4S4  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.86, 
upon  parts  of  the  subject.  That  there  are  comparatively  few  or  no 
wholly  massive  rocks  in  this  complex  is  precisely  what  would  he  ex- 
pected under  any  theory.  Its  history  is  too  long.  Whether  originally 
igneous  or  aqueous,  it  could  not  he  hoped  that  there  would  be  found 
the  characteristic  lithological  forms  of  igneous  or  aqueous  agencies. 
Many  or  all  of  these  rocks,  not  only  subject  to  the  movements 
which  have  taken  place  since  Paleozoic  time,  but  to  the  movements 
which  have  occurred  in  the  far  greater  length  of  previous  time — if  not 
too  deeply  buried  to  be  beyond  the  influence  of  the  outer  foldings,  iu 
Avhich  case  they  were  buried  beyond  the  crushing  strength  of  rocks — 
were  latently  plastic,  and  were  probably  at  a  high  temperature,  If 
originally  massive  and  igneous  in  the  ordinary  sense,  dynamic  action 
has  obliterated  the  regularity  of  the  arrangement  of  the  constituent 
particles  and  has  given  them  a  more  or  less  laminated  or  schistose  struc- 
ture. If  sedimentary,  all  trace  of  that  original  sedimentary  structure 
has  been  obliterated  by  the  repeated  foldings,  contortions,  and  perhaps 
high  degree  of  heat  to  which  they  have  at  various  times  been  subjected. 
Of  a  necessity,  through  this  complex  have  passed  all  subsequent  erup- 
tives.  Doubtless  at  A^arious  places  and  times  in  its  history,  parts  of  it 
have  become  practically  fluid  and  from  this  condition  it  has  again 
crystallized  in  the  forms  characteristic  of  eruptives. 
DELIMITATIONS   OF   ARC  HE  AN. 
It  is  generally  accepted  that  the  Archean  has  no  limit  downward.  It 
is  the  oldest  system,  and  surely  includes,  if  such  rocks  exist,  all  of  the 
original  crust  of  the  earth.  But  as  denudation  xuogresses,  material  far 
within  the  earth  approaches  its  surface,  not  by  intrusion  but  by  gradu 
ally  rising  as  a  whole.  Before  reaching  the  surface  the  material  has 
become  crystallized.  This  original  crystallization  may  have  taken  place 
in  or  even  later  than  Algonkian  time;  hence,  if  these  rocks  are  to  bej 
considered  as  belonging  to  the  age  in  wThich  they  crystallized,  the 
Archean  grades  below  into  the  Algonkian,  even  as  it  is  believed  in; 
places  to  grade  above  into  the  Algonkian.  The  truth  of  this  position 
is  not  lessened  by  the  fact,  if  fact  it  be,  that  the  earth  as  a  whole,  sub 
ject  to  sudden  strain,  acts  as  a  rigid  body.  Even  if  true,  it  is  equally 
certain  that  the  crust  of  the  earth,  under  continued  strain,  adaptsl 
itself  to  it,  thus  showing  real  plasticity.  But  in  any  case  it  can  not  be; 
assumed  that  the  rock  material  deep  within  the  earth,  under  pressuroj 
far  beyond  the  crushing  strength  of  any  known  material,  and  at  a  higro 
temperature,  exists  as  crystallized  minerals.  We  only  know  that  it  has] 
these  forms  when  the  material  rising  by  erosion  nears  the  surface. 
The  upper  limit  of  the  Archean  is  not  easy  to  define,  and  the  task  isj 
rendered  more  difficult  because  geologists  are  not  agreed  upon  the! 
origin  of  the  Archean.  If  either  the  sedimentary  or  the  subcrustall 
fusion  theory  of  its  origin  be  accepted,  there  *will  be  found  gradations  j 
from  rocks  constituting  the  ancient  complex  described  to  rocks  having* 
