Lindgren.]     PRODUCTION    OF   SILVER  IN   UNITED   STATES  IN   1904.  43 
not  positively  known.  The  remainder  of  the  production  is  scatter- 
ing, most  of  it  being  derived  from  the  various  counties  in  the  San 
Juan  region,  and  probably  in  many  cases  being  concentrates  from 
veins  of  late  Tertiary  age. 
The  principal  source  of  silver-lead  ores  in  Utah  is  the  Park  City 
district  in  Summit  County.  The  production  has  decreased  somewhat 
during  190-t,  but  still  remains  very  large.  A  smaller  part  of  the 
lead  ores  are  derived  from  the  Tintic  veins  in  Juab  County. 
The  silver-lead  ores  in  Idaho  are  derived  from  two  widely  distant 
regions.  The  first  and  more  important  is  the  Coeur  d'Alene  district, 
which  is  believed  to  have  produced  6,000,000  ounces  during  the  past 
year.  The  ores  which  carry  galena  and  zinc  blende,  with  a  relatively 
small  amount  of  silver,  occur  in  probably  pre-Cambrian  slates  near 
the  contact  with  a  granite  mass  of  intrusive  origin.  The  age  of  these 
replacement  veins  is  probably  late  Mesozoic.  A  somewhat  different 
class  of  deposits,  which  consist  of  veins  containing  galena  and  tetra- 
hedrite,  or  gray  copper,  and  which  are  very  rich  in  silver,  occur  in 
Blaine  County,  in  the  central  part  of  Idaho,  the  region  being  usually 
known  as  the  Wood  River  district.  These  veins  also  occur  in  slates 
near  the  contact  of  intrusive  granite  and  are  considered  to  be  of  late 
Mesozoic  age ;  their  part  in  the  production  of  1904  may  be  estimated 
at  500,000  ounces. 
The  production  of  lead-silver  ores  in  Montana  has  steadily  de- 
creased, and  during  the  last  year  the  amount  of  silver  derived  from 
this  source  was  given  in  the  report  of  the  Director  of  the  Mint  as 
only  450,000  ounces.  In  most  cases  these  deposits  are  veins  or 
irregular  masses  occurring  as  replacement  of  limestone. 
Arizona,  California,  Washington,  and  New  Mexico  procluce  small 
amounts  of  silver-bearing  lead  ores,  but  the  production  in  no  case 
exceeds  200,000  ounces. 
To  sum  up,  the  argentiferous  lead  ores  are  believed  to  have  con- 
tributed 23,000,000  ounces  to  the  production  of  silver  during  the  past 
year. 
There  can  be  no  doubt  that  a  very  large  proportion  of  silver-bear- 
ing lead  ores  are  derived  from  deposits  of  comparatively  great  age. 
Like  the  copper  ores,  most  of  them  are  of  late  Mesozoic  or  early  Ter- 
tiary age,  while  it  would  appear  as  if  the  Miocene  and  post-Miocene 
deposits  contained  a  comparatively  small  amount  of  lead. 
