|rving.]        ORE    DEPOSITS    OF    THE    NORTHERN    BLACK    HILLS.  125 
of  original  sedimentary  banding,  as  can  still  be  seen  in  many  places. 
Numerous  closely  spaced  folds  are  also  seen  to  exist. in  the  series,  but 
the  high  degree  of  alteration  that  the  rocks  have  undergone  has  now 
almost  completely  obliterated  their  original  structure. 
Above  the  metamorphic  crystallines  are  the  rocks  of  the  Cambrian 
system.  This  system  is  composed  of  a  basal  conglomerate  or  coarse 
bed  of  cemented  gravel,  a  thick  layer  of  quartzite,  and  a  series  of  alter- 
nating limestones,  limy  sandstones,  and  shales  with  some  quartzite — in 
all  a  thickness  of  about  400  feet.  Above  these  and  still  farther  out 
from  the  center  of  the  hills  there  is  a  yellowish  limestone  showing 
purple  spots  and  belonging  to  the  Silurian  age.  This  is  about  80  feet 
thick  and  comprises  the  third  system.  Above  it  comes  the  fourth 
system,  a  series  of  very  heavily  bedded  gray  limestones,  pinkish  at  the 
base  and  averaging  about  600  feet  in  thickness.  These  are  of  Carbon- 
iferous age  and  cover  the  other  rocks  throughout  the  entire  western 
portion  of  the  uplift. 
There  are  present,  besides  these  sedimentary  rocks,  eruptive  rocks 
of  several  different  varieties.  The  most  abundant  of  these  are  rhyolite, 
either  fine  grained  and  white  as  in  the  vicinit}r  of  the  Homestake  mine, 
or  coarse  grained  and  darker  colored  as  at  other  localities;  syenite- 
porphyries,  occurring  chiefly  in  the  vicinity  of  Deadwood  and  Two  Bit 
Gulch ;  monzonite-porphyries;  and  phonolite.  The  latter  is  generally  a 
dark-greenish  or  bluish  rock,  sometimes  very  coarse  but  usually  exceed- 
ingly dense  and  tine  grained.  Other  intermediate  varieties  of  eruptive 
rock  are  also  present  in  different  places.  The  eruptive  rocks,  when 
found  in  the  schist  series,  usually  occur  either  as  dikes  which  are 
parallel  to  the  lamination  of  the  schists  or  as  large  and  more  irregular 
masses  which  have  no  definite  form.  When  in  the  Cambrian  rocks 
they  are  generally  sills  or  sheets  which  have  spread  out  laterally  to 
great  distances  along  the  planes  of  sedimentation;  while  in  the  Car- 
boniferous they  are  less  regular  in  form,  sometimes  occurring  in  short, 
thick  sheets,  again  in  dikes,  and  still  more  frequently  in  very  irregular 
masses. 
Much  discussion  has  taken  place  as  to  the  probability  that  the 
phonolites  which  are  present  in  the  Black  Hills  indicate  a  recurrence  of 
the  types  of  ore  deposits  found  in  Cripple  Creek.  While  there  are 
certain  cases  in  which  tellurides  of  gold  have  been  found  in  the  Black 
Hills,  associated  with  phonolites  bearing  some  resemblance  to  Cripple 
Creek  ores,  the  existence  of  phonolites  themselves  in  this  region  does 
not  indicate  that  there  is  likely  to  be  found  a  second  Cripyne  Creek. 
Phonolites  occur  in  many  localities  in  the  world  (in  Europe,  Mexico, 
and  elsewhere),  and  are  in  most  cases  not  associated  with  ore  bodies. 
That  they  may  indicate  future  mineral  wealth  in  the  Black  Hills  is 
j  possible,  but  not  at  all  essential. 
