BROWN    ORES    OF    VIRGINIA.  193 
abundant  or  on  steep  slopes  where  erosion  was  particularly  rapid,  reduction  and  solution 
coull  not  keep  pace  with  the  erosion  and  the  red  residue  appeared  at  the  surface.  The 
process  outlined  above  resulted  in  a  concentration  of  the  iron,  which  was  removed  from  the 
sphere  of  surface  erosion,  while  the  other  insoluble  substances  were  exposed  to  it. 
The  other  method  of  concentration  in  the  residue  was  mainly  mechanical.  It  is  recog- 
nized by  some  of  the  leading  mining  men  of  the  district  that  areas  where  limestone  sinks 
are  abundant  are  favorable  for  deep  ore  deposits.  When  the  roof  of  a  cavern  in  the  lime- 
stone falls  in  the  residue  follows  and  a  funnel-shaped  depression  is  created.  Into  this  the 
surface  waters  pour  and  remove  in  suspension  the  fine  material  and  leave  the  lump  ore. 
Abundant  waterworn  pebbles  of  a  recrystallized  quartzite  occur  normally  at  the  surface  of 
the  residue.  These  are  found  at  the  bottom  of  the  deepest  mines.  Apparently  in  sonic 
cases  limestone  sinks  have  been  an  important  means  of  concentration  of  the  ore. 
If  it  is  conceded  that  the  "limonite"  ores  have  their  origin  in  the  limestone,  it  is  not  neces- 
sary to  go  beyond  this  source  for  the  origin  of  the  ores  which  are  now  associated  with  the 
shales  and  sandstones.  In  this  case  all  traces  of  the  limestone  except  the  ores  have  been 
removed  and  the  ores  let  down  upon  the  underlying  rocks. 
There  are  other  ores  in  this  region.  On  both  margins  of  the  limestone  small  deposits 
occur  along  the  faults.  The  faults  furnished  a  little  better  channel  for  water  circulation, 
and  the  fault  planes  became  the  locus  of  a  deposit,  but  these  ores  need  not  be  distinguished 
in  origin  from  the  other  ores  with  which  they  are  associated. 
At  Radford  Furnace,  near  the  southeastern  edge  of  this  area,  there  is  a  deposit  of  limonite 
ore  which,  at  one  place  at  least,  is  associated  with  an  iron  carbonate.  This  ore  may  be  the 
oxidized  outcrop  of  a  bedded  carbonate.  The  mines  have  not  been  worked  for  some  time, 
and  the  present  limited  showing  prevents  definite  statements  in  regard  to  this  point. 
In  Draper  Mountain,  on  the  northwestern  border  of  this  area,  five  mines  are  now  in  opera- 
tion. This  mountain  is  made  up  largely  of  Devonian  and  Lower  Carboniferous  sediments, 
chiefly  sandstones  and  shales.  Some  of  these  ores  are  different  in  form,  mode  of  occurrence, 
and  chemical  composition  from  the  other  ores.  The  Lower  Carboniferous  contains  coal 
measures  and  considerable  disseminated  iron.  This  may  be  looked  to  as  the  source  of  the 
iron  ore.  The  iron  was  carried  in  solution  and  deposited  in  favorable  situations,  which 
varied  in  their  nature  in  the  individual  cases. 
Near  Abingdon  are  scattered  deposits  of  ferruginous  limestone,  hematite,  and  magnetite 
which  have  been  mined  in  a  small  way.  The  limestone  as  mined  carried  about  30  per  cent 
of  metallic  iron.  The  deposits  occur  at  the  top  of  the  Cambro-Ordovician  limestone. 
Athens  shales  a  lie  in  synclincs  of  the  limestone.  The  ores  occur  at  the  outcrop  of  the  con- 
tact. Iron  leached  from  the  shale  has  been  precipitated  in  the  limestone  as  hematite  and 
magnetite.  At  one  place  the  limestone  has  been  much  brecciated,  and  here  the  iron  has 
been  irregularly  deposited  in  the  limestone  both  as  ferrous  and  as  ferric  iron.  The  ferrous 
iron  is  clearly  a  replacement  of  the  limestone. 
Summary. — The  iron  ores  of  the  valley  region  of  southwestern  Virginia,  with  one  excep- 
tion, are  limonite.  They  occur  in  the  residual  clay  and  usually  constitute  about  7  per  cent 
of  this  material. 
Classified  according  to  their  probable  origin,  there  are  four  classes  of  ores.  Named  in 
order  of  their  importance  they  are  as  follows: 
1.  The  limonite  ores  of  the  valley,  associated  with  the  limestone.  Iron  originally 
disseminated  in  the  limestone. 
2.  The  limonite  ores  of  Draper  Mountain.  Iron  leached  from  the  Lower  Carboniferous 
sediments. 
3.  The  limonite  ore  at  Radford  Furnace.     Oxidation  in  situ  of  carbonate-  of  iron. 
4.  Hematite,  magnetite,  and  ferruginous  limestone  near  Abingdon.  Iron  leached  from 
Athens  shale. 
Classes  1  and  2  are  mined  at  present,  and  the  average  monthly  production  for  the  last  half 
of  1905  was  20,000  tons  b  for  the  New  River-Cripple  Creek  area. 
"Campbell,  M.  R., Geologic  Atlas  U.  S.,  folio  59,  IT.  S.  Geol.  Survey,  1899. 
''This  figure  is  based  on  records  of  ore  shipments  kindly  furnished  by  the  Norfolk  and  Western 
Railway. 
