vanhise.]  LAKE    SUPERIOR    REGION.  135 
rial  as  porphyry,  both  lion-quartziferous  and  quartziferous,  felsite, 
augite-syenite,  granitell  and  granite,  but  there  are  also  often  ton  ml 
pebbles  of  the  basic  rocks,  and  in  some  cases  particles  of  gneiss  and 
granite  from  the  underlying  series.  This  is  thought  to  be  due  to  the 
fact  that  such  viscous  material  would  solidify  into  more  or  less  bulky 
erect  masses  of  relatively  small  area,  and  thus  be  most  favorably  situ- 
ated for  degradation.  Between  the  several  kinds  of  original  rocks  there 
are  no  sharp  lines,  but  a  continuous  series  of  kinds  from  the  most  basic 
to  the  most  acid, 
The  lithology  of  the  different  members  of  the  series  is  given  in  detail. 
The  basic  original  rocks  include  granular,  porphyritic,  and  glassy  kinds, 
the  most  abundant  of  which  are  gabbro,  diabase,  melaphyre,  and  por- 
phyrite.  The  acid  original  rocks  include  quartzless  porphyry,  quartz- 
iferous porphyry,  and  felsite,  augite-syenite,  granite-porphyry,  and 
granite.  Here  is  included  the  so-called  "jaspers,"  which  have  been  re- 
garded by  many  as  metamorphosed  sedimentary  rocks.  The  basic  crys- 
talline rocks  make  up  the  greater  part  of  the  thickness  of  the  series, 
the  beds  varying  from  a  few  feet  to  several  hundred  feet  in  thickness. 
Each  of  these  beds  often  has  a  twofold  division,  an  upper  amygdaloidal 
portion,  and  a  lower  compact  portion,  which,  however,  grade  into  each 
other.  The  amygdaloids  not  infrequently  resemble  beds  of  sedimentary 
origin,  but  they  never  show  any  trace  of  fragmental  character,  and 
the  stratiform  condition  is  seen  to  be  due  to  a  succession  of  thin  flows 
and  two  fluidal  structures.  Laterally  the  beds  are  not  of  indefinite  ex. 
tent,  and  are  far  less  extensive  than  sedimentary  beds  of  the  same 
thickness.  It  is  generally,  however,  difficult  to  prove  the  continuity 
or  noncontinuity  of  a  single  flow  over  a  great  distance,  but  on  the  Min- 
nesota shore  individual  layers  were  traced  with  certainty  for  10  or  15 
miles,  while  other  beds  almost  certainly  have  an  extent  of  nearly  30 
miles,  while  groups  of  layers  of  allied  characters  are  recognizable  over 
much  longer  stretches.  The  more  massive,  thicker  beds  generally  occur 
in  the  lower  part  of  the  series.  Numerous  dikes  cut  the  basic  rocks. 
These  are  generally  small,  commonly  not  more  than  10  feet  in  width, 
but  in  the  immediately  underlying  series  on  the  north  shore,  the  Lower 
Copper-bearing  or  Animikie  group,  are  dikes  of  great  magnitude.  Of 
the  original  acid  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  acid  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  <>r  Kewee- 
naw point,  for  instance,  is  traced  from  the  eastern  extremity  of  the 
point  to  the  Bad  river  in  Wisconsin,  a  distance  of  at  least  170  miles, 
although  its  thickness  in  this  distance  varies  from  less  than  100  to  as 
much  as  4,000  feet.  Thinner  conglomerates  have  been  traced  for  as 
great  a  distance  as  50  miles. 
