LAKE    SUPERIOR   REGION.  273 
The  Nipigon  formation,  resting  with  apparent  unconformity  upon 
the  Animikie  formation,  is  characterized  by  reddish  marls,  sandstones, 
and  conglomerates,  with  a  large  proportion  of  variously  colored  trap- 
pean  beds  and  masses,  a  considerable  part  of  which  is  amygdaloidal. 
The  Nipigon  formation  occupies  a  great  area  about  Nipigon  Lake 
and  considerable  areas  at  the  east  end  of  Lake  Superior  and  on  Michi- 
picoten  Island. 
On  the  east  side  of  Hudson  Bay  and  on  the  islands  off  the  coast 
volcanic  and  sedimentary  rocks  are  largely  developed,  comprising 
conglomerates,  sandstones,  limestones,  chert  breccias,  shales,  quartz- 
ites,  argillites,  porphyries,  crystalline  traps,  amygdaloids,  tuffs,  etc. 
The  upper  part  of  these  may  correspond  to  the  Nipigon  and  the  lower 
to  the  Animikie. 
The  sandstones  of  Sault  Ste.  Marie,  of  the  peninsula  between  Gou- 
lais  and  Batchawana  bays,  Isle  Parisienne,  etc.,  seem  to  be  of  Pots- 
dam age.  These  sandstones  are  mostly  red,  but,  unlike  the  Nipigon 
formation,  they  appear  to  be  free  from  local  disturbance  and  lie 
almost  Hat.  Although  they  resemble  some  of  the  sandstones  of  the 
Nipigon  series  at  Mamainse  in  red  color,  they  are  believed  to  be 
newer  and  probably  unconformable  to  them. 
Laavson,255  in  1891,  states  that  the  granite  of  Saganaga  Lake  is 
found  with  abundant  and  clearly  observed  evidences  of  eruption, 
breaking  through  the  Keewatin  rocks,  including  the  upper  Vermilion 
fragmental  rocks  of  Ogishki  Lake  with  their  associated  slates  and 
grits.  It  is  concluded  that  the  break  between  the  upper  and  lower 
Vermilion,  described  by  Van  Hise,  is  within  the  Keewatin  group, 
dividing  it  into  an  upper  and  a  lower  series,  and  that  this  break  is 
therefore  below  the  Animikie.  It  is  further  said  that  the  conglom- 
erates of  the  upper  Kaministiquia  series  come  out  close  to  the  shores 
of  Thunder  Bay  and  form  the  basement  upon  which  the  undisturbed 
Animikie  rocks  rest  with  strongly  marked  unconformity.  The  fol- 
lowing succession  for  the  region  northwest  of  Lake  Superior  is  pre- 
sented :  Keweenawan  or  Nipigon  group ;  unconformity ;  Animikie 
group  (possibly  Huronian)  ;'  unconformity;  upper  Keewatin  series; 
unconformity;  lower  Keewatin  series;  unconformity  (?);  Coutchi- 
ehing  group;  eruptive  unconformity;  Laurentian  system,  the  granites 
and  gneisses  of  which  cut  both  Keewatin  and  Coutchiching  groups. 
Smyth  (PI.  L.),256  in  1891,  describes  the  structural  geology  of 
Steep  Rock  Lake,  Ontario.  The  lake  is  roughly  in  the  shape  of  a 
letter  M,  the  top  is  to  the  north,  and  its  arms  conform  to  the  strike  of 
the  rock  series.  The  rocks  are  divisible  into  three  principal  groups. 
The  lower  consists  of  granites  and  gneisses,  and  is  designated  the 
Basement  Complex.  Besting  upon  the  Basement  Complex  is  a  series 
of  rocks  about  5,000  feet  in  thickness,  composed  of  nine  persistent  for- 
*  55721— Bull.  300—09 1 8 
