248  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1904.        [bull.  200. 
It  is  a  singular  fact  that,  while  in  the  Horn  Silver  deposit  lead  and 
zinc  have  been  the  predominant  metals,  copper  occurring  as  an  impor- 
tant constituent  only  when  it  has  been  reconcentrated  by  secondary 
enrichment,  in  the  Cactus  mine  copper  and  the  omnipresent  iron  sul- 
phides are  practically  the  only  metals  present.  It  will  be  of  interest 
to  see  whether  a  geological  study  of  the  region  will  succeed  in  estab- 
lishing a  genetic  reason  for  this  association  of  minerals  and  deter- 
mining whether,  as  suggests  itself  at  first  glance,  it  is  due  to  the  dif- 
fering character  of  the  country  rocks  that  inclose  the  respective 
deposits. 
