TRIEm  >NS  TC  7    LOGY.  lvN>4.        [n 
:ing  the  gneiss  1  _         :i  diminished  in 
ss  mineralized,  and  very  shortly  dissipated  itself  by 
g.    This  stration  tends  to   show  that   when 
hard  the  porphyry  has  afforded  as  good  ns- 
-  ■-     -  :  -  3  of  1  strict 
:wo  porphyry  wall-  sually  unfavorable  to  ore  in 
ptional.     -         _  5  are  found  parallel 
to  porphyry  dikes  and  the  conl  :  porphyry  with 
-      ie  mineralized  parallel  vein  is  not  immedi 
atel  several  feet  away,  entirely  in  gi.    ss 
granite.    In  s  ses,  as  in  S 
and  Colorado  Central  mines,  the  porphyry  dik  decom] 
and  ther  :  have  been  the  source  of  the  leaching. 
along  porphyry  contacts  frequently  diverge  from  the  dike?  anj 
them.     3         _  ntirely  unconnected  with  ai  rphyi?} 
umerous  and  prodv  s  those  that  ai 
:hlv:;a:  alteration  of  ores 
. — The  greater  part  of  the  immer  sses  o 
:  _  istrict  undoubtedly  I 
primary  ores  :  the  chalcopyri*  PJ™{ 
ted  with  these  lead  ai  sulphi-  <abl| 
with  the  primary  mineral-.     Th 
i   I     •-_::-     .         rse.  zii    - 
earn      _  galena  and  a  small  ai  :  _ 
:ng  a  la:  _  :  galena.  Less  zinc 
P  ssibly  the   _  of  th  I 
-     f  primal  m  part  to  the  fact  that  a  porticJ 
of  t:  rahedrite  found  ir. 
.-Unary:,  riginal  sulphide  ores.     Wit.'J 
t  the  larc  _    ide  pyrite  and  copper-pyrite 
found   :  workings   about   Empire   ah 
pri: 
Pol  rgen  t  i  i 
_  ay  cop  _  Dtain  the  value-  of  t^ 
of  *  _  R  silver  _  md  pro   stite 
hough 
gely  dependent  up 
upon  tl.  Irea 
.  •'         -  ■     ' 
1  _     '  of  i  min erah 
finally  la:  _  _  rid  zinc 
by    1'];' 
