138  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
dized  quartz  is  of  extremely  high  grade.  Such  ore  appears  to  con- 
tain mostly  pyrite  as  metallic  mineral,  but  in  view  of  the  fact  that 
tellurium  is  known  to  be  present  in  the  district,  it  is  probable  that 
gold  telluride  is  present.  Doctor  Ilillebrand  has  determined  tel- 
lurium and  gold  (probably  gold  telluride)  as  well  as  a  copper  mini 
era  1  in  a  specimen  of  ore  of  this  class  from  the  January  mine.  These 
are  probably  residual  minerals  similar  to  those  characteristic  of  the 
unoxidized  zone,  as  microscopic  work  by  the  writer  indicates.  Most! 
of  the  hard  quartz  in  the  pay  shoots  of  the  different  mines  shows,, 
when  thus  examined,  pyrite,  tetrahedrite  (?),  and  free  gold,  the  last* 
two  closely  associated  and  often  intergrown.  The  tellurides  have  not 
yet  been  identified  microscopically,  probably  because  of  the  difficulty, 
of  distinguishing  them  from  pyrite. 
It  appears,  then,  that  the  rich  oxidized  ores  owe  their  richness,  noti 
primarily  to  concentration  during  oxidation  (though  this  process  has^ 
certainly  been  very  important),  but  to  the  existence  of  shoots  of  ricj| 
antecedent  (sulphide)  ore. 
Probability  of  rich  ore  in  depth. — Concerning  the  origin  of  these 
rich  sulphide  ores,  it  is  probable  that  some,  so  far  as  can  be  yet  seen 
are  purely  primary,  while  evidence  points  out  other  cases  as  ha  vim 
been  formed  subsequent  to  the  main  silicification  of  the  reef,  as  in  thu 
Combination  mine.  Here  the  rich  auriferous  sulphides  have  former 
in  a  broken  zone  (breccia -zone)  in  the  silicified  barren  reef,  and  oecu 
as  seams,  and  often  as  coatings  on  the  pebbles  in  the  breccia.  ThiJ 
question  arises,  however,  as  to  whether  this  subsequent  mineralization 
was  the  result  of  descending  or  ascending  waters.  Concerning  tlii  j 
the  evidence  is  not  conclusive;  but  there  is  no  sufficient  evidence  tha  1 
these  rich  ores  have  been  concentrated  from  the  lean  antecedent 
quartz  mass,  and  the  presence  of  elements  like  arsenic,  antimony,  an  lj 
tellurium  in  the  subsequent  sulphide  ore  suggests  a  deep-seated  origii  j 
Moreover,  microscopic  examination  of  such  rich  ores  shows  they  ha\ 
as  gangue  cherty  silica  like  that  of  the  first  period  of  silicificatioi 
and  the  associated  evidence  favors  the  idea  that  the  conditions  of  th 
subsequent  deposition  were  very  much  like  those  of  the  first. 
Besides  the  elements  mentioned  above,  bismuth  occurs  in  the  or  \ 
In  the  January  mine  it  occurs  in  the  oxidized  ores  in  the  form  ( 
silvery  scales,  which  is,  as  described  by  Doctor  Hillebrand,  bismutl 
perhaps  the  oxide  bismite.     In  the  Combination  long  needle-like  cry  i 
tals  have  been  found,  which,  according  to  the  manager,  Mr.   Cc 
lins,  give  the  chemical  tests   for  bismuth   sulphide    (bismuthinite 
In  the  January  mine  the  silvery  mineral  above  noted  is  sometim 
arranged  in  long  rod-like  forms,  and  these  are  very  likely  pseud) 
morphs  after  the  sulphide.     This  silvery  mineral  is  usually,  but  n  >; 
