LAKE    SUPERIOR   REGION.  163 
The  evidence  concerning  the  source  of  the  Keweenawan  lavas  is 
considered,  and  it  is  concluded  that  they  may  probably  have  come 
from  a  higher  level  somewhat  back  from  the  edges  of  the  present 
Keweenawan  basin. 
With  this  probability  in  mind  the  following  hypotheses  are  sug- 
gested : 
1.  The  irregularities  in  the  lower  beds  of  the  Keweenaw  series  in 
the  Portage  Lake  area,  contrasted  with  the  greater  regularity  of  the 
higher  part  of  the  series,  suggest  that  in  this  area,  near  the  contact 
between  the  Keweenaw  series  and  the  Eastern  sandstone,  we  are  on 
the  edge  of  an  early  Keweenawan  or  pre-Keweenawan  basin. 
2.  If  the  lower  beds  of  the  Keweenaw  series  near  Portage  Lake  rested 
on  the  sides  of  a  basin,  the  later  beds  of  the  series  from  here  eastward 
lay  at  a  higher  altitude  and,  excepting  those  of  the  south  trap  range, 
were  eroded  in  pre-Potsdam  time,  together,  possibly,  with  a  part  of 
the  underlying  Archean. 
3.  The  porphyries  found  on  Keweenaw  Point  at  the  contact  be- 
tween the  Keweenaw  series  and  the  Potsdam  sandstone  may  be  in 
part  either, 
a.  Marginal  facies  of  the  underlying  Archean; 
b.  Intrusive  in  the  early  Keweenawan; 
c.  Early  interbedded  flows  of  the  Keweenaw  series;  or 
d.  Remnants  of  late  Keweenawan  intrusions  by  which  the  eastern 
margin  of  the  series  was  broken  up  and  its  degradation  hastened. 
Seaman  and  Sutton,81  in  1898,  discovered  east  of  Little  Presque 
Isle  River,  in  the  Gogebic  district,  a  quartzite  and  conglomerate 
beneath  the  limestone  of  the  Lower  Huronian  series.  The  conglom- 
erate rests  with  sharp  contact  on  steeply  inclined  green  schists  of 
the  Archean.  The  unconformity  between  the  Lower  Huronian  and 
the  Archean  had  been  previously  inferred  from  general  field  relations 
but  had  not  been  actually  observed.  This  exposure  is  diagrammatic 
in  its  clearness.  The  quartzite  is  believed  to  correspond  to  the  Mes- 
nard  quartzite  beneath  the  dolomite  in  the  Marquette  district. 
Clements,  Smyth,  Bayley,  and  Van  Htse,82  in  1899,  describe  the 
Crystal  Falls  iron-bearing  district  of  Michigan. 
The  rocks  of  the  district  comprise  two  groups,  separated  by  uncon- 
formities. These  are  the  Archean  and  the  Algonkian.  The  Algon- 
kian  includes  both  the  Lower  Huronian  and  the  Upper  Huronian 
series,  and  these  are  also  separated  by  unconformities.  The  terms 
Lower  Huronian  and  Upper  Huronian  arc  applied  to  the  series  which 
occur  in  this  district  because  they  are  believed  to  belong  to  the  same 
geological  province  as  the  Huronian  rocks  of  the  north  shore  of  Lake 
Huron,  and  to  be  equivalent  to  the  Lower  Huronian  and  Upper  Huro- 
nian series  which  there  occur. 
