LAKE    SUPERIOR   REGION.  255 
having  sometimes  basal  conglomerates,  the  pebbles  being  of  quartz, 
red  jasper,  and  slate.  The  upper  part  of  3  strongly  resembles  the 
upper  slates  at  Lake  Temiscaming.  The  succession  of  conglomerate 
and  pebbly  slates  at  Gros  Cap  has  a  total  thickness  of  1,700  feet.  In 
group  5  are  the  succession  of  rocks  at  Michipicoten  Island,  a  portion 
of  those  at  Thunder  Bay,  and  at  other  localities.  At  Michipicoten 
the  thickness  of  the  interbedded  volcanics  and  water-deposited  elas- 
tics is  not  less  than  10,000  or  12,000  feet.  Group  5  exhibits  an  uncon- 
formity to  the  granite.  The  conclusion  that  the  Copper-bearing  series 
is  older  than  the  Potsdam  sandstone,  arrived  at  by  Houghton  in  1841, 
is  thought  to  be  probably  true. 
Murray,-'20  in  1847,  in  an  account  of  Kaministiquia  and  Michipi- 
coten rivers,  divides  the  geological  formation  into  three  groups:  (1) 
Granite,  syenite,  gneiss,  micaceous  and  chloritic  schist;  (2)  blackish 
argillaceous  slates,  with  associated  trap;  (3)  drift  clays  and  sands. 
At  the  portage  of  the  Kaministiquia  a  massive  syenite  passes  into  a 
gneissoid  syenite,  upon  which  rest  conformably  dark-colored  altered 
slates,  one  rock  passing  imperceptibly  into  the  other.  The  junction  of 
1  and  2  was  not  observed. 
Logan,221  in  1852,  finds  the  rocks  of  the  north  shore  of  Lake  Supe- 
rior to  have  the  following  succession :  Granite,  syenite,  and  gneiss,  or 
micaceous  and  hornblende  gneiss,  which  are  succeeded  by  chloritic  and 
talcose  slates,  interstratified  with  obscure  conglomerates  with  a  slaty 
base.  Upon  these  rest  unconformably  bluish  slates,  with  belts  of 
chert  and  limestone  toward  the  bottom,  and  thick  flows  of  greenstone 
trap  at  the  top.  Above  these  are  alternations  of  sandstones,  con- 
glomerates, amygdaloids,  and  traps,  the  whole  thickness  of  the  upper 
series  above  the  unconformity  being  not  less  than  12,000  feet.  The 
conclusion  is  reached  that  this  upper  series  is  the  equivalent  of  the 
Potsdam  sandstone,  which  rests  unconformably  upon  the  tilted  beds 
of  the  Lake  Huron  series,  and  that  both  are  contemporaneous  with 
the  Cambrian  series  of  the  British  Isles. 
Bigsby,222  in  1852,  finds  the  crystalline  strata  of  the  Lake  of  the 
Woods  to  conform-  in  strike  with  those  several  hundred  miles  south- 
ward on  the  river  Mississippi.  Granite  occupies  the  axis  in  the 
northeastern  part  of  the  Lake  of  the  Woods,  and  is  always  the  lowest 
rock,  the  gneiss,  mica  schist,  and  greenstone  dipping  away  from  it 
on  both  sides.  Greenstone  is  perhaps  the  most  abundant  rock  in 
this  part  of  the  lake,  and  greenstone  conglomerates  are  found  which 
contain  black  masses  of  greenstone  lying  with  their  greater  length 
parallel  'to  the  strike.  The  granites  and  mica  slates  are  intimately 
associated,  and  the  granite  includes  greenstone  in  a  thousand  tortuous 
masses,  tongues,  and  slender  veins.  Also  the  granite  is  cut  by 
greenstone. 
