170  PRE-CAMBRIAN    ROOKS    OF    NORTH    AMERICA.  [bull.  86. 
and  porphyries  perhaps  crystallized  in  the  form  of  granite.  It  may 
well  be  that  large  masses  of  intrusive  granite  may  be  of  Keneewanan 
or  Animikie  age.  Even  if  this  were  the  ease  and  there  are  two  epochs 
of  granitic  eruptives  later  than  the  upper  Huronian  elastics,  this  would 
be  no  evidence  of  the  absence  of  an  ancient  floor  composed  mainly  of 
granite-gneiss  upon  which  the  oldest  Huronian  has  been  deposited. 
ORIGIN   OF   THE   IRON   ORES. 
Before  taking  up  the  correlation  of  the  pre-Keweenawan  lake  Supe- 
rior series,  one  further  point  remains  to  be  considered,  whether  the  iron 
ores  and  associated  rocks  are  eruptive  or  are  sedimentary,  for  upon 
this  point  depends  the  correctness  of  many  structural  determinations. 
If,  as  believed  by  Wadsworth  and  Foster  and  Whitney,  the  iron  ore  is 
partly  igneous,  and  by  the  first  named  that  a  part  of  the  jasper  has  the 
same  genesis,  they  may  occur  at  any  horizon  up  to  that  of  their  eruption 
and  can  not  be  used  as  guides  in  working  out  the  structure.  Those  who 
believe  the  iron-bearing  formations  are  sedimentary  have  regarded  them 
as  persistent  members,  and  the  striking  and  peculiar  lithologieal  charac- 
ters of  these  belts  have  furnished  key  horizons  to  which  to  refer  the 
associated  elastics. 
Foster  and  Whitney  saw  that  the  masses  of  iron  ore  and  jasper  have 
none  of  the  characteristics  of  vein  deposits,  and  believed  that  the  sup- 
position that  they  resulted  from  the  decomposition  of  pyrites  or  the 
metamorphism  of  bog  ore  is  wholly  inadequate  to  account  for  the 
accumulation  of  such  vast  masses  as  occur,  or  to  explain  the  relations 
to  the  associated  rocks.  If  not  of  this  origin,  they  could  only  conceive 
that. they  are  igneous.  The  frequent  association  of  the  ore  with  erup- 
tives and  the  tine  banding  led  to  the  conclusion  that  these  facts  can 
hardly  be  explained  except  by  igneous  action.  These  writers  saw, 
however,  that  when  the  ore  is  found  in  beds  in  clearly  metamorphic 
strata  having  a  common  bearing  and  inclination  they  must  be  sedimen- 
tary, and  such  deposits  are  regarded  as  having  been  derived  from  the 
destruction  of  previously  formed  igneous  masses  and  their  present 
association  to  have  resulted  from  aqueous  deposition-,  so  that  even  in 
this  case  the  iron  has  an  igneous  source.  At  the  present  time  tew 
would  assert  that  the  iron  ores  are  vein  deposits,  or  the  result  of 
decomposition  of  pyrites,  or  the  metamorphism  of  bog  ores;  but  there 
are  other  explanations  overlooked  by  Foster  and  Whitney  which  may 
apply  before1  being  driven  to  the  igneous  hypothesis. 
The  reasons  given  by  Foster  and  Whitney  for  an  igneous  origin  of 
the  ore-bearing  formations  are  of  a  negative  character,  and  the  only 
case  in  which  positive  evidence  is  given  the  rocks  are  recognized  as 
detrital.  But  Wadsworth  brings  forward  positive  evidence  as  to  the 
origin  of  the  ores.  Many  instances  are  cited  showing  the  way  in  which 
the  jasper  and  ore  have  eruptive  contacts  with  the  associated  schists. 
The  tacts,  however,  indicate  the  eruptive  character  of  the  ore  and  jas- 
