ransome.]  COEUK    d'aLENE    DISTRICT,  IDAHO.  281 
coincide,  as  in  the  excellent  exposures  by  the  roadside  just  east  of 
Kellogg. 
This  formation  is  the  prevalent  rock  along  the  South  Fork  from 
Kellogg  to  Osburn  and  occupies  almost  the  entire  drainage  basin  of 
Prichard  Creek.  It  is  essentially  the  gold-bearing  formation  of  the 
district,  though  it  contains  also  some  promising  lead-silver  veins, 
mainly  in  the  transitional  beds  of  its  upper  part. 
The  Burke  formation  consists  principally  of  thin-bedded,  often 
shaly,  fine-grained,  sericitic  quartzites  of  prevailingly  light  tint. 
With  these,  however,  are  included  some  more  massive  quartzites,  such 
as  form  the  summit  of  Tiger  Peak,  and  thin  beds  of  grayish-purple 
argillaceous  quartzite  or  graywacke.  The  latter  are  a  more  promi- 
nent feature  of  the  Burke  formation  in  the  eastern  part  of  the  area 
than  in  the  western  part.  Near  the  eastern  border  of  the  district 
these  purplish-gray  beds  form  the  greater  part  of  the  upper  portion 
of  the  Burke. 
The  Burke  is  not  a  very  well-defined  formation,  as  the  thin-bedded 
sericitic  quartzites  and  quartzitic  shales  that  characterize  it  pass 
gradually  downward  into  Prichard  slate  and  gradually  upward  into 
the  massive  white  quartzites  of  the  Revett  formation.  It  is,  how- 
ever, exceeding  important  from  an  economic  standpoint  as  it  con- 
tains the  principal  lead-silver  deposits. 
The  formation  is  typically  developed  along  Canyon  Creek  from 
Burke  to  Gem,  in  the  vicinity  of  Wardner,  and  in  many  other  parts 
of  the  district,  particularly  along  its  eastern  border. 
The  Revett  quartzite  is  fairly  homogeneous  and  is  composed  almost 
entirely  of  moderately  thick  beds  of  white  or  pale  greenish-gray 
quartzite.  The  greenish  tint  is  due  to  the  presence  of  sericite,  which 
forms  a  considerable  part  of  some  of  the  beds,  while  others  con- 
sist of  nearly  white,  pure  quartzite.  The  softer  sericitic  beds  are 
characteristic  of  the  upper  and  lower  parts  of  the  formation  and  are 
in  reality  transition  beds  with  reference  to  underlying  and  overlying 
formations.  The  thick  white  medial  beds  of  the  Revett  quartzite  are 
exposed  chiefly  in  the  eastern  part  of  the  district  near  the  Idaho- 
Montana  divide.  They  contain  the  copper  deposit  of  the  Snowstorm 
mine,  and  surround  Lake  Revett,  whence  the  formation  derives  its 
name. 
The  St.  Regis  formation  consists  of  siliceous  shales  or  argillites, 
shaly  sandstones,  and  impure,  fine-grained  quartzites,  characterized 
throughout  by  features  indicative  of  shallow-water  deposition,  and 
by  rather  bright-green  and  purplish-gray  tints.  An  irregular  slaty 
cleavage  is  fairly  common.  The  formation  is  far  from  uniform  in 
aspect,  however,  and  shows  considerable  lithological  variation  in 
different  parts  of  the  field.     It  contains  beds  not  ordinarily  dist in- 
