LAKE    SUPERIOR   REGION.  159 
It  grades  below  into  the  Goodrich  quartzite,  Bijiki  schist,  or  Clarks- 
burg formation.  The  thickness  can  not  be  accurately  estimated,  but 
it  is  probably  as  much  as  2,000  feet. 
The  Clarksburg  formation  is  composed  predominantly  of  volcanic 
materials,  embracing  basic  lava  flows,  tuffs,  ashes,  and  breccias,  which 
locally  are  interleaved  with  or  grade  into  slate,  graywacke,  or  conglom- 
erate. Much  of  the  material  has  been  profoundly  metamorphosed, 
and  schist  conglomerates,  mica  schists,  and  hornblende  schists  have 
resulted.  All  of  these  rocks  are  cut  by  dikes  and  masses  of  green- 
stone. The  formation  is  confined  to  the  south-central  part  of  the  dis- 
trict. The  volcanic  material  was  poured  out  from  a  number  of 
vents,  the  more  important  ones  which  have  been  recognized  being 
located  near  Clarksburg,  Greenwood,  and  Champion.  The  formation 
grades  into  the  Ishpeming  formation  or  the  Michigamme  formation 
below,  and  into  the  Michigamme  formation  above.  The  Clarksburg 
formation  belongs  in  age  either  between  the  Goodrich  quartzite  and 
the  Michigamme  formation  or  near  the  base  of  the  latter.  No 
estimate  of  the  thickness  can  be  given. 
The  igneous  rocks,  other  than  those  of  the  Clarksburg  formation, 
are  divided  for  convenience  in  discussion  into  two  classes,  in  the  first 
of  which  are  placed  those  associated  exclusively  with  the  beds  below 
the  Clarksburg  formation,  and  in  the  other  those  cutting  also  the 
beds  above  the  Clarksburg.  The  rocks  are  all  basic.  The  older  rocks 
occur  as  dikes,  bosses,  sheets,  and  tuff  beds,  although  the  latter  two 
are  subordinate.  The  post-Clarksburg  greenstones  comprise  only 
dikes  and  bosses.  It  is  conjectured  that  these  later  greenstones  may 
be  the  equivalents  of  some  of  the  Keweenawan  eruptives. 
Evidence  of  the  unconformity  between  the  Lower  Marquette  series 
and  the  Basement  Complex  is  clear  and  abundant.  At  numerous 
places  in  the  district  the  actual  contacts  of  the  basal  conglomerate  of 
the  Marquette  series  and  the  Fundamental  Complex  may  be  seen.  In 
all  of  these  cases  the  detritus  is  most  distinctly  waterworn,  and,  while 
the  major  part  of  the  material  in  each  case  has  been  derived  from  the 
immediately  subjacent  part  of  the  Basement  Complex,  other  material 
not  occurring  in  the  immediate  neighborhood  is  found,  thus  showing 
conclusively  that  these  rocks  are  not  reibungs  or  fault  breccias.  The 
principal  localities  at  which  contacts  are  well  exposed  may  be  men- 
tioned. 
At  the  east  end  of  the  south  side  of  the  Marquette  district  there 
are  several  localities,  from  Lake  Superior  to  west  of  Lake  Mary, 
where  a  conglomerate  is  found  bearing  numerous  bowlders  of  granite, 
gneiss,  and  schist,  identical  with  the  rocks  constituting  the  Basement 
Complex  immediately  adjacent.  In  sees.  22  and  28,  T.  47  N.,  R.  26 
W.,  are  two  islands  of  the  Basement  Complex,  about  which  are  found 
magnificent  exposures  of  great  bowlder  conglomerate  and  recomposed 
