70  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [butj,^. 
rior,  the  base  of  which  is  60 miles  in  length  and  the  arms  40  and  80  miles, 
respectively. 
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  trappean 
beds  and  masses,  a  considerable  part  of  which  is  amygdaloidal.  The 
Mpigon  formation  occupies  a  great  area  about  Nipigon  lake  and  con- 
siderable areas  at  the  east  end  of  lake  Superior  and  on  Michipicoten 
island. 
On  the  east  side  of  Hudson  bay  and  on  the  islands  off  the  coast  vol- 
canic and  sedimentary  rocks  are  largely  developed,  comprising  con- 
glomerates, sandstones,  limestones,  chert  breccias,  shales,  quartzites, 
argillites,  porphyries,  crystalline  traps,  amygdaloids,  tufas,  etc.  The 
upper  part  of  these  may  correspond  to  the  Mpigon  and  the  lower  to 
the  Animikie. 
The  sandstones  of  Sault  Ste.  Marie,  of  the  peninsula  between  Goulais 
and  Batchawana  bays,  isle  Parisienne,  etc.,  seem  to  be  of  Potsdam 
age.  These  sandstones  are  mostly  red,  but,  unlike  the  Mpigon  forma- 
tion, they  appear  to  be  free  from  local  disturbance  and  lie  almost  flat. 
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. 
Lawson,40  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  con- 
cluded that  the  break  between  the  upper  and  lower  Vermilion,  de- 
scribed by  Van  Hise,  is  within  the  Keewatin  group,  dividing  it  into  an 
upper  and  lower  series,  and  that  this  break  is  therefore  below  the 
Animikie.  It  is  further  said  that  the  conglomerates  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  following  succession  for  the  region 
northwest  of  lake  Superior  is  presented:  Keweenawan  or  Nipigon 
group;  unconformity 5  Animikie  group  (possibly  Huronian);  uncon- 
formity; upper  Keewatin  series,  unconformity;  lower  Keewatin 
series;  unconformity  (?) ;  Coutchiching  group;  eruptive  unconformity; 
Laurentian  system,  the  granites  and  gneisses  of  which  cut  both  Kee- 
watin and  Coutchiching  groups. 
Smyth  (H.  L.),41  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  as  the  basement  complex. 
Resting  upon  the  basement  complex  is  a  series  of  rocks  about  5,000  feet 
in  thickness,  composed  of  nine  persistent  formations,  which  together 
