vanhise.]  LAKE    SUPERIOR    REGION.  115 
found  directly  superimposed  in  a  small  gorge  upon  the  melaphyre.  The 
melaphyre  is  cut  by  vertical  planes  of  division  which  are  quite  smooth 
and  uniform;  but  there  is  another  persistent  set  which  is  much  less 
smooth,  but  persistent  and  constant  in  direction.  These  planes  are 
usually  m  detail  slightly  uneven  and  undulatory,  and  separated  by  sev- 
eral feet.  They  are  believed  to  represent  the  dip  of  the  igneous  beds. 
It  is  upon  the  persistence  of  these  inclined  beds,  taken  in  connection 
with  their  parallel  lithological  habit,  that  the  determinations  of  dip  are 
made.  Outside  of  the  district  the  northward  dipping  diabase  is  found 
on  one  side  of  the  Numakagon  river,  while  upon  the  other  is  seen  the 
Laurentian  granite. 
Brooks,140  in  1880,  gives  the  geology  of  the  Menominee  iron  region. 
The  Lower  Silurian  sandstone  is  found  capping  the  older  rocks  near 
lake  Eliza.  The  Laurentian  granite,  gneiss,  and  crystalline  schist  series 
is  not  subdivided.  No  limestones,  dolomitic  marbles,  conglomerates, 
calcareous  or  arenaceous  chloritic  schists  are  considered  as  belonging 
to  the  Laurentian  system.  It  is  not  certain  whether  this  series  occurs 
in  Wisconsin  within  the  area  surveyed. 
The  Huronian  series  is  divided  into  lower,  middle,  and  upper  divi- 
sions. The  lower  division  comprises  the  lower  quartziteof  great  thick- 
ness, the  great  marble  formation,  and  the  great  iron  ore  horizon  con- 
taining magnetic,  hematitic,  and  jaspery  schists  as  well  as  deposits  of 
iron.  The  middle  Huronian  comprises  quartzites,  clay-slates,  and  ob- 
scure soft  schists.  The  upper  Huronian  includes  mica-schists,  gneisses, 
and  granite,  the  last  of  which  may  possibly  be  eruptive,  but  is  the 
topmost  member  of  the  Huronian  succession.  Interstratified  with  the 
Huronian  are  diorites,  diabases,  gabbros  and  greenstones,  and  green- 
stone schists,  which  are  believed  to  be  conformable  beds  of  metamor- 
phosed sediments.  They  are  never  found  in  the  form  of  dikes.  The 
thickness  of  the  Huronian  in  the  Menominee  is  not  far  from  10,000  to 
15,000  feet.  There  is  great  difficulty  in  ascertaining  exactly  the  thick- 
ness on  account  of  the  sharp  folds,  where  thick  beds  double  back  upon 
themselves.  This  especially  affects  the  quartzites,  clay-slates,  and 
greenstones.  The  relative  proportions  of  the  different  kinds  of  rocks  of 
the  Huronian  and  a  correlation  of  the  successions  in  various  districts  of 
the  Menominee  are  given,  and  the  twenty  members  (including  the  upper 
granite)  are  correlated  in  detail  with  successions  north  of  lake  Huron, 
md  in  the  Marquette,  Gogebic,  Penokee,  and  central  Wisconsin  regions. 
The  resemblance  between  the  Marquette,  Menominee,  Sunday  Lake, 
bird  Penokee  series  are  so  numerous  as  to  point  unmistakably  to  their 
laving  been  formed  in  one  basin  under  essentially  like  conditions. 
Detailed  sections  and  maps  of  the  rock  exposures  are  given  at  numerous 
joints.  The  youngest  Huronian  member,  tin'  granite,  hasa  wide  extent. 
¥hile  this  is  true,  granite  dikes  are  rare  in  the  Menominee  Huronian 
nd  have  never  been  observed  in  the  Marquette  series.  No  rocks 
ffording  the  slightest  suggestion  of  conglomeratic  structure  have  been 
