LAKE    SUPERIOR  REGION.  229 
they  are  brecciated  and  occur  in  steeply  inclined  lenses  between  walls 
of  schist,  extending  to  an  indefinite  depth. 
Elftman,193  in  1895,  finds  that  the  great  gabbro  of  northeastern 
Minnesota  has  a  rude  arrangement  of  the  rock  in  parallel  layers 
similar  to  the  layers  of  sedimentary  rocks.  This  structure  usually 
dips  to  the  south.  It  does  not  depend  upon  the  differentiation  of  the 
mineral  components  of  the  rocks,  but  seemingly  is  due  to  secondary 
causes  which  acted  upon  the  rock  after  it  had  solidified.  This  sheeted 
structure  is  a  common  phenomenon  along  the  northern  limits  of  the 
mass.  The  gabbro  has  also  a  banded  structure  due  to  the  parallel 
arrangement  of  the  mineral  constituents.  The  bands  are  not  regu- 
larly arranged,  appearing  and  disappearing  in  a  manner  which  shows 
them  to  be  not  independent  of  the  secondary  causes.  This  structure 
is  present  to  a  marked  degree  in  the  central  portion  of  the  gabbro. 
Large  feldspar  masses  occur  in  the  gabbro  in  the  southeastern 
parts  of  T.  61  N.,  Es.  10  and  11  W.  The  mass  in  the  latter  township 
has  a  marked  banding.  The  line  of  division  between  the  feldspar 
masses  and  the  normal  rock  is  sharp  in  the  field  and  in  the  hand 
specimen.  Both  are,  however,  regarded  as  differentiations  from  the 
same  magma. 
In  the  southern  part  of  T.  62  N.,  E.  10  W.,  the  eastern  part  of  T.  61 
N.,  E.  11  W.,  the  greater  part  of  T.  61  N.,  E.  10  W.,  and  in  adjacent 
townships  is  a  considerable  area  of  dark,  reddish-colored  olivine 
gabbro  or  troctolite,  which  has  both  a  sheeted  and  a  banded  appear- 
ance. This  rock  and  the  normal  gabbro  have  not  been  seen  in  con- 
tact, but  wherever  they  closely  approach  each  other,  often  within  a 
few  feet,  both  preserve  their  characteristic  structure  and  there  is  no 
sign  of  the  transition  of  the  one  into  the  other.  The  olivine  rock 
appears  to  be  above  the  ordinary  gabbro. 
Smyth  and  Finlay,194  in  1895,  describe  the  western  part  of  the 
Vermilion  range.  The  sedimentary  rocks  fall  into  two  divisions. 
The  older  is  a  fragmental  slate  formation,  while  the  younger  is  an 
iron-bearing  formation  lithologically  identical  with  certain  phases  of 
the  lower  iron-bearing  formation  of  the  Marquette  district.  To  all 
appearances  it  is  devoid  of  clastic  material.  It  is  believed,  from  anal- 
ogies with  other  iron-bearing  districts  of  the  Lake  Superior  region, 
that  the  jasper  of  the  Vermilion  district  is  derived  from  a  cherty  iron 
carbonate  or  from  a  glauconitic  greensand,  or  both.  However,  as  the 
jasper  is  a  final  product  of  the  alterations,  it  is  not  possible  to  show 
this. 
Intrusive  igneous  rocks  are  very  abundant,  cutting  or  being  inter- 
leaved with  the  sedimentary  rocks  in  masses  running  from  the  thick- 
ness of  a  knife  blade  to  those  100  feel  across.  In  quantity  the  igneous 
rocks  exceed,  perhaps  several  times,  the  sedimentary  rocks.    The  old- 
