LAKE    SUPERIOR   REGION.  287 
If,  as  seems  probable,  these  jaspers  are  the  equivalents  of  the  west- 
ern Huronian  sandstones,  there  is  a  definite  horizon  traceable  from 
point  to  point  across  the  whole  northern  end  of  the  province,  a  dis- 
tance of  more  than  600  miles. 
At  a  number  of  places  over  this  area  conglomerates  containing 
jasper,  ferruginous  sandstone,  or  chert  pebbles,  probably  derived 
from  the  source  above  described,  are  known.  Beginning  at  the  west, 
some  of  these  conglomerates  occur  as  follows:  On  Shoal  Lake,  east 
of  Rainy  Lake;  west  end  of  Schist  Lake;  near  Mosher  Bay,  at  the 
east  end  of  Upper  Manitou  Lake;  a  mile  east  of  Fort  Frances  on 
Rainy  River;  near  Rat  Portage;  near  the  mouth  of  Dore  River;  in 
the  Original  Huronian  area,  north  of  Lake  Huron,  particularly  the 
Thessalon  area  ;  on  Lake  Temiscaming. 
It  is  assumed  that  the  iron-formation  material  can  not  be  other 
than  LoAver  Huronian,  and  that  the  conglomerates  must  represent  a 
basal  horizon  of  the  Upper  Huronian.  The  break  between  the  Upper 
and  Lower  Huronian  thus  represented  is  a  most  profound  one,  and 
affords  a  good  basis  for  the  correlation  of  the  Huronian  formations. 
It  is  further  suggested  that  this  great  unconformity  may  be  the  same 
as  that  between  the  Upper  and  Lower  Huronian  formations  on  the 
south  shore  of  Lake  Superior  and  in  Minnesota. 
Van  Hise  and  Clements,277  in  1900,  make  observations  on  the 
region  of  the  north  shore  of  Lake  Superior.  They  were  together  in 
the  areas  of  Thunder,  Black,  and  Nipigon  bays.  Lake  Nipigon  was 
examined  by  Clements  and  the  north  and  east  shore  of  Lake  Superior 
east  of  Nipigon  Bay  by  Van  Hise. 
At  Port  Arthur  the  Keewatin  rocks  are  found  to  be  the  same  as 
the  typical  Archean  rocks  of  the  Vermilion  district  of  Minnesota — 
greenstones,  green  schists,  and  mashed  porphyries.  The  iron-bearing 
phases  associated  with  these  have  the  same  complex  relations  as  in 
the  Vermilion  district,  although  the  proportion  of  black  jasper  is 
greater  than  in  the  Vermilion  district.  This  series  can  probably  be 
connected  to  the  west  with  the  Mattawin  iron  formation  and  through 
this  formation  with  that  occurring  north  of  the  Saganaga  granite  of 
the  Vermilion  district. 
About  Thunder  Bay  at  several  places  the  basal  conglomerate  of  the 
Keweenawan  is  in  actual  contact  with  the  underlying  Animikie  and 
contains  fragments  of  it,  thus  showing  conclusively  the  unconformity 
between  the  Animikie  and  the  Keweenawan.  That  the  laccolithic 
sills  of  diabase  which  so  characteristically  intrude  the  Animikie  slates 
of  this  area  are  of  Keweenawan  or  later  age  is  supported  by  the  facts 
that  the  basal  Keweenawan  conglomerate  contains  no  fragments  of 
this  diabase  and  that  at  various  places  the  diabase  is  found  cutting 
both  the  Animikie  and  the  Keweenawan. 
