PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.       705 
The  pre-Cambrian  rocks  are  divisible  into  three  great  series, 
based  upon  the  original  nature  of  the  rocks  themselves  and  on  the 
events  through  which  they  have  passed.  The  first  of  these  series 
consists  of  the  Roan  and  Carolina  gneisses,  in  which  the  structures 
due  to  metamorphism  have  largely  obliterated  the  original  textures. 
The  second  series  consists  of  the  various  granites,  massive,  schistose, 
or  gneissoid,  such  as  the  Cranberry,  Beech,  Max  Patch,  Blowing  Rock, 
and  Henderson.  In  these  the  original  characters  are  evident  through- 
out large  masses  and  the  acquired  features  of  metamorphism  are 
locally  prominent.  The  third  series  consists  of  basic  and  acidic 
lavas.  In  places  the  original  characters  of  these  rocks  are  plain, 
while  elsewhere  they  have  been  destroyed  by  metamorphism. 
The  gneisses  of  the  first  series  are  cut  by  the  granites  of  the  second 
series  in  countless  places.  The  lavas  of  the  third  series  cut  through 
the  granites  and  gneisses  and  spread  out  over  their  surfaces.  Many 
great  areas  and  separate  basins  are  now  known  in  the  mountains  and 
the  Piedmont  Plateau,  and  further  work  will  doubtless  reveal  many 
more.  They  show  an  epoch  marked  by  effusive  rocks,  in  strong  con- 
trast with  the  preceding  plutonic  rocks. 
The  interval  of  time  between  the  gneiss  and  granite  series  was 
enormous,  as  shown  by  differences  in  their  acquired  structures.  The 
older  gneisses  received  a  strong  schistosity  through  deformation. 
The  schistose  planes,  the  gneissoid  banding,  and  the  individual 
minerals  have  since  been  bent,  wrinkled,  and  variously  deformed. 
In  places  the  secondary  deformation  has  produced  another  schistosity, 
cutting  across  the  earlier  planes.  The  gneisses  have,  therefore,  pa ssed 
through  two  entirely  distinct  periods  of  metamorphism.  The  rocks 
of  the  granitoid  series,  on  the  other  hand,  show  the  results  of  only  a 
single  metamorphic  period.  The  planes  of  shearing  and  schistosity 
and  the  accompanying  flakes  of  parallel  minerals  now  dip  at  angles 
which  are  similar  over  large  areas.  These  were  the  results  of  a 
great  force  acting  with  considerable  uniformity  and  thus  producing 
uniform  phenomena.  The  amount  of  metamorphism  corresponds 
in  degree  to  that  produced  in  the  group  of  gneisses  by  the  first  period 
of  deformation.  The  period  of  deformation  which  rendered  the 
granites  schistose,  however,  is  the  second  one  which  affected  the 
gneisses.  Thus  it  is  seen  that  the  series  of  gneisses  is  older  than 
the  series  of  granites  by  at  least  the  time  needed  for  a  period  of 
deformation  and  metamorphism.  Regarding  these  as  the  expression 
of  slowly  accumulating  strain  in  the  earth's  crust,  the  lapse  of 
time  must  be  considered  very  great. 
The  lavas  of  the  third  series  as  a  rule  overlie  all  of  the  other  pre- 
Cambrian  rocks.  They  were  formed  under  fairly  well-defined  condi- 
tions, which  include,  in  part,  solidification  at  <>r  near  th*'  surface 
55721— Bull.  360—00 45 
