216  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1903.  [bull. 225 
the  dolomite  formation  underlying  the  Traders  member  of  the  Vulcan  formation 
(b)  by  a  slate  constituting  the  lower  part  of  the  Traders  member,  and  (c)  by  the  i 
Brier  slate  between  the  Traders  and  Curry  members.  The  dolomite  formation  is 
especially  likely  to  furnish  an  impervious  basement  where  its  upper  horizon  has  been 
transformed  into  a  talc-shist,  as  a  consequence  of  folding  and  shearing  between  the 
formations.  Smaller  ore  deposits  occur  at  contacts  between  the  different  members 
and  al  ]  .laces  within  the  iron-bearing  members  where  severe  brecciation  has  occurred. a 
The  ores  are  largely  of  Bessemer  grade.  ' '  The  rich  ores  are  usually 
bluish-black,  porous,  tine-grained  aggregates  of  crystallized  hematite." 
Sonic  are  highly  siliceous,  resulting  from  the  enrichment  of  jaspilites 
and  differing  very  little  in  appearance  from  them.  "The  brecciated 
ores  may  consist  of  jasper  fragments  in  a  mass  of  hematite,  or  of 
hematite  fragments  in  a  mass  of  dolomite,  or  they  may  be  composed 
of  fragments  of  ore,  jasper,  and  slate  in  a  mass  consisting  largely  of 
slate  debris  that  has  been  strongly  ferruginized."  The  ores,  when 
exposed  to  the  action  of  the  atmosphere,  become  coated  with  a 
white  efflorescence,  consisting  of  a  mixture  of  the  sulphates  of  sodium, 
magnesium,  and  calcium,  in  which  the  first-named  is  greatly  in  excess. 
The  forms  of  the  ore  bodies  vary  with  their  positions.  They  con- 
form in  a  general  way  to  the  shape  of  the  foundation  on  which  they 
rest.  The  deposits  in  troughs  have  in  general  a  U-shaped  cross  sec- 
tion, very  thick  at  the  bottom.  Where  much  compressed,  the  arms  of 
the  U  may  unite  at  the  center  and  produce  a  lens-shaped  deposit. 
Contact  deposits  are  usually  broad  and  sheet-like,  with  irregular  pro- 
jections extending  from  their  upper  surfaces. 
The  shape  and  structure  of  the  ore  deposits,  their  locations  along 
lines  of  major  water  circulation  confined  below  by  impervious  base- 
ments, and  association  with  minerals  of  aqueous  origin,  point  to  the 
concentration  of  much  of  the  ore  through  the  agency  of  downward- 
percolating  waters.  But  the  iron -bearing  formation  of  the  Menominee 
district,  and  especially  the  Traders  member,  differs  from  that  of  other 
districts  in  the  Lake  Superior  region  in  having  a  comparatively  large 
quantity  of  original  detrital  ore.  Principally  from  the  close  similarity 
of  the  nondetrital  ores  of  the  Menominee  district  to  the  nondetrital 
ores  of  the  Marquette  and  Gogebic  districts  it  is  believed  that  the  non- 
detrital ores  of  the  Menominee  district  are  largely  a  result  of  the 
secondary  alteration  of  an  original  cherty  iron  carbonate,  although 
the  alteration  has  been  so  thorough  that  little  or  none  of  this  material 
now  remains  in  the  iron  formation.  In  the  slate  overlying  the  iron 
formation  there  still  remains  iron  carbonate  showing  characteristic 
alterations,  on  a  small  scale,  to  chert,  jasper,  and  iron  ore,  thus  afford- 
ing corroborative  evidence  that  iron  carbonate  originally  existed  in 
the  iron  formation  itself.  In  both  the  Curry  and  Traders  members 
pseudomorphs   of   concretions   of   iron   carbonate   are   found.      The 
"  Bayley.W.  S.,The  Menominee  iron-bearing  district  of  Michigan:  Mon.  U.  S.  Geol.  Survey,  vol.46, 
1904,  p.  392. 
