40  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
PLACERS. 
It  is  well  known  that  all  placer  gold  contains  a  small  amount  of 
silver.  The  amount  derived  from  this  source  is  very  slight  and  may 
be  estimated  at  64,000  ounces  in  1904,  of  which  by  far  the  larger  part 
is  divided  between  Alaska  and  California,  each  probably  contribut- 
ing about  80.000  ounces.  A  large  portion  of  this  silver  is  never 
separated  from  the  gold  in  which  it  occurs. 
QUARTZOSE,   OR  DRY,   ORES. 
This  division  embraces  the  silver  derived  from  quartz  veins  of 
various  ages  and  different  characters.  A  large  part  of  the  silver 
from  this  series  is  free  or  alloyed  with  gold.  Many  of  the  veins  of 
this  type  contain,  however,  pyrite,  copper  ores,  or  lead  ores,  in  rela- 
tively small  amounts.  In  many  cases  the  concentrates  from  these 
veins  have  been  classed  as  copper  ores  and  lead  ores.  In  the  present 
state  of  the  statistics  relating  to  this  subject  it  is  difficult  to  avoid 
this  source  of  confusion.  It  is  not,  however,  believed  to  seriously 
affect  the  figures  given. 
According  to  their  age  the  quartz  veins  may  be  subdivided  into 
three  groups,  the  pre-Cambrian,  Mesozoic,  and  Tertiary.  The  pre- 
Cambrian  quartz  veins  of  the  Appalachian  States  yield  principally 
gold,  but  also  a  small  amount  of  silver,  probably  about  31,000  ounces. 
There  are  a  number  of  veins  in  South  Dakota  and  Wyoming  which 
probably  belong  to  this  class,  but  the  only  one  producing  a  notable 
amount  is  the  celebrated  Homestake  mine,  in  the  Black  Hills,  to  which 
approximately  48,000  ounces  shoudld  be  credited.  It  will  be  seen  that 
the  total  amount  derived  from  pre-Cambrian  quartz  veins  is  very  | 
small,  scarcely  exceeding  the  quantity  derived  from  the  placers.  The 
amount  estimated  is  79,000  ounces. 
A  strongly  developed  belt  of  quartz  veins  of  late  Mesozoic  age 
extends  through  California,  Oregon,  Idaho,  and  Alaska.  There  are 
probably  veins  of  the  same  age  and  character  in  Montana  and  Ari- 
zona. It  is  possible  that  some  of  the  Alaska  veins  may  be  older  than  j 
those  of  California,  and  it  is  likewise  possible  that  among  the  deposits 
of  Montana  referred  to  this  class  there  may  be  some  of  early  Tertiary 
age.  All  these,  however,  are  principally  gold  bearing,  and  the  total 
amount  of  silver  derived  from  veins  of  this  class  is  not  very  large. 
Alaska  contributes  about  107.000  ounces,  California  150.000  ounces, 
and  Oregon  130,000  ounces,  while  the  other  States  mentioned  supply  i 
smaller  amounts  from  this  source.  The  total  amount  of  silver  derived 
from  the  Mesozoic  belt  of  gold-quartz  veins  in  1904  is  believed  to  be 
860,000  ounces. 
The  third  subdivision  of  the  dry  silver  ores  comprises  those  derived 
