34  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  200. 
impetus  in  Utah,  augmenting  the  gold  output  in  that  State  by  $1,000,- 
000.  These  four  factors,  together  with  moderate  gains  in  the  pro- 
duction of  Alaska,  Montana,  and  South  Dakota,  are  sufficient  to 
account  for  the  increase  of  nearly  $11,000,000  in  the  production  of 
the  last  year  compared  with  that  of  1903. 
In  the  following  pages  a  first  attempt  will  .be  made  to  classify  the 
gold  production  according  to  its  derivation  from  placers,  dry  or 
quartzose  ores,  copper  ores,  and  lead  ores.  As  far  as  possible  a  fur- 
ther classification  according  to  types  and  age  of  deposits  will  be  made. 
GOLD  DERIVED  FROM  PLACERS. 
The  production  of  placer  gold  for  1904  may  be  estimated  at  $12,- 
900,000,  an  increase  of  $660,000  compared  with  the  figures  of  the 
Mint  Report  of  1903.  Alaska  is  the  largest  producer,  and  should 
show  a  gain  of  at  least  $200,000,  the  output  being  estimated  at 
$5,800,000.  California  will  show  an  increase  which  may  reach 
$800,000  ($-1,800,000  being  the  estimated  figure  for  1904).  partly  due 
to  a  favorable  season  for  hydraulic  mining,  but  chiefly  to  the  great 
development  of  the  dredging  industry.  Up  to  1900  this  branch  of 
work  was  unimportant.  From  then  on  began  a  rapid  increase,  as 
shown  by  the  returns  of  dredger  gold  as  follows:  1900,  $200,000; 
1901,  $500,000;  1902,  $800,000;  1903,  $1,500,000.  Unexpectedly 
large  areas  in  Yuba,  Sutter,  Nevada,  Butte,  and  Sacramento  coun- 
ties have  proved  to  be  suitable  for  the  dredging  process,  while  the 
dredging  machines  have  been  greatly  improved  and  enlarged  and 
the  expenses  correspondingly  reduced.  At  the  same  time  the  output 
due  to  drift  mining  and  hydraulic  mining  is  on  the  whole  slowly 
decreasing.  In  1903  hydraulic  mining  yielded  $1,064,243;  surface 
placers,  $761,823,  and  drift  mining,  $581,397. 
Idaho,  Montana,  Colorado,  and  Oregon  have  probably  maintained: 
their  production  at  the  value  reached  for  a  great  number  of  years, 
During  1903  they,  respectively,  yielded  $750,000,  $482,000,  $400,000  | 
and  $207,000.     Since  1902  New  Mexico  has  contributed  over  $100,00C  j 
annually  in  placer  gold,  derived  from  veins  of  unknown  age  and  ob- 
tained by  dredging  operations  in  Colfax  County,  but  none  among  the 
other  States  has  exceeded  $100,000  in  output. 
Considered  from  a  geological  standpoint,  it  is  important  to  stat( 
that  the  placers  of  Alaska,  California,  Montana,  Idaho,  and  Oregoi 
are  derived  almost  entirely  from  the  erosion  and  concentration  o: 
deposits  the  origin  of  which  antedate  the  Tertiary  system.  In  Cali 
fornia  the  placers  are  generally  derived  from  quartz  veins  of  Juras 
sic  age,  and  the  same  is  probably  true  of  most  placers  in  Idaho  an< 
Oregon.  In  Montana,  and  especially  in  Colorado,  a  part  of  th<  ! 
placers  are  more  likely  derived  from  veins  of  early  Tertiary  age;  bu 
