88  XTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906,  PART    I. 
The  gravel  containing  the  gold  and  platinum  is  usually  well  worn 
and  small,  affording  ideal  conditions  for  dredging  and  with  a  large 
enough  plant  and  intelligent  handling  may  be  made  to  pay.     The; 
possible  margin  of  profit,  however,  working  for  the  gold  content  alone, 
would  be  small  and  unattractive,  unless  associated  values  of  gold  or 
platinum  not  apparent  to  ordinary  methods  of  saving  can  be  recovered. 
That  platinum  in  metallic  form  is  associated  with  the  gold  in  these 
gravel  beds  can  not  be  questioned,  for  while  it  can  rarely  be  seen  in] 
panning  it  invariably  shows  in  cleaning  amalgam.     In  the  operation 
of  the  Sweet zer-Burroughs  dredge  near  Minidoka,  it  was  always  ob-j 
served  at  clean-up  time,  appearing  as  ashy  gray  metallic  particles 
floating  on  the  "quick"  when  the  hard  amalgam  was  thinned  down  with 
more  "quick"  for  tin1  purpose  of  separating  foreign  matter  from  thej 
gold.     A  quarter  of  an  ounce  of  clean  platinum   recovered  in  this 
manner  is  now  in  the  possession  of  Lewis  Sweet  zer,  of  Rupert,  Idaho. 
It  is  perfectly  clean  gray  metal  in  scaly  particles  and  about  as  fine  as 
the  fine  gold. 
SOURCE   OF   GOLD. 
The  source  of  the  Snake  River  fine  Hour  or  flake  gold  is  unknowrn. 
The  fineness  of  the  individual  particles  and  the  abrasion  marks  seen! 
on  some  of  the  pieces  suggest  that  they  have  been  carried  for  some 
distance.  From  the  evidence  gathered  within  the  area  studied  this 
season  it  seems  likely  that  the  gold  was  carried  from  regions  lying 
farther  north  and  northeast.  It  was  probably  derived  from  the  older 
rocks  of  the  Teton  and  Gros  Ventre  ranges  or  from  the  region  of  the 
later  intrusives  of  the  upper  Snake  Valley.  The  old  bars  and  benches 
containing  the  gold  were  built  up  in  much  the  same  manner  as  those 
now  forming  in  the  river,  and  some  of  the  gold  particles  have  been 
worked  over  and  over  again  by  the  river  until  they  are  finally  mine! 
in  their  present  places. 
The  thickness  and  value  of  the  pay  streaks  is  no  doubt  dependent 
<m  the  local  conditions  under  which  they  were  deposited.  The 
gradient  of  the  stream  as  well  as  the  volume  of  the  water,  its  turbid- 
ity, and  the  time  during  which  it  (lowed  in  the  same  channel  would 
affect  the  distribution,  thickness,  and  richness  of  the  pay  streaks. 
Unlike  other  gold  placers  those  of  Snake  River  do  not  increase  in  gold 
values  as  bed  rock  is  approached,  for  the  gold  is  usually  more  plentiful 
in  the  gravel  banks  between  present  and  former  high  and  low  water- 
marks than  at  the  deeper  horizons.  By  the  completion  of  the  Miiner 
and  Minidoka  dams  the  flow  of  the  stream  has  been  absolutely  stopped 
for  several  days  at  two  points  along  its  course  within  the  past  two* 
years,  and  its  bed  rock  has  thus  been  laid  bare  for  miles  during  periods 
in  which  its  potholes  and  crevices  were  searched  in  vain  for  paying 
gold  values. 
