vanhisk.]  LAKE    SUPERIOR    REGION.  53 
are  all  apparently  from  hypogene  rocks.  Formations  4  and  5  rest  un- 
conformably  upon  1,  2,  and  3,  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  Temiscamang.  The  suc- 
cession 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  elastics  is  not  less  than  10,000  or  12,000  feet. 
Group  5  exhibits  an  unconformity  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,5  in  1847,  in  an  account  of  the  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  gneiss- 
oid  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,6  in  1852,  finds  the  rocks  of  the  north  shore  of  lake  Superior 
to  have  the  following  succession:  Granite,  syenite,  and  gneiss,  or  mi- 
caceous 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,  conglomerates, 
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  sand- 
stone, 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,7  in  1852,  finds  the  crystalline  strata  of  the  lake  of  the  Woods 
to  conform  in  strike  with  those  several  hundred  miles  southward  on 
the  river  Mississippi.  Granite  occupies  the  axis  in  the  northeast  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.  Green- 
stone 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  in- 
cludes greenstone  in  a  thousand  tortuous  masses,  tongues,  and  slender 
veins.    Also  the  granite  is  cut  by  greenstone. 
Bigsby,8  in  1852,  divides  the  rocks  about  lake  Superior  into  (1)  Meta- 
morphic,  including  greenstone,   slates,  schists,   gneisses,   quartz  it  es, 
