rving.]        ORE    DEPOSITS    OP    THE    NORTHERN    BLACK    HILLS.  129 
ater  than  the  intrusion  of  the  dikes  and  bodies  of  rhyolite.  It  is 
)robable  that  this  belt  has  been  the  seat  of  much  fracturing'  and  crush- 
ng  from  very  early  geological  time  until  the  present,  constituting  a 
ine  of  weakness  along  which  mineralizing  waters  were  permitted  to 
dreulate  more  freely  than  elsewhere.  Impregnation  of  the  country 
■ock  at  successive  periods  with  vein  minerals  and  small  amounts  of 
rold  has  thus  given  rise  to  a  workable  zone  of  gold-bearing  rock. 
In  the  earlier  days  of  the  mine  the  ore  was  completely  free  milling 
md  of  higher  grade  than  that  now  mined.  It  was  highly  oxidized, 
md  contained  little  or  no  sulphurets.  As  the  workings  penetrated 
ieeper,  oxidized  material  gave  place  gradually  to  sulphides,  and  more 
tnd  more  of  the  values  in  the  ores  failed  to  yield  to  amalgamation. 
mr  a  time  the  concentrates  from  the  mine  were  sold  to  smelting  com- 
mnies,  but  experimentation  on  their  treatment  gradual ly  led  to  the 
;onstruction  of  a  cyanide  plant  with  a  view  to  treating  the  more 
•efractory  portions  of  the  ore. 
It  has  been  assumed  by  many  who  have  written  upon  the  geology 
)f  these  ore  bodies  that  the  rhyolite-porphyries  which  occur  in  inti- 
nate  association  with  them  have  enriched  the  ore,  but  there  is  no 
svidence  to  support  this  theory. 
The  Homestake  mine  has,  since  its  opening,  been  an  illustration  of 
he  manner  in  which  a  large  body  of  low-grade  ore  handled  on  a  large 
icale  could  be  made  to  yield  great  profit.  Its  successful  operation  has 
>een  due  chiefly  to  the  care  with  which  it  has  been  managed  and  the 
jreat  business  ability  of  those  who  have  handled  it. 
Clover  Leaf  mine. — The  other  mine  which  has  been  operated  on 
Ugonkian  ores  is  known  as  the  Clover  Leaf  mine  (formerly  the.  Uncle 
lam),  and  is  not  far  from  Perry  station,  on  the  Black  Hills  and  Fort 
^ierre  Railroad.  Compared  with  the  Homestake  belt  its  production 
b  comparatively  small,  but  it  is  of  singular  geological  interest.  The 
re  body  is  a  large,  saddle-shaped  mass  of  quartz  inclosed  in  the  meta- 
aorphic  schists,  with  its  apex  striking  N.  64°  W.  and  pitching  to  the 
outheast  at  an  angle  of  40°.  The  horizontal  section  of  the  quartz 
ody  as  exposed  on  the  250-foot  level  has  the  appearance  of  the  letter 
I,  with  slightly  flaring  arms.  The  northern  arm  strikes  N.  40°  W., 
he  southern  S.  75°  W.  This  quartz  body  is  thickest  at  the  crest,  and 
m  lamination  of  the  inclosing  schist  is  parallel  to  its  surface,  curving 
round  it  so  as  to  give  to  the  mass  the  appearance  of  a  folded  lens  at 
|.ie  crest  of  a  southeast-pitching  anticlinal  fold  in  the  Algonkian  schist. 
jk>th  of  the  arms  of  this  quartz  mass  when  followed  out  from  the  apex 
ecome  much  narrower  than  the  main  body,  the  northwesterly  having 
In  average  width  of  20  feet,  the  south  westerly  of  about  10  to  12  feet. 
he  gold  is  contained  chiefly  in  the  quartz,  in  which  it  often  appears 
pee  and  generalty  associated  with  small  particles  of  galena.  The 
Bull.  225—04 9 
