256  PRE-CAMBRIAN    GEOLOGY    OF    NOKTH    AMERICA. 
Bigsby,223  in  1852,  divides  the  rocks  about  Lake  Superior  into  (1) 
metamorphic,  including  greenstone,  slates,  schists,  gneisses,  quartzites, 
jaspers,  and  crystalline  limestones;  (2)  aqueous,  including  the  cal- 
ciferous  and  Cambrian  sandstones  and  conglomerates;  (3)  igneous, 
including  granite,  syenite,  and  trap.  The  lake  is  a  trough  or  basin  of 
Cambrian  or  Silurian  sandstone,  surrounded  by  two  irregular  and  im- 
perfect zones,  the  inner  consisting  of  traps  with  conglomerates,  the 
outer,  of  metamorphic,  flanked  by  igneous  rocks.  The  metamorphic 
rocks,  with  the  exception  of  quartzite  aiid  jasper,  the  oldest  in  the 
lake,  support  unconformably  the  sandstone.  These  rocks  have  been 
upheaved  and  altered  by  the  intrusion  of  igneous  rocks.  The  sand- 
stone is  generally  horizontal,  except  near  the  intrusive  rocks,  where  it 
rises  at  high  angles  and  passes  into  jasper,  porphyry,  gneiss,  or 
quartzite.  The  conglomerate  is  of  the  same  age  as  much  of  the  sand- 
stone, and  is  between  it  and  the  trap;  and  there  is  reason  to  believe 
that  the  sandstone  is  interleaved  with  trap.  The  igneous  rock,  gran- 
ite, everywhere  forms  the  nucleus  of  the  anticlinal  axes.  The  trap 
rocks  are  divided  into  crystalline  mountain  masses — sometimes  anti- 
clinal and  syenitic,  into  bedded  traps,  and  into  dikes  intersecting 
igneous  and  metamorphic  rocks ;  but  all  are  portions  of  one  long  series 
of  volcanic  operations. 
Bigsby,224  in  1854,  finds  the  geology  of  Rainy  Lake  to  be  as  follows : 
Chloritic  and  greenstone  slates,  gneiss,  and  mica  slate,  in  proportional 
quantities  in  the  order  here  set  down,  seem  once  to  have  occupied  the 
lake  basin,  with  an  east-northeast  strike,  and  a  north-northwest  dip, 
at  a  high  angle  usually.  But  subsequently  a  very  extensive  outburst 
of  granite,  with  some  syenite,  has  taken  place,  to  the  great  disturbance 
of  the  stratified  rocks,  and  penetrating  them  both  in  intercalations 
and  crosswise.  These  intrusive  rocks  occupy  a  very  large  portion  of 
the  lake,  most  of  the  western  shore,  nearly  all  of  the  eastern  trough  or 
arm,  and  much  of  the  east  end  of  the  lake  about  Stokes  and  Hale  bays. 
Intercalations  of  syenite  and  hornblende  greenstone  are  numerous, 
and  so  are  veins  of  porphyritic  granite  traversing  the  gneiss  in  all 
directions.  The  chlorite  slate,  greenstone,  gneiss,  and  mica  slate  are 
conformable  with  one  another. 
Dawson  (Sir  William),225  in  1857,  finds  between  Sault  Ste.  Marie 
and  Mamainse  three  of  the  oldest  formations  in  America  or  the  world. 
Beginning  at  the  top  are :  (X)  Potsdam  sandstone ;  (2)  an  enormously 
thick  formation  of  conglomerates,  sandstones,  slate,  and  trap,  consti- 
tuting the  Huronian  series  of  Logan;  (3)  a  still  older  Laurentian 
series,  represented  here  principally  by  syenitic  rocks  which  have  af- 
forded the  material  of  the  Huronian  conglomerates.  The  sandstones 
and  conglomerates  of  the  second  series  probably  unconformably 
underlie  the  Potsdam  sandstone,  as  is  indicated  by  their  high  inclina- 
tion and  disturbed  condition. 
