LAKE    SUPERIOR  REGIOtf.  249 
interesting  phases  are  especially  well  developed  in  the  vicinity  of 
Akeley  Lake.  In  general  the  formation  has  a  monoclinal  dip  to  the 
south-southeast  at  a  low  angle.  It  has  been  extremely  metamor- 
phosed by  the  Duluth  gabbro,  and  where  most  metamorphosed  the 
rocks  are  composed  of  coarsely  crystalline  bands  of  quartz,  of  vary- 
ing width,  alternating  with  coarsely  crystalline  bands  of  magnetite 
ore,  reported  to  vary  from  1  inch  up  to  10  or  12  feet  in  thickness,  and 
of  bands  of  dark-green,  brown,  or  black  rocks  that  consist  of  combina- 
tions of  quartz,  augite,  hypersthene,  hornblende,  olivine,  and  mag- 
netite as  the  principal  minerals,  but  associated  occasionally  with  some 
ferruginous  carbonate,  actinolite,  and  griinerite. 
The  Duluth  gabbro  and  the  Logan  sills,  referred  to  the  Keweena- 
wan,  occur  in  the  eastern  portion  of  the  district.  The  gabbro  is  found 
to  metamorphose  all  of  the  sediments  already  enumerated,  and  is 
thus  shown  to  be  one  of  the  youngest  rocks  of  the  district.  It  is  also 
found  to  be  intrusive  in  the  Keweenawan  volcanics.  A  number  of 
facts  are  enumerated  to  show  that  the  gabbro  and  the  Logan  sills  are 
of  essentially  the  same  petrographic  character,  although  they  exhibit 
minor  differences  that  are  readily  explicable  when  one  considers  the 
relative  amounts  of  the  two  rocks.  After  a  consideration  of  these 
facts  and  of  the  stratigraphic  relationship  of  the  rocks  the  conclusion 
is  reached  that  the  gabbro  and  the  sills  are  of  essentially  the  same 
composition  and  age,  having  been  derived  from  the  same  parent  mass 
or  magma.  In  certain  localities  in  the  Duluth  gabbro  are  masses  of 
titaniferous  magnetite  of  varying  but  small  size,  with  some  associ- 
ated minerals.  These  masses  grade  into  the  surrounding  gabbro  and 
were  formed  as  the  result  of  processes  of  segregation. 
Cutting  the  Duluth  gabbro  are  acidic  dikes  and  dikes  of  basalt  and 
diorite. 
The  entire  district  has  been  much  folded  and  metamorphosed,  re- 
sulting in  a  north-of-east  and  south-of-west  trend  of  the  Archean 
and  Lower  Huronian  formations,  marked  principally  by  schistosity. 
Winchell  (X.  H.),212  in  1903,  summarizes  some  results  of  the  work 
of  the  late  Minnesota  Geological  Survey.  Those  referring  to  the 
pre-Cambrian  are  as  follows  (the  numbers  are  WinchelFs)  : 
5.  The  discrimination  of  two  iron-bearing  formations  in  northern 
Minnesota,  thus  separating  the  Mesabi  range  stratigraphically  from 
the  Vermilion.  This  observation  was  continued  into  Wisconsin  and 
Michigan  by  a  visit  to  those  States,  and  the  same  duality  was  pointed 
out  in  the  iron  regions  of  those  States  and  was  announced  for  the  first 
time  in  the  Minnesota  report  for  1888.  It  has  since  been  discovered 
(hat  there  is  still  a  third  iron  horizon  in  northeastern  Minnesota,  not 
mentioning  the  titanic  iron  ore  of  the  gabbro.  It  is  the  Upper  Keewa- 
tin,  the  others  being  in  the  Lower  Keewatin  and  the  Taconic. 
