132  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 225. 
of  the  Cambrian  to  lap  over  onto  the  mineralized  roek  of  the  Algon- 
kian.  A  general  downward  inclination  of  the  schist  surface  toward 
the  northeast  may  also  be  observed. 
It  is  not  possible  to  give  exact  boundaries  to  the  original  extent  of 
these  gold-bearing-  gravels  on  account  of  the  dissected  nature  of  the 
areas  which  now  remain.  The  gravel  is  composed  of  rounded,  water- 
worn  pebbles  of  quartz  and  quartz-schist  and  a  few  fragments  of  softer 
schist,  which  seem  to  decrease  in  abundance  as  the  distance  increases 
from  the  Homestake  lode,  that  is,  from  the  old  pre-Cambrian  shore 
line.  The  gold-bearing  portions  of  the  gravel  may  be  at  once  distin- 
guished from  tho.se  which  are  barren  by  the  character  of  the  material 
which  cements  the  pebbles.  In  the  gold-bearing  portions  this  is  chiefly 
oxide  of  iron  when  weathered,  or  pyrite  when  it  has  not  suffered  alter- 
ation. The  nongold-bearing  portions,  on  the  other  hand,  have  a  rather 
sandy,  quartzose  matrix,  or  are  in  some  instances  slightly  calcareous. 
The  gold  in  the  richest  portions  of  the  conglomerate — those  first 
mined — is  chiefly  placer  gold,  for  it  is  rounded  and  worn  by  attrition 
and  is  concentrated  near  the  bed  rock.  It  was  undoubtedly  derived 
from  the  erosion  of  gold-bearing  lodes  in  the  Algonkian  rocks  and 
mechanically  deposited  in  depressions  along  the  old  shore  line.  Some 
of  it  has  been  dissolved  by  ferric  sulphate,  which  has  resulted  from 
the  oxidation  of  the  pyrite,  and  has  been  redeposited  from  this  solution 
in  thin  films  in  the  lamina?  of  the  underlying  schists.  This  has  also 
produced  an  enrichment  of  the  lowermost  layers  of  the  conglomerate. 
Besides  these  two  types  of  gold  which  occur  in  these  conglomerates, 
it  is  also  possible  that  gold  was  introduced  with  the  pjaite  which  either 
cements  or  once  cemented  the  pebbles.  The  introduction  of  pyrite 
was  subsequent  to  the  deposition  of  the  conglomerates,  since  it  pene- 
trates fractures  in  the  quartz  pebbles  and  is  probably  a  replacement  of 
the  original  quartzose  cementing  material.  Intrusions  of  rhyolite  cut 
the  conglomerate  in  many  places,  and  are  often  heavily  impregnated 
with  pyrite.  The  close  relation  between  these  gravel  deposits  and  the 
Homestake  lode,  together  with  their  absence  along  its  line  of  outcrop, 
seems  to  indicate  that  the  Homestake  zone  projected  above  the  level 
of  the  surrounding  rocks  and  formed  in  the  old  Cambrian  sea  a  reef 
about  which  these  gravels  were  deposited.  The  greater  portion  of 
their  gold  was  thus,  with  little  question,  derived  from  the  disintegra- 
tion of  the  old  Homestake  lode.  They  are  not  exactly  equivalent  to 
the  gold-bearing  sands  in  the  Nome  district  of  Alaska,  but  are  some- 
what exceptional,  not  only  because  they  are  the  only  representatives 
of  what  may  be  termed  fossil  placers,  but  because  the}^  were  not  uni- 
formly deposited  along  the  shore,  having  been  confined  to  the  vicinity 
of  an  outcrop  of  a  large  gold  lode,  and  the  detrital  material  from  that 
lode  being  held  in  irregular  depressions  in  the  submarine  surface  in  its 
vicinity. 
