42  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
of  these  silver-bearing  copper  deposits  is  not  known ;  they  appear  as 
replacement  veins  in  clioritic  rocks.  The  copper  deposits  of  Cali- 
fornia are  principally  located  in  Shasta  County,  the  two  most  impor- 
tant producers  being-  the  Mountain  Copper  Company  mines,  near 
Reading,  and  the  Bully  Hill  mines.  Both  of  these  are  working  on 
pyritic  ores  that  appear  to  be  replacement  veins  of  Mesozoic  age  con- 
nected with  eruptive  rocks  of  the  same  age.  The  amount  yielded  by 
California  is  estimated  at  (>00,000  ounces. 
On  account  of  defective  statistics  the  amount  which  should  be 
credited  to  copper  ores  in  Colorado  is  difficult  to  estimate.  Much  of! 
it  is  produced  by  the  Leadville  deposits,  which,  in  general,  are  re- 
placements of  limestone  and  are  considered  to  be  of  late  Mesozoic 
age.  The  remaining  amount  is  derived  from  widely  scattered 
sources,  and  part  of  it  is  no  doubt  obtained  from  concentrates  of  Ter- 
tiary gold-quartz  A^eins.  In  a  preliminary  way  Colorado  has  beem! 
credited  with  1,500,000  ounces  derived  from  copper  ores. 
Utah  is  next  to  Montana  in  the  production  of  silver  from  copper 
ores.  Most  of  the  amount,  which  is  estimated  to  be  4,000,000  ounces, 
comes  from  the  smelting  of  copper  ores  from  Bingham,  Salt  Lake 
County,  but  a  part  is  obtained  from  the  old  mining  district  of  Tin- 
tic,  in  Juab  County.  At  both  places  the  deposits  are  replacement 
veins,  or  forms  allied  to  these.  Their  age  is  believed  to  be  early  Ter- 
tiary. 
New  Mexico  and  Washington  add  smaller  amounts  of  copper  ores, 
which,  however,  together  do  not  yield  more  than  185,000  ounces. 
On  the  whole,  it  may  be  said  that  the  greater  part  of  the  production 
of  silver  from  this  source  is  derived  from  replacement  veins  of  late 
Mesozoic  or  early  Tertiary  age.  The  Tertian7  veins  of  later  date 
do  not,  as  a  rule,  contain  much  copper. 
LEAD  ORES. 
Silver  is  usually  most  intimately  connected  with  lead,  so  that  it 
will  not  occasion  surprise  to  learn  that  nearly  one-half  of  the  total 
output  is  derived  from  lead  ores.     The  three  largest  producers  are  j 
Colorado,  Utah,  and  Idaho,  the  first  two  yielding  each  about  8,000,000 
ounces,  while  (*>,500,000  ounces  are  derived  from  Idaho. 
In  Colorado  the  greater  part  of  the  production  still  comes  from 
Leadville,  where  the  ores  occur  as  irregular  replacement  deposits  j 
in  limestone  and  are  believed  to  be  of  late  Mesozoic  age.  Much  of 
the  remaining  amount  is  supplied  by  Aspen,  the  mines  of  which  do 
not,  however,  produce  nearly  as  much  as  in  former  years.  Here 
again  the  lead  ores  appear  as  replacement  deposits  in  limestones, 
more  or  less  distinctly  connected  with  fissure  veins,     Their  age  is 
