LAKE    SUPERIOR   REGION.  257 
Logan,226  in  1863,  finds  on  the  north  shore  of  Lake  Superior  crystal- 
line stratified  rocks  which  occur  in  extensive  tracts  about  Rainy  Lake 
and  Lac  la  Croix,  as  well  as  adjacent  to  Lake  Superior,  which  are 
probably  of  Laurentian  age.  There  are  three  areas  of  Huronian  along 
the  northeast  coast  of  Lake  Superior,  and  a  narrow  strip  of  Huronian 
rocks  is  seen  along  Thunder  Bay. 
The  Laurentian  gneiss  is  succeeded  b}^  green  or  gray  slates,  which 
at  the  base  appear  to  be  interstratified  with  feldspathic  beds  of  the 
reddish  color  belonging  to  the  subjacent  gneiss.  Higher  in  the  series 
the  dark-green  slates  become  interstratified  with  layers  holding  a 
sufficient  number  of  pebbles  of  different  kinds  to  constitute  conglom- 
erates. Often  the  pebbles,  unless  they  are  of  white  quartz,  are  very 
obscurely  distinguishable  on  fractured  surfaces,  both  the  pebbles  and 
the  matrix  having  a  gray  color  and  showing  very  little  apparent  dif- 
ference in  mineral  character. 
The  Dore  section,  composed  of  strata  inclined  only  10°  or  15°  from 
the  vertical,  1,700  feet  thick,  consists  mainly  of  green  slate  rock  and  a 
green  slate  conglomerate  the  pebbles  of  which  are  of  granite,  gneiss, 
syenite,  etc.,  and  some  of  the  bowlders  of  which  are  a  foot  in  diameter. 
At  the  Dore  the  lower  part  of  the  section  assumes  more  the  character 
of  a  gneiss  and  becomes  interstratified  with  feldspathic  layers.  The 
Laurentian  appears  to  be  conformable  with  and  to  grade  into  the 
Huronian  on  Kaministiquia  River. 
The  Huronian  formation  of  Lake  Superior  is  unconformably  over- 
lain by  a  second  series  of  copper-bearing  rocks,  which  may  conven- 
iently be  divided  into  two  groups,  the  lower  consisting  of  bluish  slates 
or  shales,  interstratified  with  chert  beds,  sandstones,  and  trap,  and 
the  upper  consisting  of  a  succession  of  sandstones,  limestones,  mails, 
and  conglomerates,  also  interstratified  with  trap,  which  is  often 
amygdaloidal.  At  the  top  of  the  lower  group  is  a  crowning  over- 
flow of  trap  200  or  300  feet  thick,  and  the  whole  of  it  has  a  thickness 
of  1,500  or  2,000  feet.  The  upjjer  group  contains  a  great  quantity  of 
trap  layers  and  has  an  enormous  trappean  overflow,  the  total  volume 
having  a  thickness  of  between  0,000  and  10,000  feet.  Dike  rocks, 
consisting  of  greenstone,  porphyry,  and  syenite,  are  found  to  be  of 
two  ages.  The  lower  group  composes  the  whole  country,  both  islands 
and  mainland,  between  Pigeon  River  and  Fort  William.  The  upper 
group  occupies  the  coast  and  islands  from  the  south  side  of  Thunder 
Cape  to  the  east  end  of  Battle  Islands,  east  of  Nipigon  Bay.  It  also 
covers  a  large  part  of  Isle  Royal  and  Michipicoten,  at  which  latter 
island  the  total  volume  of  the  formation  is  at  the  most  moderate  esti- 
mation 12,000  feet.  On  the  cast  coast  of  Lake  Superior  this  group  is 
found  at  Cape  Choyye,  Cape  Gargantua,  Point  aux  Mines,  Maniainse, 
where  the  breadth  across  the  measures  is  sufficient  to  give  a  thickness 
55721— Hull.  :5(K)— 00 IT 
