LAKE   SUPERIOR  REGION.  327 
clinal.     Thus  the  Upper  Keweenawan  and  Lake  Superior  sandstones  are  not 
entirely  undisturbed.0 
To  sum  up,  I  have  not  been  able  to  find  other  strata  or  unconformities  or  indi- 
cations of  time  interval  to  represent  the  Middle  and  Lower  Cambrian  between 
the  Lake  Superior  sandstone  and  the  Upper  Keweenawan. 
The  distribution  and  relations  of  the  Ajibik  and  Mesnard  quartz- 
ites,  as  shown  by  recent  work,  are  discussed.  Faulting  is  believed  to 
be  more  important  than  previously  thought.  At  one  place  the  so- 
called  Laurentian  of  the  Marquette  district  has  been  found  to  be 
intrusive  into  the  Huronian.  Intrusive  and  effusive  igneous  rocks 
are  to  be  found  in  formations  other  than  those  mapped  by  the  United 
States  Geological  Survey,  and  more  work  nee'ds  to  be  done  in  separat- 
ing the  intrusive  from  the  effusive  phases. 
Observations  on  the  grain  of  rock,  based  on  the  hypothesis  that  the 
grain  shows  a  definite  relation  to  distance  from  margin  and  tempera- 
ture of  the  intruded  rock,  show  that  the  Laurentian  granites  visited 
by  the  committee,  and  especially  those  of  the  Rainy  Lake  district, 
are  in  all  respects  analogous  to  plutonics  generally.  While  the  in- 
truded strata  were  nearly  or  quite  fused  and  softened,  the  intruding 
rocks  were  considerably  superfused  and  were  therefore  not  the  imme- 
diately subjacent  strata. 
Leith,340  in  1906,  summarizes  the  general  geology  of  the  Lake 
Superior  region,  with  particular  reference  to  the  iron-bearing  series. 
SECTION    6.     SUMMARY  OF  PRESENT  KNOWLEDGE  OF 
IiAKE  SUPERIOR  REGION. 
INTRODUCTION. 
The  geology  of  the  Lake  Superior  region  is  of  peculiar  interest 
from  both  an  economic  and  a  scientific  standpoint.  The  presence  of 
large  and  rich  iron  and  copper  deposits  has  made  a  knowledge  of 
the  geology  of  the  region  necessary  for  purposes  of  exploration  and 
exploitation.  The  ores  of  the  region  are  contained  in  rocks  of  pre- 
Cambrian  age,  which  present  an  unusually  full  succession  and  great 
variety.  It  has  been  possible,  with  the  large  expenditures  which  the 
magnitude  of  the  mining  industry  warrants,  to  work  out  the  stratig- 
raphy more  satisfactorily  than  in  almost  any  other  area  of  pre- 
Cambrian  rocks. 
The  real  complexity  of  Lake  Superior  geology,  the  confusion  of 
names,  and  the  multiplicity  of  reports  covering  small  parts  of  the 
region  are  such  that  interested  persons,  other  than  geologists  directly 
engaged  in  the  mapping,  may  well  hesitate  to  attempt  to  comprehend 
"  In  fact,  the  Lake  Superior  basin  has  probably  been  a  concave  portion  of  the  earth 
crust  and  therefore  incapable  of  accumulating  notable  stress  through  all  geologic  time, 
and  thus  always  susceptible  to  slight  adjustments.  See  Chambeiiin  and  Salisbury, 
Geology,  vol.   1,  p.  561. 
