LAKE   SUPERIOR   REGION.  347 
southeast  of  Keweenaw  Point,  but  wherever  close  local  studies  have 
been  made,  as  on  the  northwest  part  of  Keweenaw  Point,  northern 
Wisconsin,  and  on  Isle  Royal  and  elsewhere,  complicated  and  some- 
times extensive  faults  have  been  shown  to  exist. 
THICKNESS. 
The  maximum  thickness  of  the  Keweenawan  was  estimated  by 
Irving  at  the  Montreal  River  section  to  be  45,000  feet.  Lane  con- 
firms this  estimate  in  the  Black  River  section.  These  figures  are 
probably  too  large,  since  the  apparent  thickness  may  have  been  in- 
creased by  faulting  and  the  dips  may  in  part  have  been  initial. 
The  great  quantity  of  material  of  the  Keweenawan  does  not  of 
necessity  mark  a  period  longer  than,  or  perhaps  even  one  as  long  as, 
the  lower  Huronian  or  upper  Huronian,  for  the  greater  part  of  it 
is  of  igneous  origin  and  doubtless  accumulated  rapidly.  The  lava 
flows  in  their  extent  and  thickness  are  to  be  compared  with  the  great 
volcanic  plateaus  of  the  Far  West  rather  than  with  local  volcanoes, 
such  as  Vesuvius,  or  the  local  volcanoes  of  the  upper  Huronian  and 
lower  Huronian. 
COPPER   DEPOSITS. 
Keweenaw  Point  is  especially  noted  as  being  the  locality  at  which 
the  valuable  native  copper  deposits  of  the  Keweenawan  series  are 
found.  The  copper  ores  occur  (1)  in  sandstones  and  conglomerates 
interstratified  with  lava  beds,  (2)  in  the  scoriaceous  parts  of 
amygdaloids,  and  (3)  in  veins  cutting  the  igneous,  rocks.  In  all  of 
these  deposits  the  copper  occurs  both  in  the  original  openings  and  as 
replacements  in  the  igneous  and  clastic  rocks. 
GENERAL   STRUCTURE   AND    DISTRIBUTION    OF    LAKE   SUPERIOR 
ROCKS. 
For  the  region  as  a  whole  it  is  difficult  to  make  a  brief  statement 
adequately  covering  the  complex  structure  and  distribution  of  the 
pre-Cambrian  rocks,  but  consideration  of  a  few  general  features  may 
aid  in  fixing  the  reader's  ideas  of  the  general  geology  of  the  region. 
By  mapping  under  one  symbol  all  greenstones  and  under  another 
symbol  all  granites,  whether  of  Archean  or  of  Algonkian  age,  the 
granite  and  greenstone  areas  may  be  easily  remembered  in  their  gen- 
eral distribution.  By  so  doing  also  we  avoid,  the  necessity  of  drawing 
boundary  lines  between  granite  and  greenstone  of  different  ages  in  the 
parts  of  the  Lake  Superior  region  in  which  a  separation  has  not  been 
satisfactorily  made.  For  stratigraphic  purposes  the  granite  and 
greenstone  areas  may  be  practically  eliminated  from  consideration. 
There  then  remain  principally,  as  the  rocks  with  which  we  are  pri- 
marily concerned  in  comprehending  the  general  stratigraphy  of  the 
