66  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  L'tiu. 
beds,  and  it  can  not  be  proved  that  any  two  of  the  ore  bodies  in  them 
occur  in  the  same  bed.  This  may  readily  be  understood  from  the 
geological  section,  fig.  1  (p.  56),  although  even  here  it  has  not  been 
possible  to  represent  all  of  the  thinner  quartzite  beds. 
The  Dakota  quartzite,  in  which  the  majority  of  occurrences  are 
found,  is  a  dense  white  quartzite,  varying  from  25  to  100  feet  in  thick- 
ness and  containing  intercalated  beds  of  shale,  usually  at  the  center, 
but  not  constant  either  in  position  or  amount.  In  the  upper  portions 
it  becomes  finer  grained,  the  bedding  planes  are  closer  together,  fine 
shaly  partings  are  observable,  and  it  is  slightly  blackish,  owing  to 
included  carbonaceous  material.  Above  it  is  a  series  of  black  shales 
extremely  thin  bedded,  and  so  highly  charged  with  coaly  matter  as  to 
strongly  resemble  those  which  are  usually  associated  with  coal.  It  is 
underlain  by  the  greenish  shales  of  the  McElmo,  and,  owing  to  the 
comparative  ease  with  which  these  are  disintegrated  and  worn  away, 
it  usually  forms  an  abrupt  cliiT.  It  outcrops  on  either  side  of  the 
Uncompahgre  Canyon,  about  1,800  feet  above  the  bottom  of  the  val- 
ley, and  follows  the  indentations  made  by  the  steep  gulches  on  either 
side. 
Distribution. — The  ores  are  widely  distributed  along  the  outcrop 
of  the  formation.  Among  the  mines  and  prospects  are  the  America! 
Nettie  mine,  the  Valle}^  View,  Rock  of  Ages,  and  Stenographer.  Ir 
form,  mode  of  occurrence,  and  contained  values  all  of  these  deposit; 
are  precisely  similar,  differing  only  in  unimportant  details.  In  bu 
one  mine,  however,  have  the  ore  bodies  yet  proved  sufficiently  exten 
sive  to  be  of  commercial  value.  This  is  the  American  Nettie  mine 
The  general  description  of  this  class  of  ores  would  therefore  best  b 
accomplished  by  a  description  of  this  mine. 
The  American  Nettie  mine. — The  openings  of  this  mine  are  locate* 
near  the  lower  portion  of  the  steep   cliff  which  forms  the  top  o 
the  canyon  wall  on  the  east  side  of  the  Uncompahgre,  1,800  feet  abov 
the  bottom  of  the  valley.     The  buildings  which  have  been  erect© 
near  the  entrance  are  about  100  feet  above  the  base  of  the  perpendicv 
lar  bluff,  and  so  little  support  does  the  rock  face  afford  that  they  ar  j 
partly  kept  in  place  by  wooden  brackets,  and  except  for  their  moder  i 
construction  might  well  pass  for  cliff  dwellings.     The  position  0(  j 
cupied  by  the  country  rocks  may  most  readily  be  understood  fro] 
fig.  4.     Above  the  mine  is  an  immense  sheet  of  quartz-monzonite-pol 
phyry,  aggregating  about  500  feet  in  thickness.     Below  this  are  -I 
to  50  feet  of  fine  black  shales,  very  heavily  charged  with  carbonaceoi  | 
matter  and  containing  a  few  thin  beds  of  black  sandstone. 
Conformably  below  these  shales  is  the  ore-bearing  quartzite! 
massive  white  rock,  fine  grained  and  blackish  at  the  top  and  white  ai 
more  coarse  grained  below.  It  is  separated  into  two  portions  som 
what  below  the  middle  by  a  thin  parting  of  light-colored  shales. 
