136  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  Fbvll.86. 
The  Keweenaw  series  is  stratigraphically  separated  into  two  grand 
divisions,  an  upper  member,  made  Nvkolly  of  detrital  material,  for  the 
most  part  red  sandstones  and  shale ;  and  a  lower  division,  made  chiefly 
of  a  succession  of  flows  of  basic  rocks,  but  including  layers  of  conglom- 
erate and  sandstone  nearly  to  the  base,  and  also  original  acid  rocks. 
The  line  of  separation  between  the  two  divisions  is  somewhat  arbitrary, 
for  the  sandstone  gradually  increases  in  quantity  upward,  but  above 
the  highest  known  eruptive  member  is  a  maximum  thickness  of  15,000 
feet  of  detrital  material.  The  chief  characteristics  of  the  lower  division 
are,  first,  that  coarse  grained  basic  rocks  in  very  heavy  beds  are  much 
more  common  at  lower  horizons;  second,  amygdaloidal  texture  is  more 
frequent  and  highly  developed  at  high  horizons,  this  being  more  charac- 
teristic of  the  thinner  beds;  third,  the  gabbros  are  more  often  found  at 
lower  horizons  while  the  ordinary  diabases  and  melaphyres  affect  higher 
horizons;  fourth,  the  acid  rocks  are  found  especially  in  low  horizons, 
rarely  reaching  above  the  middle  of  the  lower  division;  and,  fifth,  the 
detrital  beds,  although  seen  all  the  way  to  the  base,  are  rare  in  the 
lower  third  of  the  series  and  increase  in  thickness  and  frequency  toward 
the  top.  The  thickness  of  the  lower  division  is  placed  in  round  num- 
bers at  25,000  to  30,000  feet,  while  at  the  Montreal  river  its  apparent 
thickness  is  33,000  to  35,000  feet,  but  a  part  of  this  may  be  due  to  the 
westward  continuation  of  the  Keweenaw  fault. 
Detailed  descriptions  are  given  of  the  rocks  of  Keweenaw  point,  of 
the  region  between  Portage  lake  and  the  Ontonagon  river,  of  the  South 
rauge,  of  the  region  between  the  Ontonagon  river  and  Numakagon  lake 
including  the  Porcupine  mountains,  of  northwestern  Wisconsin  and  the 
adjoining  part  of  Minnesota,  of  the  Minnesota  coast,  of  isle  Eoyale  and 
Mpigon  bay,  of  Michipicoten  island  and  the  east  coast  of  lake  Superior. 
Silver  mountain,  belonging  to  the  South  range,  is  composed  of  diabase, 
dipping  at  an  angle  of  30°,  and  appears  to  be  surrounded  by  horizontal 
sandstone.  On  the  west  branch  of  the  Ontonagon  are  found  cliff's  of 
horizontal  sandstone  almost  in  proximity  with  ferruginous  slate  supposed 
to  belong  to  the  Huronian,  and  but  a  short  distance  from  diabases  re- 
garded as  Keweenawan.  The  isolated  position  of  the  South  range  is 
regarded  as  due  to  a  fault,  as  there  is  no  evidence  whatever  of  a  fold, 
and  to  regard  this  part  of  the  series  as  a  continuous  conformable 
succession  with  the  Keweenawan  rocks  to  the  north  would  give  the 
series  an  incredible  thickness.  The  Porcupine  mountains  are  found 
to  be  due  to  a  subordinate  fold  in  the  series,  the  core  being  a  quartz- 
porphyry. 
All  the  known  facts  with  reference  to  the  relations  of  the  horizontal 
sandstone  to  the  Copper-bearing  rocks  of  northwestern  Wisconsin  and 
the  adjoining  part  of  Minnesota  are  recapitulated.  The  unconformity 
between  the  fossiliferous  Cambrian  of  the  St.  Croix  valley  and  the 
bedded  melaphyre  and  amygdaloids  described  by  Sweet,  Strong,  and 
Chamberlin  is  indisputable,  and  the  latter  rocks  are  identical  in  nature 
