226  PRE-C  AM  BRIAN    GEOLOGY    OF    NORTH    AMERICA. 
composed  of  peculiar  rocks,  presenting  no  resemblance  to  the  Pewabic 
quartzite  or  to  the  upper  slate.  The  upper  slates  are  of  great  thick- 
ness, and  have  at  their  base  an  impure  limestone,  often  dolomitized 
or  sideritized. 
The  part  of  the  iron-bearing  member  which  extends  from  Poke- 
o-ama  Falls  to  Embarrass  Lake  is  called  the  Western  Mesabi  Range; 
the  part  from  Embarrass  Lake  to  Gunflint  Lake,  the  Eastern  Mesabi 
Range,  and  the  part  from  Gunflint  Lake  east,  the  International 
Boundary  area.  The  description  of  the  iron-bearing  member  below 
applies  to  the  western  part  of  the  district.  It  has  a  thickness  varying 
from  500  to  1,000  feet,  with  an  average  of  about  800  feet.  The  dip 
varies  from  less  than  10°  to  as  much  as  30°,  and  the  width  of  the 
formation  varies  correspondingly  from  2  or  3  miles  to  less  than  half 
a  mile,  the  average  width  being  1  mile  and  the  average  dip  10°. 
Resting  upon  the  iron-bearing  member  is  a  great  thickness  of  fine- 
grained slates,  at  the  base  of  which  is  locally  an  impure  dolomitic 
limestone.  When  this  limestone  is  present  the  contact  between  the 
iron-bearing  member  and  the  upper  slate  can  not  be  distinctly  located. 
The  least-altered  phase  of  the  iron-bearing  member  is  a  rock  called 
taconite,  which  consits  of  a  background  of  cryptocrystalline,  pheno- 
crystalline,  and  chalcedonic  silica,  in  which  are  numerous  granules. 
These  are  composed  of  glauconite,  siderite,  hematite,  magnetite, 
limonite,  and  cryptocrystalline  silica,  in  the  very  freshest  phase,  the 
two  former  being  predominant.  The  granules  in  one  of  these  fresher 
phases  showed  by  analyses  about  35  per  cent  of  siderite  and  65  per  cent 
of  glauconite,  or  about  22  per  cent  of  ferrous  oxide  in  the  form  of 
siderite,  and  about  10  per  cent  of  ferrous  and  ferric  oxide,  two-thirds 
being  the  former  in  the  glauconite.  Other  analyses  gave  similar  re- 
sults. Analyses  showed  a  very  little  calcium  and  magnesium.  In 
the  freshest  phase  found  were  seen,  in  thin  section,  probably  detrital 
original  grains  of  carbonate,  recognized  by  their  cleavage  as  calcite 
or  dolomite.  From  the  taconite,  by  a  complicated  series  of  metaso- 
matic  changes,  there  have  developed  cherts  and  jaspers,  which  are 
sideritic,  hematitic,  magnetitic,  or  actinolitic,  or  twTo  or  more  of  these 
combined.  During  the  process  the  chert  and  iron  oxides  were  largely 
concentrated  in  alternating  bands.  The  cherts  and  jaspers  are  in 
numerous  places  concretionary  and  brecciated.  Many  of  them  have 
a  prismatic  jointing  and  horizontal  parting. 
These  transformations  were  caused  by  downward-percolating  wa- 
ters, carrying  as  the  chief  agents  oxygen  and  carbonic  acid,  and  as 
subordinate  agents  sulphuric  acid  and  alkalies.  In  the  changes  from 
glauconite  and  siderite  to  the  oxides  there  was  an  important  shrink- 
age of  the  mass,  and  this  has  resulted  in  the  brecciation,  prismatic 
jointing,  horizontal  parting,  and  banding.  The  prismatic  jointing- 
is  analogous  in  its  formation  to  the  shrinkage  of  basaltic  columns  of 
