156  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA..  [bull. 86^ 
silicified  gabbro  lias  been  traced  by  this  geologist  southwest  through 
sees.  25,  35,  34,  T.  65  K,  E.  5  W.,  to  Mitch  i  gam  me  lake.  The  same 
silicified  gabbro  belt  has  been  found  by  Merriam  at  lake  Gobbemichig- 
omog. 
Merriam,208  in  1888  and  1889,  in  a  detailed  systematic  study  of  parts 
of  the  Marquette  district,  ascertained  that  about  many  of  the  massel 
of  basic  eruptives  the  clastic  rocks  bow  in  a  quaquaversal  manner,  in- 
dicating that  many  of  the  diabases,  gabbros  and  diorites  are  intrusive 
subsequent  to  the  formation  of  the  Marquette  series,  and  that  the  local 
strikes  and  dips  are  often  due  to  this  cause. 
Van  Hise,209  in  1890,  examined  the  rock  succession  at  Iron  moun- 
tain, Michigan.  Overlying  the  ore  formation  of  the  Ludington  and 
Chapin  mines  is  a  con  glomerate  which  bears  fragments  of  ore  and  jasper. 
It  therefore  appears  that  after  this  material  reached  its  present  condi- 
tion in  the  ore-bearing  series  it  was  eroded  and  furnished  debris  for  a 
newer  series. 
Pumpelly  and  Van  Hise,210  in  1891  and  1892,  find  that  in  places 
the  ore  formation  of  the  Menominee  and  Felch  mountain  districts  passes 
down  into  the  limestone.  .This  gradation  may  be  well  seen  in  the  Me- 
nominee district  at  a  quarry  east  of  the  Chapin  mine.  Also  in  the: 
deeper  workings  of  the  Chapin,  the  ores  resting  almost  directly  upon 
the  limestone  are  found  to  bear  a  considerable  percentage  of  carbonates, 
including  iron,  calcium,  and  magnesium.  The  Metropolitan  ore  deposits 
in  the  Felch  mountain  district  are  found  associated  with  or  within  the 
limestone.  At  one  pit  the  ore  and  jasper  may  be  seen  interlaminated 
with  and  grading  down  into  a  limestone.  It  is  therefore  probable  that 
the  ore  formation  of  these  districts,  in  part  at  least,  is  but  an  upward 
continuation  of  the  limestone  formation,  perhaps  differing  from  it  origi- 
nally only  in  that  the  upper  part  contained  a  greater  quantity  of  origin 
nal  carbonate  of  iron. 
Above  the  ore  formation  at  Quiniiesec,  test  ruts  show  the  presence 
of  a  typical  chert  and  jasper  conglomerate,  in  every  respect  like  the 
basement  conglomerates  of  the  Upper  Marquette. 
SECTION   VII.      SUMMARY    OF   RESULTS. 
The  lake  Superior  region  is  the  one  in  America  about  which  most 
has  been  written  and  which  has  furnished  the  most  definite  knowledge 
of  the  structural  relations  of  the  pre-Cambrian  rocks.  Contained  in  the 
foregoing  summary  of  literature  is  potentially  much  that  follows,  but 
it  seems  desirable  to  put  together  the  conclusions  which  may  be  con- 
sidered as  determined  and  see  how  far  the  various  positions  taken 
by  the  different  writers  are  really  in  harmony.  Unless  otherwise  stated, 
the  cited  positions  of  the  various  authors  are  their  later  expressions; 
oftentimes  earlier  and  different  views  have  been  held.  For  the  sources 
from  which  the  conclusions  are  drawn  it  will  be  necessary  to  turn  to 
