LAKE    SUPERIOR   REGION.  235 
northeast,  and  the  dip  in  general  about  10°  east  of  south.  The  thick- 
ness varies  from  a  knife  edge  to  4,000  feet.  The  Animikie  is  sepa- 
rable into  four  conformable  divisions:  (1)  the  lower  or  quartzite 
member,  called  the  Pewabic  quartzite;  (2)  the  iron-bearing  or  taco- 
nite  member;  (3)  the  black  slate  member;  (4)  the  graywacke  slate 
member. 
(1)  The  quartzite  member  is  well  developed  in  Itasca  County,  but 
disappears  before  reaching  the  eastern  side  of  St.  Louis  County. 
(2)  The  rocks  of  the  iron-bearing  member  are  similar  to  those  in 
St.  Louis  County  on  the  western  end  of  the  range,  described  by 
Spurr.a  They  differ,  however,  in  two  features.  They  are  more  com- 
pletely crystalline,  and  the  iron  is  magnetite  instead  of  hematite. 
The  rocks  consist  chiefly  of  jaspers,  amphibole  (griinerite)  schists, 
greenish  siliceous  slates,  cherts,  cherty  carbonates,  and  magnetite 
slates.  It  is  believed  that  these  rocks  were  originally  glauconitic 
greensands;  that  the  ore  has  been  derived  from  the  iron  in  the  glau- 
conite,  and  that  the  ore  bodies  result  from  concentration  and  replace- 
ment. In  this  part  of  the  Mesabi  range  no  ore  bodies  have  yet  been 
found  which  are  at  the  same  time  both  rich  enough  and  large  enough 
for  profitable  mining,  although  vast  quantities  of  magnetite  ore  occur 
at  or  near  the  surface. 
The  dip  of  this  formation  varies  from  an  average  of  45°  to  50°  on 
the  west  to  less  than  15°  on  the  east,  and  the  thickness  varies  from  650 
feet  or  less  on  the  west  to  900  feet  on  the  east. 
(3)  The  black  slate  is  essentially  a  fine-grained,  black,  more  or  less 
siliceous,  apparently  carbonaceous  slate. 
(4)  The  graywacke  slate  member  is  composed  of  black  to  gray 
slates  and  fine  graywackes,  with  some  flinty  slates;  the  upper  part 
shows  coarser  detrital  material,  and  the  highest  beds  seen  are  fine- 
grained quartzites  and  quartz  slates.  This  member  is  well  exposed  on 
the  south  shore  of  Loon  Lake. 
Associated  with  all  of  the  strata  of  the  Animikie  are  diabase  sills, 
and  bounding  the  Animikie  rocks  on  the  south  is  the  great  gabbro 
mass.  These  are  igneous  rocks  of  later  date  than  the  Animikie.  Near 
the  contact  with  the  gabbro  the  Animikie  rocks  show  marked 
metamorphism  and  usually  complete  recrystallization.  The  gabbro 
varies  from  a  nearly  pure  plagioclase  rock  to  titaniferous  magnetite. 
The  pre- Animikie  rocks  here  described,  according  to  the  nomen- 
clature used  by  the  United  States  Geological  Survey,  belong  to  the 
Lower  Huronian  series  of  the  Algonkian  system,  and  probably  also  in 
part  to  the  older  Archean  or  Basement  Complex;  the  Animikie  is 
regarded  as  the  equivalent  of  the  Upper  Huronian  series  of  the  Algon- 
kian, and  the  gabbro  as  the  lower  part  of  the  Keweenawan  series  of 
the  Algonkian. 
"Bull.  Geol.  and  Nat.  Hist.  Survey  Minnesota,  vol.  10,  1894. 
