162  PRE-CAMBRIAN    GEOLOGY    OF    NORTH    AMERICA. 
tains  present  phenomena  which  indicate  their  eruption  subsequent  to 
the  formation  of  the  main  deposits  of  the  region,  although  the  ques- 
tion is  as  yet  open.  Subsequent  to  the  deposition  of  the  Keweenawan 
a  fissure  was  formed  near  the  contact  of  the  Eastern  sandstone  and 
the  lavas,  along  which  normal  faulting  occurred,  the  copper-bearing 
series  forming  the  overhanging  side  of  the  fault.  As  to  the  nature  of 
the  displacement,  however,  more  evidence  is  needed.  The  copper  oc- 
curs in  fissure  veins,  in  melaphyres,  and  in  conglomerates. 
Boss,78  in  1898,  describes  the  dikes  associated  with  the  ore  deposits 
of  the  Gogebic  iron  range.  They  are  dioritic  and  more  or  less  altered ; 
they  are  approximately  at  right  angles  with  the  dip  of  the  formation 
they  cut,  and  the  greater  number  of  them  have  an  average  easterly  dip 
of  15°  to  18° ;  sometimes  they  are  folded  in  such  a  manner  as  to  form 
long  synclinal  basins  with  eastward  pitch.  In  a  majority  of  cases 
mining  exploitation  has  shown  a  succession  of  dikes,  one  below  the 
other,  and  ferruginous  quartzite  of  varying  thickness  immediately 
underlying  each  dike  and  forming  the  cap  of  the  succeeding  deposit 
of  ore. 
Lane,79  in  1898,  gives  a  detailed  account  of  the  geology  and  petrog- 
raphy of  Isle  Royal,  Lake  Superior.  The  island  trends  north  of  east 
and  the  edges  of  the  strata  outcrop  in  approximately  this  direction. 
The  rocks  are  interbedded  conglomerates,  sandstones,  and  traps  of 
Keweenawan  age,  dipping  in  a  southerly  direction  at  angles  varying 
from  8°  to  32°,  the  higher  dips  in  general  to  the  north.  Faults  are 
shown  to  exist  at  various  places  with  directions  approximately  north- 
east-southwest and  northwest-southeast,  and  the  probable  existence  of 
other  more  extensive  faults  running  entirely  across  the  island  is  indi- 
cated. Detailed  sections,  with  correlations  with  the  Keweenawan  of 
other  parts  of  Lake  Superior,  are  given. 
Hubbard,80  in  1898,  discusses  the  geology  of  Keweenaw  Point  with 
particular  reference  to  the  felsites  and  their  associated  rocks.  The 
term  felsite  is  used  to  include  all  the  very  fine-grained  and  highly 
acidic  igneous  rocks.  These  occur  at  a  number  of  horizons  below  the 
Bohemian  conglomerate,  so  called  from  the  fact  that  it  skirts  the 
northern  side  of  the  Bohemian  range  near  the  northeast  end  of  Ke- 
weenaw Point.  The  outcrops  of  felsite  studied  occur  in  sec.  30,  T 
58,  R.  27  (New  England  or  Keystone  location)  ;  sec.  25  ( ?),  sec.  35 
(Fish  Cove),  sees.  26  and  27  (Little  Montreal  River),  all  in  T.  58,  R. 
28;  sees.  29  and  30,  T.  58,  R.  28  (Bare  Hill  and  westward  therefrom)  ; 
sees.  23  and  24,  T.  58,  R,  29  (Mount  Houghton)  and  both  eastward 
and  westward  therefrom;  sec.  10,  T.  57,  R.  31  (Suffolk  location, 
Praysville)  ;  sec.  4,  T.  5G,  R.  32  (Allouez  Gap,  east  of  the  Kearsarge 
and  Wolverine  mines)  ;  sec.  30,  T.  56,  R.  32  (falls  on  branch  of  Trap 
Rock  River)  ;  sec.  36,  T.  56,  R.  33  (Douglass  Houghton  Falls),  and 
sec.  1,  T.  55,  R.  33  (Hecla  and  Torch  Lake  Railroad). 
