  bulletin  was  published  the  stratigra- 
phy of  these  crystalline  rocks  had  not  been  worked  out,  and  detailed 
mapping  had  been  done  in  but  limited  areas.  Since  that  time  great 
progress  has  been  made  in  the  solution  of  the  problems  of  the  succes- 
sion and  structure  of  the  region.  Among  those  who  have  made  im- 
portant contributions  to  the  knowledge  of  the  region  are  Keith,  Wat- 
son, Mathews,  and  Bascom.  In  the  foregoing  pages  each  of  these 
geologists  gives  a  summary  of  the  state  of  knowledge  of  the  districts 
respectively  studied.  Of  these  four,  Keith  has  longest  studied  the 
region  and  has  mapped  the  most  extensive  areas. 
For  the  central  part  of  the  district  Keith  gives  an  admirable  sum- 
mary of  the  succession  which  he  has  worked  out  for  a  great  area  of 
the  Piedmont  Plateau,  the  Blue  Ridge,  and  the  [Jnakas.  I  lis  studies 
lead  him  to  conclude  that  the  pre-Cambrian  rocks  are  divisible  into 
three  great  series.  The  oldest  of  these  is  an  extensive  set  of  gneisses, 
which  are  known  as  the  Roan  and  Carolina  gneisses.  The  latter  is.  at 
least  in  part,  of  sedimentary  origin.  Very  much  later  than  this  series, 
and  separated  from  it  by  at  least  one  great  period  of  deformation 
and  metamorphism,  are  extensive  granite  masses,  to  which  he  has 
given  the  names  Cranberry,  Beech,  Max  Patch,  Blowing  Rock,  and 
Henderson.  These  rocks  intrude  the  first  series  in  a  most  complex- 
fashion,  and  they  themselves  had  been  metamorphosed  by  profound 
dynamic  action  before  the  appearance  of  the  third  series  of  rocks. 
This  latest  series  consists  of  basic  and  acidic  lavas.  In  most  places 
these  rocks  have  been  so  little  metamorphosed  that  the  original  char- 
acters are  plain,  but  in  other  places  these  characters  have  been  de- 
stroyed by  metamorphism.  Keith  thinks  that  the  difference  in  age  of 
each  of  these  series  is  very  great,  but  he  emphasizes  the  great  change 
in  the  nature  of  the  formations  indicated  by  the  appearance  of  the 
third  series,  the  volcanics.  lie  refers  the  ancient  gneisses  and  gran- 
ites to  the  Archean.  and  places  the  lavas  in  the  Algonkian  to  empha- 
size the  dual  classification,  but  without  emphasis  on  specific  con-ela- 
tion with  Archean  and  Algonkian  elsewhere. 
Associated  with  the  rocks  which  are  certainly  Archean  or  Algon- 
kian  are   many   belts  of   more  or   less   metamorphosed   sedimentary 
