vanhise.]  LAKE    SUPERIOR    REGION.  55 
northeast  coast  of  lake  Superior,  and  a  narrow  strip  of  Huronian  rocks 
are  seen  along  Thunder  bay. 
The  Laurentian  gneiss  is  succeeded  by  green  or  gray  slates,  which  at 
the  base  appear  to  be  mterstratified  with  feldspathic  beds  of  the  red- 
dish color  belonging  to  the  subjacent  gneiss.  Rising  in  the  series  the 
dark  green  slates  become  inter  stratified  with  layers  holding  a  sufficient 
number  of  pebbles  of  different  kinds  to  constitute  conglomerates.  It 
often  happens,  unless  the  pebbles  are  of  white  quartz,  that  they  arc 
very  obscurely  distinguishable  on  fracturing  the  rock,  both  the  pebbles 
and  the  matrix  having  a  gray  color,  and  showing  very  little  apparent 
difference  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  being  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  mterstratified  with  feldspathic  layers.  The  Lau- 
rentian appears  to  be  conformable  with  and  to  grade  into  the  Huronian 
on  the  Kaministiquia  river. 
The  Huronian  formation  of  lake  Suj)erior  is  unconformably  overlain 
by  a  second  series  of  copper-bearing  rocks,  which  may  conveniently  be 
divided  into  two  groups,  the  lower  consisting  of  bluish  slates  or  shales, 
interstratified  with  chert  beds,  sandstones  and  trap,  and  the  upper  con- 
sisting of  a  succession  of  sandstones,  limestones,  marls,  and  conglom- 
erates, also  interstratified  with  trap  which  is  often  amygdaloidal.  At 
the  top  of  the  lower  group  is  a  crowning  overflow  of  trap  200  or  300 
feet  thick  and  the  whole  of  it  has  a  thickness  of  1,500  or  2,000  feet. 
The  upper  group  contains  a  great  quantity  of  trap  layers  and  has  an 
enormous  trappean  overflow,  the  total  volume  having  a  thickness  of 
between  6,000  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  Mpigon  bay.  It  also  covers  a  large  part  of  isle  Royale  and 
Michipicoten,  at  which  latter  island  the  total  volume  of  the  formation 
is  at  the  most  moderate  estimation  12,000  feet.  Upon  the  east  coast  of 
lake  Superior  this  group  is  found  at  cape  Choyye,  cape  Gargantua, 
point  aux  Mines,  Mamainse,  where  the  breadth  across  the  measures  is 
sufficient  to  give  a  thickness  not  far  from  10,000,  and  at  three  other 
places  on  the  coast  between  the  last  point  and  Sault  Ste.  Marie. 
As  to  the  age  of  this  series,  the  fact  that  the  generally  moderate  dips  ot 
the  red  sandstone  at  Sault  Ste.  Marie  contrasts  with  the  higher  inclina- 
tions of  the  copper-bearing  rocks,  while  none  of  the  many  dikes  are 
known  to  intersect  the  Sault  Ste.  Marie  sandstone,  leads  to  the  suspicion 
that  the  latter  may  overlie  unconformably  the  rocks  which,  associated 
