300  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
pean  rocks  onty  are  left.     The  age  and  relations  of  the  sandstones  on 
Lake  Superior  to  the  trappean  rocks  and  Azoic  slates  are  again  de- 
scribed as  before. 
Schoolcraft,311  in  1855,  states  that  the  granite  strata  of  the 
Thousand  Isles  reappear  on  the  north  shore  of  Lakes  Huron  and 
Superior,  underlie  the  bed  of  the  latter  and  are  found  on  the  rough 
coast  between  Chocolate  River  and  Keweenaw,  and  cross  the  Mis- 
sissippi near  the  Falls  of  St.  Anthony.  The  Strait  of  St.  Marys  ap- 
pears to  be  the  ancient  line  of  junction  between  the  great  calcareous 
and  granitic  series  of  rocks  on  the  continent.  The  island  of  St. 
Joseph  is  chiefly  Primitive  rocks,  and  at  its  south  end  is  largely 
loaded  with  granitic,  porphyritic,  and  quartzitic  bowlders.  The  north 
shore  of  the  river,  opposite  the  island,  is  entirely  of  the  granite 
series,  which  continues  to  Gros  Cap  on  Lake  Superior.  The  red 
sand  rock  of  Lake  Superior  is  regarded  as  the  Old  Red  sandstone. 
The  formation  of  red  jasper  in  white  quartz  exists  on  the  southern 
foot  of  Sugar  Island.  In  the  granitic  conglomerates  are  seen  red 
feldspathic  granite,  black  shining  hornblende  rock,  white  fatty 
quartz,  and  striped  jasper,  all  held  together  firmly.  Volcanic  action 
appears  to  have  thrown  up  the  trap  rocks  of  the  Pic,  of  the  Porcu- 
pine chain,  of  Isle  Royal,  and  of  the  long  peninsula  of  Keweenaw. 
The  sandstone  of  the  southeastern  coast  exhibits  undulations  of  8° 
or  10°  at  several  places.  Two  instances  of  this  are  the  Point  des 
Grands  Sables,  beginning  with  the  horizontal  strata  of  the  Pictured 
Rocks,  and  at  Grand  Island. 
Whitney,312  in  1856,  describes  the  northeastern  side  of  Lake  Su- 
perior, from  Gros  Cap  to  Nipigon  Bay,  as  consisting  of  rocks  of  the 
Azoic  system.  On  the  south  shore  of  the  lake,  and  along  the  north- 
west shore  as  far  as  the  northeastern  extremity  of  Nipigon  Bay,  are 
found  the  shales,  sandstones,  conglomerates,  and  trappean  rocks  of 
the  Potsdam  system,  except  at  Thunder  Bay  and  Carp  River,  where 
the  Azoic  appears.  The  south  side  of  the  Azoic  on  the  north  side  of 
the  lake  runs  from  Kakabikka  Falls  on  the  Kaministiquia  in  an 
almost  straight  line  southwest,  keeping  a  few  miles  from  the  lake. 
Thunder  Cape  consists  of  thinly  bedded  slates  for  800  feet  of  its 
thickness,  above  which  is  a  sheet  of  trappean  rock  200  or  300  feet 
thick. 
Whitney,313  in  1856,  maintains  that  the  iron  ores  of  Lake  Superior, 
Scandinavia,  Missouri,  and  northern  New  York  form  a  class  by  them- 
selves belonging  to  the  Azoic  age,  and  that  they  have  been  poured  out 
like  other  igneous  rocks  from  the  interior  in  a  molten  or  plastic  state. 
Besides  the  purest  ores  are  others  interlaminated  with  bands  of  quartz 
which  are  distinctly  bedded  and  probably  of  sedimentary  origin. 
The  iron  ore  in  these  may  have  been  introduced  either  by  sublimation 
during  the  deposition  of  the  siliceous  particles  or  by  precipitation 
