490  PRE-CAMBRIAN    ROCKS    OF    SORTH    AMERICA'.  [bpll.8& 
• 
equivalent  to  the  more  crystalline  of  the  rraginental  rocks  about  lake 
Superior,  here  called  Lower  Huronian,  was  early  suggested  by  Cham 
berlin.  Excluding  from  the  Laurentiau  the  small  areas  of  elastics  here 
referred,  and  applying  the  term  to  the  vast  areas  of  rock  referred  to 
this  system  during  the  last  forty  years  would  seem  now  to  be  the  pref- 
erable course.  This  usage  accords  with  the  principles  of  good  nomen- 
clature in  that  it  makes  the  Laurentiau  a  definite  unit  and  avoids  the 
anomaly  of  including  under  one  term  two  radically  different  groups  of 
rocks. 
The  tine  grained,  dark  colored  schists  belonging  to  the  basal  complex 
and  antedating  the  Algonkian  should  have  a  name  assigned  to  them 
coordinate  with  Laurentiau  as  the  other  main  lithological  division  of 
the  Archean.  If  Lawson's  Coutehiching  series  belongs  here  this  term 
has  priority  for  this  place.  Law  son.  however,  regards  this  series  as 
probably  sedimentary,  although  upon  this  point  additional  evidence  is 
needed.  If  it  turns  out  that  the  Coutehiching  is  sedimentary  under 
th<^  classification  proposed  in  this  paper  the  series  belongs  with  the 
Algonkian.  As  a  provisional  name  for  this  second  division  of  the 
Archean  is  proposed  the  term  Marenisean.  This  term,  like  Laurentiau, 
is  a  geographical  one  derived  from  Marenisco  township,  Michigan,  south 
of  the  Gogebic  range,  where  these  rocks  have  a  typical  development. 
The  Archean  as  a  whole  naturally  occupies  a  group  place  in  the  clas- 
sitication.  and  its  Laurentiau  and  Marenisean  would  be  systematic  in 
their  value  if  they  were  structural  terms.  But  until  the  Archean  can 
be  separated  on  a  structural  basis,  if  it  ever  can  be.  it  will  be  necessary 
for  the  purposes  of  atlas  sheet  mapping  to  treat  the  group  as  a  unit, 
except  that  lithological  divisions  may  be  made.  With  reference  to 
Archean.  we  are  in  the  same  difficulty  as  with  Agnotozoie  or  Protero- 
zoic,  considered  below.  This  group  can  at  present  have  but  a  single 
system  division,  the  Algonkian.  because  we  are  not  yet  able  to  subdi- 
vide the  group  into  systems  which  can  be  shown  to  be  general  for  the 
whole  of  America. 
The  anticlinal  structure  described  as  generally  characteristic  of  the 
Archean  ranges  of  the  west  has  been  based  Upon  the  belief  that  folia- 
tion represents  bedding,  and  also  on  the  obsevations  that  the  overlying 
sedimentary  rocks  often  dip  away  from  the  axes.  The  anticlinal  struct- 
ures of  tin'  sedimentary  rocks  and  that  of  the  Archean  are  independent 
questions.  If  the  Archean  as  a  whole  be  regarded  as  of  igneous  origin, 
it  would  be  expected  that  a  gradation  from  massive  rocks  in  the  cores 
to  schistose  rocks  upon  the  flanks  would  be  found,  for  these  outer  zones 
are  the  places  where  the  most  powerful  effects  of  dynamic  action  are 
felt,  and  also  the  parts  where  greater  interior  accommodations  of  the 
constituent  particles  are  necessary.  In  dynamo-metamruphic  eruptives 
of  post- Archean  age  precisely  these  relations  prevail.  The  determina- 
tion of  the  structure  of  the  Archean  cores  must  wait  until  the  origin  of 
the  Archean  has  been  determined;  in  short,  until  it  is  known  whether 
structural  methods  are  applicable  at  all. 
