706  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
of  the  earth  and  comparative  freedom  of  horizontal  flow.  The  ig- 
neous rocks  of  the  granitoid  series  were  solidified  at  great  depths 
beneath  the  surface  and  under  heavy  loads.  These  conditions  are 
entirely  diverse  from  those  which  accompanied  the  production  of 
the  lavas.  When,  therefore,  as  is  the  rule,  the  lavas  are  seen  to  lie 
upon  the  deep-seated  granites,  it  is  evident  that  an  enormous  change 
in  the  conditions  has  taken  place.  This  would  result  only  from 
erosion  of  the  surface,  which  slowly  exposed  the  deeper  granite 
masses.  The  interval  of  time  implied  by  this  is  enormous.  The 
total  change  in  the  character  of  the  rocks,  from  plutonic  to  effusive, 
also  shows  a  physical  revolution  of  the  first  order.  The  inference 
is  warranted,  therefore,  that  the  lavas  are  very  much  younger  than 
the  rocks  of  the  granitoid  series. 
The  application  of  the  differences  of  age  thus  defined  is  far  from 
simple.  The  lavas  are  separated  from  the  Cambrian  by  a  great  un- 
conformity. They  form  a  totally  different  class  of  rocks,  and  all  the 
conditions  surrounding  them  Avere  different  from  those  which  pro- 
duced the  Cambrian  sediments.  These  differences  justify  the  sepa- 
ration of  the  lavas  into  a  different  epoch,  either  Algonkian  or 
Archean.  In  favor  of  Algonkian  is  the  marked  grouping  of  the 
areas  of  lava  and  of  Cambrian  sediments.  In  one  region  in  North 
Carolina  there  is  even  a  small  contemporaneous  lava  flow  near  the 
base  of  the  Cambrian.  The  lapse  of  time  and  the  reversal  of  condi- 
tions between  the  lava  series  and  the  granitoid  series  are  also  suffi- 
cient to  remove  the  granites  into  a  period  older  than  the  lavas — 
that  is,  Archean.  Between  the  granites  and  the  gneisses,  however, 
the  lapse  of  time  is  equally  great,  and  the  gneisses  might  properly 
be  put  into  a  still  older  period,  if  there  Avere  such.  Absence  of  that 
makes  it  necessary  to  call  the  gneisses  also  Archean,  referring  them 
to  the  earlier  part  of  that  era.  It  is  clear  that  these  assignments 
are  strictly  relate;  still  the  differences  betAveen  the  three  series 
are  of  the  same  order  as,  and  of  far  greater  magnitude  than,  the 
differences  on  which  later  geologic  periods  are  founded.  The  broad 
facts  are  that  the  three  series  are  separated,  first,  by  great  periods 
of  deformation ;  second,  by  total  reversal  in  all  the  conditions  of 
formation,  and,  third,  by  visible  unconformities. 
SECTION  11.     SUMMARY  OF  PRESENT  KNOWLEDGE  OF 
THE   REGION. 
In  the  southeastern  part  of  the  United  States  is  a  belt  of  noncrys- 
talline rocks,  in  its  broader  parts  more  than  100  miles  Avide,  Avhich 
extends  from  Alabama  continuously  to  Maryland,  and  with  only  a 
slight  break  into  Pennsylvania,  and  finally  into  New  Jersey  as  far  as 
Trenton.  The  Piedmont  Plateau  is  almost  altogether  occupied  by 
these  rocks,  as  are  also  the  Blue  Ridge  and  the  Unaka  Mountains. 
