vANiirsE.]  LAKE    SUPERIOR    REGION.  173 
iug  tlie  schistose  rocks  having  eruptive  contacts  with  the  ores  and  jas 
pers  as  the  intrusives,  which  in  many  cases  they  demonstrably  are.  In 
his  most  recent  paper  Wadsworth  himself  suggests  that  the  supposed 
eruptive  jasper  and  ore  may  be  "  truly  fragmental,  their  present  rela 
tions  being  due  to  sedimentary  and  chemical  action  and  the  squeezing 
together  of  the  jaspilite  and  schist." 
It  was  suggested  by  Brooks  that  the  ore  deposits  of  the  Now  Bng 
land-Saginaw  range  and  of  the  lake  Superior  mine  in  the  Marquette 
district  are  secondary  concentrations  due  to  the  removal  of  silica  and 
the  deposition  of  iron  oxide.  The  same  position  was  taken  by  Romiuger 
for  the  entire  Marquette  district.  Later  I  have  shown  that  it  is  a  general 
truth  for  the  lake  Superior  region  that  the  iron  ore  deposits  are  second- 
ary concentrations,  produced  by  downward  percolating  waters  along 
the  paths  of  great  water  channels,  and  particularly  at  places  where  the 
waters  are  converged  by  tilted  impervious  basement  formations. 
THE    BASIC    ERUPTIVES    AND    STRATIGRAPHY. 
At  one  other  point  the  problem  of  lake  Superior  stratigraphy  has 
been  made  more  difficult  by  certain  of  the  geologists  than  was  neces- 
sary. The  diabases,  diorites,  and  gabbros  were  by  several  writers  in 
early  days  regarded  as  metamorphic  sedimentary  rocks.  Logan,  Mur- 
ray, and  Foster  and  Whitney  are  notable  exceptions.  Partly  as  a  con- 
sequence of  this  fact  came  the  minute  subdivision  of  the  Marquette, 
Menominee,  Penokee,  and  other  series.  All  now  regarding  these  rocks 
as  intrusives,  the  facts  that  they  are  often  in  boss-like  masses,  and  when 
interleaved  do  not  necessarily  continue  for  any  considerable  distance, 
present  no  difficulty;  while  a  great  formation  like  the  Upper  Slate  of 
the  Penokee  and  Marquette  districts  is  left  as  a  whole  rather  than  di- 
vided into  a  number  of  members  separated  by  layers  of  greenstone. 
Also  it  is  nowT  known  that  the  "  dioritic  schists"  are  ancient  eruptives, 
in  part  contemporaneous  and  interbedded  with  the  sedimentary  rocks. 
The  volcanic  character  of  these  rocks  was  suggested  by  Foster  and 
Whitney,  and  their  igneous  origin  was  appreciated  by  Kominger.  Later 
investigations  by  Wadsworth  and  Williams,  with  the  microscope,  lead 
to  the  same  conclusion. 
The  intrusions  of  the  diabases,  gabbros,  and  diorites,  as  suggested  by 
Kominger,  Wadsworth,  and  Merriam,  have  oftentimes  had  an  important 
influence  upon  the  structure  of  the  sedimentary  beds,  although  Rom- 
inger holds  that  the  folding  is  primarily  due  to  the  intrusive  granite. 
Strong  and  tvominger  also  suggested,  in  the  St.  Louis  river  district, 
that  the  eruptives  were  the  pipes  which  furnished  the  outflows  of  Ke- 
weenawan  time.  This  idea  is  not  only  plausible  for  this  district,  but 
is  probably  true  for  the  entire  lake  Superior  region.  A  closer  study  will 
probably  demonstrate  that  the  fresher  eruptives  in  both  the  upper  and 
lower  Huronian,  including  the  great  intrusive  beds  of  the  Animikie,  are 
really  Keweenawan  in  age.   Also  the  numerous  dikes  of  like  character 
