110  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1905. 
The  Keweenaw  Peninsula,  the  site  of  the  copper  mine  of  Lake  Superior,  is  40  miles  across 
from  east  to  west  and  projects  about  70  miles  northeastward  into  the  lake.  The  central 
portion  of  this  peninsula  is  formed  of  a  succession  of  lava  beds  interbedded  with  lay<  is 
of  sandstone  and  conglomerate.  The  lavas  are  of  two  kinds,  the  prevailing  variety  being 
a  dark  basaltic  rock,  having  the  texture  of  diabase  and  most  conveniently  designated  as 
trap.  The  light-colored  lavas  are  quartz-porphyries  or  rocks  corresponding  to  andesite 
in  composition.  The  conglomerates  are  composed  of  rounded  fragments  of  igneous  rock, 
the  light-colored  reddish-chocolate  and  yellowish  quartz-porphyries  predominating.  The 
entire  series  has  a  thickness  of  from  25,000  to  30,000  feet  and  is  flanked  on  both  sides  by 
conglomerate  and  sandstone.  To  the  west  these  sediments  appear  to  be  conformable 
dipping  with  the  lava  beds.  To  the  east  there  is  a  fault  and  the  sandstones  are  nearly 
horizontal. 
Copper  occurs  native  in  these  rocks.     It  was  first  found  in  fissures  cutting  across  tht 
rocks  and  later  in  veins  which  cut  the  bedded  series  at  an  acute  angle  to  the  dip.     At  the 
present  time,  however,  the  great  production  of  the  region  comes  from  the  amygdaloida 
lava  beds  and  the  so-called  "ash  beds'r  of  nearly  similar  character  and  from  the  copper 
bearing  conglomerate,  the  latter  rock  furnishing  one-half  of  the  total  production. 
The  so-called  "Copper  Range  "  is  a  belt  from  4  to  0  miles  wide,  lying  on  the  eastern  sid. 
of  the  series  of  bedded  trap  rocks.  The  differenl  members  of  this  series  vary  from  a  fev 
feet  to  100  feet  in  thickness,  the  individual  lava  flows  or  layers  being  distinguished  \r 
the  amygdaloida!  or  vesicular  nature  of  the  upper  portion  and  in  many  cases  being  separate< 
by  beds  of  conglomerate.  The  igneous  rocks  have  been  altered  to  dark-green  rocks  i:; 
which  the  original  hornblende  and  augite  have  become  chlorite  and  epidote  and  the  hole 
or  vesicles  have  been  filled  by  quartz,  calcite,  and  other  minerals,  including  native  coppei 
Both  the  amygdaloids  and  the  conglomerates  are  heavily  impregnated  with  epidote.  Th 
copper  occurs  in  crystalline  form  and  as  casts  or  fillings  having  the  shape  of  the  amygdul 
which  it  fills.  The  conglomerates  often  inclose  seams  of  fine-grained  sandstone,  but  i 
such  cases  the  copper  impregnation  is  confined  to  the  conglomerate.  The  conglomerate 
consist  largely  of  chocolate-colored  and  reddish  porphyry,  with  a  few  fragments  of  diabas 
and  amygdaloid.  These  beds  outcrop  as  ledges  having  a  general  northeasterly  coursi 
but  owing  to  glacial  erosion  and  the  covering  of  the  country  by  drift  long  outcrops  ai 
seldom  seen  and  exploration  work  consists  largely  in  stripping  oil*  the  superficial  cover  I 
find  the  underlying  trap  beds. 
The  chief  ore  production  of  the  district  comes  from  the  "Calumet  conglomerate."     Th 
bed  is  from  12  to  25  feet  thick,  and  has  a  northwest  dip  of  36°  to  39°.     The  ore  is  not    oi 
tinuous  throughout  the  entire  extent  of  the  bed,  but  is  confined  to  ore  shoots  of  grei 
length;  thus  the  Calumet  and  Hecla  property  is  productive  for  2  miles  in  length,  and 
tins  distance  there  are  but  few  lean  spots. 
The  diabase  beds  have  the  characteristics  of  extrusive  lava  sheets  whose  upper  surfa 
is  often  scoriaceous,  giving  rise  to  the  term  "ash  bed."  In  the  uplifting  of  this  gre 
series  of  trap  rocks  there  has  been  dislocation,  and  cross  veins  have  been  formed. 
The  distribution  of  the  copper  in  the  different  ore-bearing  beds  is  dependent  on  t 
structure  of  the  rock.  In  the  Quincy  mine  the  hanging-wall  rock  is  richest,  this  porti 
being  the  most  permeable.  In  the  Atlantic  ore  bed  the  copper  is  somewhat  uniform 
disseminated  throughout  the  amygdaloid.  In  the  Baltic  mine  the  copper  is  irregnlai 
distributed  and  the  ore  channel  appears  to  be  a  sheared  zone,  the  fracturing  extendi 
beyond  the  limits  of  the  amygdaloid  layer  and  into  the  incasing  trap.  Slip  planes  a 
flat  cross  joints  indicate  movements,  a  cross  vein  at  No.  4  mine  extending  for  200  f< 
across  the  beds,  and  carrying  copper  with  quartz  and  calcite.  Cross  veins,  though  frequei  | 
do  not  extend  beyond  the  limits  of  the  ore  bed  and  conform  in  strike  to  that  of  the  mi 
ore  bed.  The  occurrence  of  copper  in  these  cross  veins  and  of  sheet  copper  along  en 
joints  in  the  foot  wall  indicates  migration  of  material  after  the  original  ore  deposition  w 
