480  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
The  school  of  geologists  that  regards  massive  rocks  as  metamorphic 
without  any  intervening  time  of  fluidity,  the  granitic  and  gneissic  lay- 
ers interlaminated  with  the  schistose  being  selectively  metamorphosed, 
and  the  fragments  of  schist  and  gneiss  within  the  massive  rocks  as 
residual  unmetamorphosed  material,  while  nearly  gone,  still  has  repre- 
sentatives. That  the  matrix  of  a  fragmental  rock  could  become  slowly 
heated  to  such  a  temperature  or  be  subject  to  such  other  conditions  as 
are  necessary  in  order  that  it  should  crystallize  as  a  coarsely  granular 
granitoid  gneiss  or  granite,  and  not  at  the  same  time  destroy  the 
bowlders  and  pebbles  which  it  contains,  seems  incredible.  The  explana- 
tion of  these  rocks  and  of  the  interlaminations  of  granite  with  slate 
and  schist  by  metamorphism  implies  not  only  that  the  fragments  and 
the  bands  of  slate  and  schist  have  been  able  to  resist  the  forces  of 
change  during  the  slow  processes  which  have  been  sufficient  to  produce 
coarsely  crystalline  material  adjacent,  but  that  in  situ  they  have  con- 
tinued to  resist  these  forces  during  all  the  time  required  by  the  matrices 
to  pass  once  more  into  ordinary  conditions.  The  processes  embodied 
in  such  "  selective  metamorphism  "  certainly  need  explanation. 
The  need  of  an  exact  definition  of  metamorphism  is  evident.  It 
appears  to  the  writer  that  it  should  not  be  applied  to  a  rock  which  has 
actually  suffered  igneous  fusion.  Shall  it  apply  to  sedimentary  mate- 
rial which  has  been  free  to  recrystallize  under  aqueo-igneous  fusion? 
May  not  rocks  under  pressure  beyond  the  crushing  strength  of  rocks 
and  saturated  with  hot  water  recrystallize  at  a  much  lower  temperature 
than  is  required  for  ordinary  fusion  ?  If  so,  where  is  the  boundary 
between  aqueo-igneous  and  ordinary  fusion  ?  Is  there  not  a  transition 
between  the  two  and  does  not  aqueo-igneous  fusion  pass  by  impercepti- 
ble steps  into  ordinary  metamorphism!  Is  there  not  here  a  transition 
between  the  extremes  just  as  there  is  between  different  rock  species,  be- 
tween basic  and  acidic  rocks,  between  organic  and  inorganic  sediments, 
between  fragmental  and  crystalline  rocks,  between  aqueous  and  sur- 
face igneous  rocks. 
(2)  All  or  a  part  of  the  Archean  is  considered  as  of  igneous  origin, 
but  later  in  age  than  the  pre  -Cambrian  elastics.  The  facts  of  those  who 
have  described  downward  gradations  from  unmistakable  elastics  into 
a  crystalline  complex  by  this  school  have  not  been  interpreted  as 
above.  It  has  declined  to  apply  the  term  metamorphism  to  a  product 
which  has  become  fluent,  and  has  insisted  upon  its  essentially  igneous 
character.  Lawson  is  conspicuous  as  having  recently  strongly  put  this 
side  of  the  case;  but  it  is  noteworthy  that  Winchell,  belonging  to  the 
first  school,  and  Lawson,  to  the  second,  have  had  essentially  the  same 
facts  before  them,  both  having  done  their  work  in  the  same  region.  The 
difference  is  one  of  definition  and  emphasis  rather  than  ideas.  Both 
schools  regard  the  granite-gneiss  as  material  which  has  resulted  from 
a  change  in  the  condition  of  original  sediments  and  has  not  moved  far. 
This  theory  that  the  Archean  or  a  part  of  it  is  the  liquefied  floor  of 
