330  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
the  geology  of  the  region  as  a  whole.  In  the  following  summary  an 
attempt  will  be  made  to  sketch  in  a  somewhat  elementary  way  the 
outlines  of  the  geology  of  the  Lake  Superior  country.     (See  PL  II.) 
Geological  information  concerning  the  region  has  been  gathered 
by  mining  companies  and  by  the  geological  surveys  of  Michigan, 
Wisconsin,  Minnesota,  Ontario,  Canada,  and  the  United  States. 
Perhaps  in  no  other  part  of  the  country  have  the  mining  men  them- 
selves spent  so  large  sums  in  purely  geological  work.  This  has  re- 
sulted in  the  publication  of  reports  and  maps  by  the  State  and 
Federal  surveys  in  much  less  time  and  at  far  less  cost  than  would 
have  been  otherwise  possible.  The  United  States  Geological  Survey 
is  the  only  organization  which  has  covered  all  of  the  districts  on  the 
United  States  side  of  the  boundary,  and  as  its  reports  include  the 
information  gathered  by  the  mining  companies  and  State  organiza- 
tions special  reference  is  made  to  these  reports.  The  Avork  of  the 
Survey  was  begun  in  the  early  eighties,  in  charge  of  R.  D.  Irving, 
and  has  been  continued  since  1888  by  C.  R.  Van  Hise  and  his  assist- 
ants. The  results  of  their  work  appear  in  a  series  of  detailed  mono- 
graphs and  maps  on  each  of  the  principal  producing  ore-bearing  dis- 
tricts. A  final  general  monograph  on  the  Lake  Superior  region, 
accompanied  by  revised  maps  of  each  of  the  districts  and  a  general 
geological  map  of  the  region,  is  in  preparation. 
Geological  work  has  been  much  less  detailed  in  the  Canadian  por- 
tion of  the  Lake  Superior  region,  largely  because  the  ore-bearing  dis- 
tricts thus  far  discovered  have  not  seemed  to  warrant  such  large 
expenditures  for  geological  work  as  have  been  made  on  the  United 
States  side  of  the  boundary,  but  also  because  the  small  appropria- 
tions made  by  the  Dominion  and  Provincial  governments  for  geo- 
logical work  must  be  made  to  cover  enormous  areas,  thus  preventing 
expensive  detailed  work  in  one  area.  The  Michipicoten  district  is 
the  only  ore-bearing  district  in  Canada  which  has  been  mapped  and 
described  in  any  considerable  detail,  although  much  has  been  written 
on  the  general  geology  of  the  north  shore  of  Lake  Superior. 
The  ore-bearing  districts  themselves  comprise  but  a  small  portion 
of  the  Lake  Superior  region  as  a  whole,  but  they  are  the  areas  in  which 
the  fullest  successions  of  pre- Cambrian  rocks  are  exposed.  The  inter- 
vening areas  are  less  well  known.  It  has  been  possible  to  correlate 
with  reasonable  certainty  the  rock  series  of  the  different  districts, 
although  the  geology  of  each  of  the  districts  has  been  worked  out  in 
large  part  independently.  But  because  of  the  considerable,  less  well- 
known  areas  separating  the  closely  studied  districts,  it  is  difficult  at 
the  present  time  completely  to  unify  the  geology  of  the  region. 
The  confusion  of  names  and  correlation  has  made  an  understand- 
ing of  Lake  Superior  geology  very  difficult.  In  the  hope  of  reaching 
a  common  basis  of  nomenclature  and  correlation,  a  joint  committee  of 
geologists  of  the  United  States,  Michigan,  Canadian,  and  Ontarian 
