102  PRE-CAMBRIAN   GEOLOGY   OF    NORTH   AMERICA. 
States.  The  peneplain  is  an  ancient  one,  which  has  undergone  dif- 
ferential elevation,  has  been  denuded,  and  subsequently  slightly  in- 
cised around  the  uplifted  margin.  At  several  places  on  the  margin, 
as  exposed  to-day,  the  dissection  may  be  regarded  as  submature.  The 
date  of  the  major  development  of  the  peneplain  is  not  determined, 
but  may  be  pre-Ordovician  or  Cretaceous.  Around  the  southern 
margin  between  Montreal  and  Winnipeg  there  are  traces  of  a  pene- 
plain (or  probably  more  than  one)  of  still  earlier  date,  upon  which 
Paleozoic  sediments  were  laid  down,  and  which  has  been  uncovered 
by  processes  of  degradation  and  denudation  since  the  differential 
uplift  of  the  latest  peneplain. 
Van  Hise,38  in  1904,  discusses  principles  of  metamorphism  appli- 
cable to  the  study  of  pre-Cambrian  and  other  metamorphic  rocks 
and  cites  many  illustrative  pre-Cambrian  rocks  and  localities. 
Willmott,39  in  1904,  finds  a  steplike  regularity  in  the  contact  of 
the  Archean  (pre-Cambrian  of  the  United  States  Geological  Survey) 
and  post- Archean  rocks  in  the  region  of  the  Great  Lakes  and  believes 
it  to  be  explained  by  a  dislocation  in  the  Archean  before  the  deposi- 
tion of  the  post- Archean  sediments. 
Darton,40  in  1905,  in  connection  with  a  discussion  of  the  artesian 
wells  of  the  central  Great  Plains,  publishes  a  map  giving  the  best 
available  information  of  the  distribution  of  the  pre-Cambrian  rocks 
of  southwestern  Minnesota,  South  Dakota,  Wyoming,  and  the  eastern 
part  of  Colorado,  and  briefly  discusses  their  distribution  and 
structural  relations. 
Willis,41  in  1906,  in  cooperation  with  the  Canadian,  Mexican,  and 
United  States  geological  surveys,  publishes  a  geological  map  of  North 
America  with  explanatory  text.  This  is  by  far  the  best  representa- 
tion of  the  distribution  of  the  pre-Cambrian  rocks  as  a  whole  that 
has  been  published  to  this  date.  The  pre-Cambrian  rocks  are  mapped 
in  part  as  unclassified,  in  part  as  Eo-Algonkian,  including  the  Lower 
Huronian  and  Keewatin  of  the  Lake  Superior  country  and  the  Gren- 
ville  series  of  eastern  Canada,  and,  third,  the  Neo-Algonkian,  includ- 
ing the  Belt  terrane  of  Montana,  the  quartzites  of  the  Colorado  Eiver, 
and  the  Keweenawan  and  Animikie  of  the  Lake  Superior  region 
and  northern  Canada.  The  gold-bearing  slates  of  Nova  Scotia,  the 
Ocoee  series  of  the  Piedmont  area,  and  the  Adams  Lake  and  Niscon- 
lith  series  of  British  Columbia  are  all  mapped  as  Cambrian,  while 
the  Belt  terrane,  with  which  they  are  unrelated  in  the  present  paper, 
is  mapped  as  Algonkian.  The  Animikie  series  of  the  Lake  Superior 
region  has  erroneously  been  mapped  with  the  Eo-Algonkian  rather 
than  with  the  Neo-Algonkian.  The  Keewatin  and  Lower  Huronian 
are  mapped  under  one  color  because  of  the  impossibility  in  the  present 
state  of  knowledge  of  separating  them  over  much  of  Canada.  The 
use  of  the  term  Algonkian  for  this  combination  is,  however,  unac- 
