LAKE    SUPERIOR  REGION.  331 
geological  surveys  visited  crucial  areas  about  Lake  Superior  in  1904. 
Their  recommendations  are  followed  in  this  report.  They  will  be 
followed  also  in  the  reports  of  the  Canadian,  United  States,  Ontarian, 
and  Michigan  geological  surveys.  The  general  succession  upon  which 
the  committee  agreed  has  already  been  given.     (See  p.  326.) 
The  table  on  pages  328-329  presents  the  detailed  succession  and  cor- 
relation of  the  series  in  the  various  Lake  Superior  districts  under  the 
classification  adopted  by  the  joint  committee.  It  presents  also  a 
major  grouping  of  the  committee's  units  into  Archean  and  Algonkian, 
concerning  which  the  committee  made  no  recommendation.  Reasons 
for  this  grouping  are  given  later  (pp.  356  et  seq.). 
ARCHEAN   SYSTEM,  OR  BASEMENT  COMPLEX. 
The  oldest  system  of  rocks  in  the  Lake  Superior  region  has  very 
great  lithological  complexity.  It  was  formerly  supposed  to  consist 
largely  of  plutonic  granites  and  gneisses  and  surface  equivalents, 
with  but  subordinate  amounts  of  basic  igneous  rocks,  but  it  is  now 
known  that  the  system  includes  large  quantities  of  intermediate  and 
basic  igneous  rocks,  both  plutonic  and  surface  phases,  and  also  one 
or  more  sedimentary  formations.  The  abundant  rocks  are  granites, 
syenites,  acidic  porphyries,  basalts,  gabbros,  peridotite,  and  a  great 
variety  of  schists  and  gneisses — micaceous,  hornblendic,  and  chloritic. 
Jaspilites,  iron  ore,  and  slates  are  present  in  subordinate  amounts. 
Many  of  the  schists  and  gneisses  result  from  the  alteration  of  the 
above-named  rocks.  It  is  not  possible  fully  to  state  the  stratigraphic 
relations  of  the  rocks,  but  it  is  certain  that  a  series  of  rocks  known 
collectively  by  the  terms  greenstones  and  green  schists  are  the  oldest, 
that  associated  with  them  and  perhaps  in  part  later  and  above  these 
is  an  iron-bearing  formation  with  associated  slate,  and  that  intrusive 
into  both  the  basal  greenstones  and  the  sedimentary  rocks  are  great 
masses  of  acidic  igneous  rocks.  The  basic  and  intermediate  igneous 
rocks,  commonly  referred  to  as  greenstones  and  green  schists  and  char- 
acterized by  a  prevailing  green  color,  with  their  associated  slates  and 
iron  formation,  will  be  called  Keewatin  series,  following  the  recom- 
mendation of  the  international  geological  committee.  The  acidic 
rocks,  including  granites,  gneisses,  and  acidic  schists,  characterized  by 
prevailing  reddish  or  light  colors,  will  be  called  Laurentian  series. 
One  of  the  conspicuous  features  of  the  greenstones  and  green  schists 
of  the  Keewatin  is  the  presence  of  ellipsoidal,  spherulitic,  and  pyro- 
clastic  phases,  indicating  their  essential  surface  volcanic  origin.  The 
basement  upon  which  they  were  deposited  has  not  been  found.  It  is 
an  interesting  fact,  then,  that  the  oldest  rock  in  the  Lake  Superior 
region,  and  perhaps  the  oldest  pre-Cambrian  rock  with  definitely  de- 
termined age  in  the  country,  is  of  surface  volcanic  origin,  and  that  the 
character  of  the  basement  upon  which  it  was  deposited  is  unknown. 
