228  PRE-CAMBETAN    GEOLOGY    OF    NORTH    AMERICA. 
Winchell  (N.  TL),1!)0  in  1895,  discusses  the  origin  of  the  Archean 
greenstones.  The  great  bulk  of  them  are  pyroclastic.  They  were 
distributed  and  somewhat  stratified  by  the  waters  of  the  ocean  into 
which  the  material  fell.  Evidence  of  their  arrangement  by  water  is 
seen  in  their  very  general  stratiform  structure,  which  can  only  be 
explained  by  the  action  of  water.  This  structure  stands  vertical  or 
nearly  so.  These  greenstones  constitute  a  distinct  terrane,  forming 
the  latest  portion  of  the  Keewatin,  at  the  top  of  the  Fundamental 
Complex  of  the  Lake  Superior  region.  Below  the  greenstones  are 
found  chloritic  slates  and  schists,  chloride  schists,  clay  slates,  gray- 
wackes,  conglomerates,  quartzites,  novaculites,  and  jaspilites.  The 
thickness  of  the  greenstones  in  Minnesota  exceeds  that  of  any  other 
Archean  terrane.  The  Keewatin  passes  gradually  down  into  crystal- 
line mica  schists  or  hornblende  schists,  and  finally  into  acidic  gneiss. 
Winchell  (H.  V.)  and  Grant,191  in  1895,  give  a  preliminary 
account  of  the  Rainy  Lake  gold  region.  Following  Lawson,  they 
separate  the  rocks  there  found  into  four  distinct  groups.  These  are, 
beginning  with  the  lowest:  (1)  Laurentian,  composed  of  granites 
and  granitoid  gneisses  and  allied  rocks;  (2)  Coutchiching,  composed 
of  mica  schists  grading  into  fine-grained  gneisses;  (3)  Keewatin, 
composed  of  hornblendic,  greenish,  and  sericitic  schists,  conglomer- 
ates, graywackes,  etc.;  (4)  diabase  dikes,  more  recent  than  and  cut- 
ting all  the  others.  The  Coutchiching  mica  schists  have  in  many 
places  rapid  alternations  in  bands  from  an  inch  to  several  feet  in 
width  of  slightly  different  mineralogical  composition,  structure,  or 
color.  The  position  of  these  bands  gives  the  strike  and  dip  of  the 
rock,  and  when  they  are  lacking  the  schistose  structure  is  taken  as 
giving  the  strike  and  dip,  as  this  seems  to  be  parallel  with  the  band- 
ing when  the  two  are  seen  together.  While  an  unconformity  between 
the  two  is  not  proved,  it  seems  quite  probable,  on  account  of  basal 
conglomerate  beds  in  places  in  the  Keewatin  resting  on  the 
Coutchiching. 
Winchell  (H.  V.)  ,192  in  1885,  gives  a  brief  sketch  of  the  iron  ranges 
of  Minnesota.  Along  the  north  side  of  the  Mesabi  range  is  a  ridge 
of  Archean  syenite  and  granite,  flanked  on  both  sides  by  crystalline 
and  semicrystalline  schists.  This  ridge  is  called  the  Giants  range. 
On  the  south  side  of  the  Giants  range,  lying  at  times  nearly  up  to  its 
summit,  are  the  outcropping  edges  of  Taconic  or  Upper  Huronian 
strata,  which  overlie  unconformably  the  syenites  and  schists.  These 
are  in  turn  overlapped  to  the  south  by  eruptive  rocks  of  Keweenawan 
age  and  by  Cretaceous  sediments.  The  ore  is  soft  hematite,  which  lies 
at  low  angles  from  the  horizontal,  usually  covered  merely  by  drift. 
The  geology  of  the  Vermilion  range  is  not  yet  understood.  The 
iron  ores  are  solid  and  massive,  except  at  the  Chandler  mine,  where 
