LAKE    SUPERIOR   REGION.  303 
tude.  Of  the  original  acidic  rocks  true  granite  has  been  observed  only 
in  the  Bad  River  region  of  Wisconsin,  intersecting  the  coarse  gabbro 
at  the  base  of  the  series.  Quartz  porphyry  and  allied  acidic  rocks 
have  a  widespread  occurrence,  two  of  the  largest  masses  being  the 
Palisades  of  Minnesota  and  the  core  of  the  Porcupine  Mountains  of 
Michigan.  The  detrital  members  have  often  a  great  extent.  The 
outer  conglomerate  of  Keweenaw  Point,  for  instance,  is  traced  from 
the  eastern  extremity  of  the  point  to  Bad  River  in  Wisconsin,  a  dis- 
tance of  at  least  170  miles,  although  its  thickness  in  this  distance 
varies  from  less  than  100  to  as  much  as  4,000  feet.  Thinner  con- 
glomerates have  been  traced  for  as  great  a  distance  as  50  miles. 
The  Keweenaw  series  is  stratigraphically  separated  into  two  grand 
divisions — an  upper  member,  made  wholly  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 
conglomerate  and  sandstone  nearly  to  the  base,  and  also  original 
acidic  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  character- 
istics of  the  lower  division  are:  First,  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  characteristic  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  acidic 
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  coarse 
gabbros  of  Bad  River,  at  the  base  of  the  series,  present  the  appearance 
of  unconformity.  Lying  immediately  upon  the  Huronian  slates,  they 
taper  out  rapidly  at  both  ends  and  seem  to  lie  right  in  the  course  of 
the  diabase  belts  to  the  east  and  the  west.  This  coarse  gabbro  must 
have  stood  up  to  a  great  height  and  the  later  flows  terminated  against 
it  until  they  had  accumulated  sufficiently  to  overflow  its  upper  sur- 
face, but  possibly  they  represent  the  slowly  solidified  and  subse- 
quently denuded  reservoirs  from  which  the  later  flows  were  in  part 
derived.  The  great  extent  of  coarse  gabbros  of  Minnesota  seems  to 
sustain  somewhat  the  same  relations  to  the  more  regularly  bedded 
portions  of  the  series.  Because  of  these  relations  the  question  arose 
whether  these  gabbros  do  not  belong  with  the  Huronian,  but  they 
appear  to  cut  across  the  Huronian  at  a  small  angle  and  thus  to  be 
unconformable  with  this  series;  hence,  because  of  the  relationship 
lithologically  and  because  sudden  breaks  in  eruptive,  series  have  not 
