322  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
Leith,335  in  1902,  compares  the  origin  and  development  of  the 
Gogebic  and  Mesabi  iron  ores.  The  ores  of  the  two  districts  occur  in 
the  same  geological  horizon;  they  result  from  the  alteration,  under 
weathering  conditions,  of  a  ferrous  compound  of  iron,  through  the 
agency  of  percolating  waters,  and  are  localized  in  channels  of  vigor- 
ous circulation  of  water.  But  the  differences  in  the  development  of 
the  ores  of  the  two  districts  are  important.  The  original  ferrous 
compound  of  iron  is  mainly  iron  silicate  in  the  Mesabi  district  and 
iron  carbonate  in  the  Gogebic  district,  although  both  substances  ap- 
pear in  each  district.  The  localization  of  the  ores  in  the  Gogebic 
district  during  their  concentration  has  been  within  clear-cut  pitching 
troughs  with  definite  shapes,  while  in  the  Mesabi  district  the  very 
gentle  folding  of  the  iron  formation,  its  fracturing,  and  the  absence 
of  intrusives  combine  to  make  the  channels  of  vigorous  flow  within 
the  iron  formation  most  devious,  resulting  in  the  curious  and  exceed- 
ingly irregular  shapes  now  to  be  observed  in  the  Mesabi  ore  deposits. 
The  original  ferrous  silicate  from  which  the  ores  develop  in  the 
Mesabi  district  is  in  minute  homogeneous  granules,  the  form  of  which 
remains  even  after  the  substance  is  changed.  Associated  with  these 
granules  are  undoubted  concretions  of  iron  oxide  and  chert  with  con- 
centric structure.  In  the  Gogebic  district  there  appear  numerous 
concretions  with  concentric  structure,  which  Van  Hise  has  shown  to 
develop  during  the  alteration  of  iron  carbonate ;  and  associated  with 
these  are  rare  granules  of  iron  oxide  and  chert  in  varying  propor- 
tions, which  may  represent  altered  ferrous  silicate  granules  similar 
to  those  of  the  Mesabi  district.  Evidences  of  the  existence  of  origi- 
nal ferrous  silicate  granules  in  the  Gogebic  district  are  not  sufficiently 
numerous  to  warrant  modification  of  Van  Hise's  conclusion  that  the 
ores  have  developed  mainly  from  the  alteration  of  iron  carbonate. 
Lawson,330  in  1902,  criticises  the  use  of  the  term  Algonkian.  He 
emphasizes  the  importance  of  the  interval  which  he  calls  the  Epar- 
chean,  between  the  "  Huronian  "  and  Animikie  series— that  is,  be- 
tween the  Lower  Huronian  and  Upper  Huronian  of  the  United  States 
Geological  Survey — and  argues  that  no  one  term,  such  as  Algonkian, 
should  include  a  break  of  this  importance.  It  is  proposed  to  restrict 
Algonkian  to  the  Animikie  and  Keweenawan  rocks  and  to  retain 
Dana's  term  Archean  for  all  rocks  below  the  Animikie — that  is,  below 
the  Eparchean  interval — and  also  to  retain  the  terms  Laurentian 
and  Huronian  as  subdivisions  of  the  Archean  with  the  significance 
originally  given  them  by  Logan.  The  correlations  of  the  Animikie 
with  the  Keewatin  of  Minnesota  and  with  the  Upper  Huronian  of 
Lake  Huron  are  regarded  as  erroneous,  because  of  dissimilarity  in 
lithology,  in  stratigraphy,  and  in  relations  to  intrusives.  Following 
Willmott  and  others,  it  is  believed  that  the  Animikie  is  younger  than 
the  (Upper)  Huronian  series  of  Lake  Huron,  and  thus  later  than  the 
