LAKE    SUPERIOR   REGION.  369 
separates  from  the  Huronian  and  puts  in  the  Laurentian  the  forma- 
tion of  dioritic  schists,  obscure  green  conglomerates,  chloritic  schists, 
etc.,  cut  by  granite  veins  in  the  Marquette  district,  which  Brooks 
placed  as  the  lowest  part  of  the  Huronian,  but  the  relations  of  which 
are  said  not  to  have  been  fully  made  out.  There  is  the  further  re- 
sultant difference  between  Brooks  and  Irving  that  Brooks  regards 
very  considerable  masses  of  granite  in  the  Menominee  district  as  the 
highest  member  of  the  Huronian.  As  this  granite  is  said  to  overlie 
conformably  the  Huronian  schists  and  to  send  dikes  into  them,  it  is 
suggested  that  toward  the  end  of  the  Huronian  period  there  was  a 
great  eruptive  outflow  of  granite.  As  has  been  seen,  these  facts 
are  explained  by  Irving  by  excluding  from  the  Huronian  the  granite 
and  the  schists  cut,  although  it  is  recognized  that  lesser  granitic  in- 
trusions have  occurred  since  Huronian  time. 
Rominger,  in  his  earlier  work  on  the  south  shore,  seeing  that  his 
dioritic  group  of  Huronian  rocks  is  cut  by  granite,  and  considering 
the  former  as  a  sedimentary  rock,  and  finding  also,  as  he  believed, 
actual  transitions  between  the  fragmental  quartzites  and  the  granites, 
placed  the  whole  complex  as  Huronian  and  regarded  the*  granite  as 
the  youngest  member.  These  positions  are,  however,  very  largely 
abandoned  in  his  later  unpublished  work.  That  the  granite  and 
gneiss  existed  prior  to  the  deposition  of  the  lowest  members  of  the 
Huronian  and  yielded  debris  to  them,  is  recognized,  although  con- 
tacts are  said  to  be  not  numerous  enough  to  make  possible  a  discrim- 
ination between  the  original  "  Primary  "  granites  and  gneisses  and 
those  of  later  eruptive  origin.  The  recomposed  character  of  the 
detrital  rocks  which  repose  upon  and  have  derived  debris  from  the 
granites  and  gneisses,  instead  of  grading  into  them,  is  now  seen.  It 
is,  however,  still  maintained  that  the  great  mass  of  the  granite  and 
gneiss  is  an  eruptive  of  later  age  than  the  detrital  rocks.  The  dioritic 
group,  which  is  so  frequently  cut  by  granite  veins,  before  considered 
as  the  bottom  of  the  Huronian,  is  recognized  as  an  igneous  rock 
which  must  be  excluded  from  the  sedimentary  series.  Rominger's 
succession  is,  then :  Lake  Superior  sandstone ;  unconformity ;  Kewee- 
nawan;  unconformity;  Huronian  sedimentary  series,  which  has, 
however,  been  disturbed  by  great  intrusions  of  granite  and  gneiss, 
with  also  basic  rocks :  unconformity ;  granite-gneiss-schist  complex. 
It  is  therefore  to  be  noted  that  Brooks,  Irving,  and  Rominger,  who 
have  done  the  most  work  in  the  detailed  mapping  of  the  rocks  of  the 
south  shore,  reach  identical  conclusions  as  to  the  succession,  the  only 
difference  being  one  of  emphasis.  Rominger  insists  on  the  great 
importance  of  the  later  granite  gneisses,  but  does  not  emphasize  the 
presence  of  the  basal  granite-gneiss-schist  complex;  Irving,  on  the 
other  hand,  reverses  the  emphasis,  while  Brooks  occupies  an  inter- 
55721— Bull.  360—09 24 
