LAKE    SUPERIOR   REGION".  251 
associated  with  the  gabbro,  and  grading  into  it,  is  of  the  Keweena- 
wan,  and  another  is  still  later.  Hence  it  was  inferred  that  the 
Archean  granites  did  not  spring  from  a  deep  source,  but  are  a  sur- 
face product  of  metamorphism  carried  to  the  extreme  of  fusion,  on 
clastic  materials  that  were  later  than  the  basal  greenstones.  Ad- 
ventitiously they  form  intrusions  in  some  of  the  later  (and  especially 
into  the  clastic)  greenstones,  but  they  are  not  known  to  penetrate  the 
oldest  greenstones.  Tentatively  the  alkaline  and  the  acidic  siliceous 
elements  in  these  early  sediments  were  supposed  to  have  been  derived 
from  the  atmosphere,  as  the  basal  crust  could  »ot  have  afforded  them. 
In  the  same  manner  the  gabbro,  which  becomes  acidic  and  grades 
into  syenite,  was  derived  from  the  metamorphism  and  fusion  of  the 
greenstones  with- their  clastic  variations.  Diabase  was  found  to  pass 
insensibly  into  gabbro,  but  on  the  other  hand  it  is  also  certain  that  it 
was  the  original  form  of  all  the  igneous  greenstones,  and  that  it  must 
have  had  a  deep-seated  source,  and  has  still  a  deep-seated  source. 
These  belts  of  intensest  metamorphism,  whether  productive  of 
granite  or  of  gabbro,  have  a  parallelism  with  one  another  and  with 
the  northwestern  rim  of  the  great  synclinorium  of  the  basin  of  Lake 
Superior,  marking  successive  continental  folds,  in  harmony  with  a 
system  which  continued  through  Archean  and  Taconic  time,  and  even 
into  the  upper  Cambrian. 
Van  Hise,  Beel,  Adams,  Mileer,  Lane,  Merriam,  Sebenius,  and 
Leith,S8  in  1904,  visited  the  Mesabi  district  of  Minnesota  and  made 
observations  confirming  the  conclusions  reached  by  Leith  in  previous 
years.  Two  unconformable  Huronian  aeries,  intruded  by  granites, 
were  observed  to  be  separated  by  unconformities  and  to  rest  on  a 
Basement  Complex  consisting  of  a  basic  Keewatin  division  and  an 
acidic  Laurentian  granite  division  intrusive  into  the  Keewatin. 
In  the  Vermilion  district  observations  were  made  confirming  the 
conclusions  previously  reached  by  the  geologists  of  the  United  States 
Geological  Survey.  One  sedimentary  series,  Lower  Huronian,  was 
observed  to  rest  upon  a  Basement  Complex  consisting  of  a  basic 
Keewatin  division  and  an  acidic  Laurentian  division. 
Leith,213  in  1907,  describes  the  geology  of  the  Cuyuna  iron  range 
of  Minnesota.  This  range  is  the  most  recently  discovered  range  in 
the  Lake  Superior  region.  It  lies  along  the  line  of  the  Northern 
Pacific  Railway,  near  Mississippi  River,  in  the  vicinity  of  the  towns 
of  Brainerd,  Deerwood,  and  Aitkin,  in  Crow  Wing,  Aitkin,  and  Mor- 
rison counties,  in  north-central  Minnesota.  Its  boundaries  are  still 
being  extended  and  few  would  care  to  draw  limits  in  any  direction. 
The  area  of  present  greatest  activity  lies  to  the  south  and  east  of 
Mississippi  River,  in  Tps.  43,  44,  45,  46,  47,  and  48  N.,  Rs.  28,  29,  30, 
31,  and  32  W. 
