76 
CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1904.        [bull.  260. 
can  be  proved  that  the  known  ore  bodies  were  formed  during  or 
later  than  the  close  of  the  Eocene  epoch.  The  conglomerates,  how- 
ever, were  formed  in  the  early  Eocene  and  were  clearly  in  existence 
before  any  of  the  known  gold  ores  were  deposited.  The  source  of 
the  placer  gold  therefore — in  case  it  were  proved  to  be  present — 
must  be  sought  in  deposits  belonging  to  an  earlier  geological  period — 
deposits  for  Avhose  existence  there  is  as  yet  no  evidence. 
Gold  may,  of  course,  have  been  deposited  in  these  gravels  long 
after  they  were  consolidated  into  rock,  but  in  that  case  the  character 
of  the  gold  present  would  show  clearly  that  it  was  not  deposited  by 
placer  action. 
With  these  facts  in  view  it  seemed  necessary  to  determine  two 
points:  (1)  Whether  the  gravels  contained  appreciable  quantities  oil 
gold;   (2)   the  physical  condition  of  the  gold,  if  present.     For  thj< 
purpose  a  series  of  nine  carefully  selected  samples  were  taken  and! 
tested  for  gold.     The  samples  were  taken  from  portions  of  the  rocM 
most  favorable  for  the  occurrence  of  gold,  and  from  widely  separatee 
localities.     Some  samples  were  collected  from  places  recommended  a: 
especially  rich;  others  at  points  where  no  prospecting  had  been  done 
The  samples  were  very  carefully  assayed  and  four  of  them  panned 
the  concentrates  being  tested  microscopically.     The  results  are  give]  n 
below : 
Analyses  of  Telluride  conglomerate,  Ouray  district,  Colorado. 
No. 
Gold. 
Value. 
Panning  test. 
Ounces. 
Cents. 
l-_ 

0.  005 
10 
Small  grains  of  magnetite  sand, 
but  no  colors. 
2__ 
.  005 
10 
Do. 
3.. 
.  005 
10 
Do. 
4 
.  005 
.  005 
10 
10 
Do. 
Do. 
5.. 
6__ 
.  005 
10 
Do. 
.  005 
.  005 
.  005 
10 
10 
10 
Do. 
Do. 
Do. 
8__ 
9 
Average  _ 
.005 
10 
While   these   determinations   are   by   no  means   the   result   of  a 
exhaustive  sampling,  they  clearly  show  that  if  gold  be  present  i 
this  rock   in   appreciable   quantities   it  must  be   contained   in  thoM 
portions  of  the  deposit  from  which  no  samples  were  taken.     A  coi 
tent  of  10  cents  per  ton  in  gold  is  so  small  that  it  can  hardly  1< 
