LAKE    SUPERIOR   REGION.  243 
Hall,207  in  1901,  describes  and  maps  the  slates  and  associated  rocks 
in  the  vicinity  of  Cloquet  and  Carlton  on  St.  Louis  River,  and  cer- 
tain hornblendic  and  micaceous  schists  associated  with  granite  and 
diabase  to  the  west  along  Mississippi  and  Snake  rivers.  He  main- 
tains that  the  slates  and  graywackes  to  the  east  and  the  hornblendic 
schists  to  the  west  belong  to  one  and  the  same  series,  and  that  the 
schists  have  resulted  from  the  metamorphism  of  graywacke  and 
slate  by  the  intrusion  of  granite.  Still  later  intrusions  of  diabase 
have  cut  both  the  granites  and  the  slates.  Accepting  Spurr's  state- 
ment that  the  Carlton  slates  are  Keewatin  or  Lower  Huronian,  the 
conclusion  is  reached  that  the  schists  to  the  southwest  are  Lower 
Huronian  and  that  the  intruding  granites  are  post-Lower  Huronian. 
If  this  conclusion  be  correct,  the  granites  and  schists  of  the  central 
and  eastern  portions  of  Minnesota  must  be  mapped  as  Algonkian 
rather  than  Archean,  as  in  the  past. 
Hall,208  in  1901,  describes  the  Keweenawan  rocks  south  and  south- 
west from  Duluth,  along  St.  Louis  and  St.  Croix  rivers,  and  shows 
that  a  series  of  alternating  lava  flows  and  sediments  lie  in  a  synclinal 
northeast-southwest  trough,  the  western  border  of  which  is  marked 
by  a  profound  fault.  To  the  east  of  the  fault  the  Keweenawan  rocks 
are  highly  tilted  to  the  southeast,  while  to  the  west  of  it  the  Cam- 
brian rocks  are  much  broken  up.  The  relations  of  the  fault  to  the 
distribution  of  the  flows  and  analogy  with  other  volcanic  regions 
seem  to  show  that  the  fault  was  a  plane  of  weakness  along  which 
most  of  the  lavas  were  originally  erupted.  The  faulting  was  pre- 
Cambrian.  Cambrian,  and  post-Cambrian,  as  shown  by  the  fact  that 
in  some  places  the  Cambrian  rests  horizontally  upon  the  upturned 
Keweenawan  rocks,  and  in  others  is  much  broken  up. 
Winchell  (N.  H.),209  in  1902,  briefly  describes  the  iron  ores  of 
Minnesota  and  incidentally  sketches  their  geological  relations.  No 
new  points  are  added  to  those  previously  presented. 
Leith,210  in  1903,  describes  and  maps  the  geology  of  the  Mesabi 
district  of  Minnesota.  The  district  is  2  to  10  miles  in  width,  extend- 
ing from  near  Grand  Rapids  on  Mississippi  River  to  Birch  Lake,  a 
distance  of  approximately  100  miles.  The  main  topographic  feature 
is  a  ridge,  known  as  the  Giants  (or  Mesabi)  range,  which  extends 
the  length  of  the  district.  The  geologic  formations  represented  in 
the  district  belong,  in  ascending  succession,  to  the  Archean,  Lower 
Huronian,  Upper  Huronian,  Keweenawan,  Cretaceous,  and  Pleisto- 
cene. They  are  all  separated  by  unconformities.  The  core  of  the 
Giants  range  is  formed  by  Archean  and  Lower  Huronian  rocks, 
except  for  the  portion  in  Rs.  12  and  13,  where  Keweenawan  granite 
forms  the  core.  On  the  south  flank  rest  the  Upper  Huronian  rocks, 
containing  the  iron-bearing  formation,  with  gentle  southerly  dips. 
