54  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
jaspers,  and  crystalline  limestones;  (2)  aqueous,  including  the  ealcifer- 
ous  and  Cambrian  sandstones  and  conglomerates;  (3)  igneous,  includ- 
ing granite,  syenite,  and  trap.  The  lake  is  a  trough  or  basin  of  Cam- 
brian or  Silurian  sandstone,  surrounded  by  two  irregular  and  imperfect 
zones,  the  inner  consisting  of  traps  with  conglomerates,  the  outer-,  of 
metamorphic,  flanked  by  igneous  rocks.  The  metamorphic  rocks,  with 
the  excei> tion  of  quartzite  and  jasper,  the  oldest  in  the  lake,  support 
unconformably  the  sandstone.  These  rocks  have  been  upheaved  and 
altered  by  the  intrusion  of  igneous  rocks.  The  sandstone  is  generally 
horizontal,  except  near  the  intrusive  rocks,  where  it  rises  at  high 
angles  and  passes  into  jasper,  porphyry,  gneiss,  or  quartzite.  The  con- 
glomerate is  of  the  same  age  as  much  of  the  sandstone,  and  is  between 
it  and  the  trap;  and  there  is  reason  to  believe  that  the  sandstone  is 
interleaved  with  trap.  The  igneous  rock,  granite,  everywhere  forms 
the  nucleus  of  the  anticlinal  axes.  The  trap  rocks  are  divided  into 
crystalline  mountain  masses— sometimes  anticlinal  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,9  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  ENE.  strike,  and  a  NNW.  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  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  frequent,  and  so  is  the  occur- 
rence of  veins  of  porphyritic  granite  traversing  the  gneiss  in  all  direc- 
tions. The  chlorite- slate  greenstone,  gneiss,  and  mica-slate  are  con- 
formable with  each  other. 
Dawson  (Sir  William),10  in  1857,  finds  between  Sault  Ste.  Marie 
and  Mamainse  three  of  the  oldest  formations  in  America  or  the  world. 
Beginning  at  the  top  are:  (1)  Potsdam  sandstone;  (2)  an  enormously 
thick  formation  of  conglomerates,  sandstones,  slate,  and  trap,  consti- 
tuting the  Huronian  series  of  Logan;  (3)  a  still  older  Lauren tian  series, 
represented  here  principally  by  syenitic  rocks  which  have  afforded  the 
material  of  the  Huronian  conglomerates.  The  sandstones  and  con- 
glomerates of  the  second  series  probably  unconformably  underlie  the 
Potsdam  sandstone,  as  is  indicated  by  their  high  inclination  and  dis- 
turbed condition. 
Logan,11  in  18G3,  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 
