LAKE   SUPERIOR  REGION. 
Seaman  diabase  dikes  of  the  Gogebic  area  are  probably  the  same  as 
those  that  cut  the  overlying  sandstones  of  the  Keweenawan,  from 
which  it  is  concluded  that  the  Keweenawan  lava  flows  are  the  effusive 
equivalents  of  the  Holyoke  diabase  dikes. 
The  soft  hematites  of  the  region  are  produced  by  secondary  enrich- 
ment at  places  where  the  water  could  best  act,  being  at  points  of  frac- 
turing or  in  basins.  The  silica  of  the  lean  material  has  been  leached 
out  and  iron  oxide  substituted.  Gold  and  silver  veins  are  discussed 
and  a  classification  of  ore  deposits  is  given. 
The  Eastern  or  Potsdam  sandstone  rests  unconformably  on  the 
Azoic.  This  includes  the  unaltered  horizontal  sandstone,  which  is 
free  from  dikes  of  eruptive  material,  and  the  Keweenawan,  which 
consists  of  lava  flows  alternating  with  sandstones  and  conglomerates. 
largely  derived  from  the  former.  Above,  and  conformably  with  the 
Eastern  sandstone,  near  L'Anse,  is  limestone  of  Silurian  age,  as  shown 
by  its  fossil  contents.  On  Keweenaw  Point  the  Eastern  standstone 
dips  toward,  and  passes  under,  the  interstratified  sandstones  and  lavas 
of  the  Keweenawan.  At  or  near  the  contact  is  a  fault,  However,  at 
Douglass  Houghton  and  Hungarian  rivers  it  is  thought  not  to  be  at 
the  contact,  and  consequently  that  the  Eastern  sandstone  underlies 
the  Keweenawan  lava,  but  the  Eastern  sandstone  may  contain  two  or 
more  sandstones  of  different  ages,  which  may  perhaps  be  considered 
as  the  most  probable  explanation  of  all  the  evidence.  In  sec.  13, 
T.  4(5  N.,  R,  41  W.,  on  the  south  trap  range,  a  nearly  horizontal,  soft, 
friable  micaceous  sandstone  is  found  near  the  interbedded  Kewee- 
nawan melaphyre  and  indurated  sandstone.  This  soft  sandstone  con- 
tains numerous  spherical  spots  very  common  in  the  Eastern  sandstone, 
but  not  found  in  the  Keweenawan.  In  the  soft  sandstone  are  found 
pebbles  and  large  angular  fragments  of  indurated  sandstone,  which 
Seaman  thinks  could  have  been  derived  only  from  the  adjacent  indu- 
rated sandstone.  The  rocks  of  the  trap  range  here  exposed  are  be- 
lieved by  Seaman  to  hold  a  position  near  the  top  of  the  Keweenawan 
series,  and  he  concluded  that  the  soft  sandstone  belongs  to  a  distinct 
and  later  geological  age  than  the  trap  range. 
The  character  and  origin  of  the  copper  deposits  are  discussed. 
Wadsworth,03  in  1893,  states  that  recent   work  renders  it   probable 
that  the  Azoic  or  Archean  of  northern  Michigan  is  divisible  into  five 
unconformable  formations.     The  tentative  arrangement,  commencing 
with  the  oldest,  with   the   parallel   formations  as  determined   by  the 
United  States  Geological  Survey,  is  as  follows: 
.United  sia!rs  Geological 
Michigan    Geological    Survey.  Survey. 
Cascade  formation Fundamental  Complex. 
Republic  formation 
ation  1  _. 
I  Lower  Marqiioitc  series. 
Mesnard  formation  j 
Holyoke  format  ion 
^T  „  K Inner  Marquette  series. 
Negaunee  formation 
t 
