irving]        ORE    DEPOSITS    OF    THE    NORTHERN    BLACK    HILLS.  139 
of  a  thick  dike  of  fine-grained,  white  porphyry.  Much  galena  also 
was  found  together  with  the  carbonates,  and  after  the  ore  was  worked 
out  a  seam  or  vertical  was  detected  extending  downward  from  the 
main  mass.  Other  pockets  of  ore  were  also  found  at  different  points, 
and  in  one  place  a  pocket  of  vanadinite  containing  1  or  5  tons  was 
encountered.  Mr.  Fowler"  reports  the  occurrence  of  the  following 
minerals:  Galena,  cerrusite,  cerargyrite,  matlockite,  wolfenite,  pyro- 
morphite,  plattnerite,  atacamite,  and  vanadinite.  This  type  of  ore 
resembles  in  its  general  character  and  in  its  association  with  porphyry 
bodies  the  deposits  of  Leadville,  Colo.,  described  by  Mr.  S.  F. 
Emmons.6  From  the  workings  now  accessible  too  little  can  be  deter- 
mined regarding  the  details  of  the  ore  occurrence  to  afford  any  more 
definite  idea  of  the  manner  in  which  it  originated  than  the  simple  fact 
that  it  is  probably  a  replacement  of  the  limestone. 
Of  the  second  type  of  occurrence  the  most  important  case  is  that  at 
the  Seabury  mine.  This  consisted  of  an  irregular  crevice  striking  S. 
85°  W.  and  running  through  the  Seabury,  Iron  Hill,  Segregated  Iron 
Hill,  and  Adelphi  mines,  with  a  possible  continuation  in  the  Spanish  R, 
a  mine  in  which  some  ore  was  obtained,  but  at  too  great  a  distance  for 
its  relation  to  the  others  to  be  clearly  made  out.  This  crevice  varies 
from  1  to  20  feet  in  width.  The  sides  consist  of  a  ferruginous  jas- 
peroid  material  which  replaces  the  limestone,  often  for  2  or  3  feet 
from  the  crevice,  and  contains  at  times  galena,  lead  carbonates,  and 
horn  silver  in  sufficient  amount  to  be  profitably  worked.  The  latter 
mineral  most  frequently  occurs  as  a  thin  film,  covering  druses  of  fine 
quartz  crystals,  which  form  linings  to  cavities. 
The  center  of  the  crevice  is  loosely  filled  by  a  soft,  ferruginous, 
gouge-like  matter  of  a  pinkish-red  color  and  containing  gold.  A  large 
quantity  of  this  ore  is  reported  to  have  been  mined  from  the  Seabur}^, 
and  also  from  the  west  side  of  the  porphyry  dike  in  the  Iron  Hill. 
Since  1891  there  seems  to  have  been  but  little  work  done  in  this  dis- 
trict, no  output  being  recorded  for  that  period.  Within  the  last  year, 
however,  a  small,  35-ton  cyanide  plant  has  been  erected  to  treat  the 
tailings  from  the  old  smelter. 
In  concluding  this  brief  review  of  the  ore  deposits  of  the  northern 
Black  Hills,  it  seems  well  to  add  a  few  words  on  the  future  prospects 
of  the  region. 
Mining  communities  are,  owing  to  the  limited  extent  of  the  deposits 
on  which  their  activity  is  based,  generally  short-lived.  That  this  is 
true  a  glance  at  the  history  of  many  western  mining  camps  will  show. 
A  few,  like  Leadville,  have  been  productive  for  many  years,  and  will 
probably  continue  so  in  the  future,  but  there  are  few,  if  any,  which, 
if  dependent  wholly  upon  mineral  production,  will  not  in  time  cease 
to  thrive  as  their  economic  resources  become  exhausted. 
a  Oral  communication. 
''Emmons,  S.  F.,  Geology  and  mining  industry  of  Leadville:  Mon.  IT.  S.  Geol.  Survey,  vol.  12,  p. 
545  et  seq. 
