192  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    L905. 
"limonites."  The  "limonites"  arc  throughout  the  more  desirable  ores.  The  ratio  of  the 
amount  of  ore  to  the  total  material  of  the  hunk  varies  widely.  In  some  of  the  ''  mountain! 
ore  mines  the  ore  constitutes  the  larger  portion  of  the  material.  In  the  "limonite"  mines 
the  ore  content  of  the  bank  varies  from  20  to  1  per  cent.  In  other  words,  it  is  necessary  to 
handle  from  5  to  25  yards  of  hank  to  get  1  yard  of  ore.  Sampl<  s  taken  by  the  writer  from 
20  mines  show  an  average  content  of  iron,  computed  to  ferric  oxide,  of  14.;".*)  percent,  with 
a  variation  between  03  and  23  per  cent  .<>  These  samples  were  washed  by  hand  and  the 
tine  material,  which  was  removed  in  suspension,  showed  an  average  of  30  per  cent  ferric 
oxide. a 
Origin  of  tin  ores.  It  seems  probable  that  these  ores  arc  concentrations  of  the  iron  which 
was  originally  disseminated  in  the  Cambro-Ordovician  limestone.  It  has  been  previous!! 
recognized  &  that  the  ores  are  associated  with  the  lower  measures  of  this  limestone.  In 
order  to  determine  the  possibility  of  the  limestone  being  the  source  of  the  iron,  i.  was  sam- 
pled in  the  immediate'  vicinity  of  a  producing  mine,  lit)  samples  being  taken  through  a 
vertical  thickness  of  600  feel  of  the  lower  measures  of  the  limestone.  The  average  of  the 
analyses  c  of  these  gave  the  follow  ing  results  for  the  more  insoluble  constituents  of  the  rock: 
Silica,  1 .74  per  cent ;  metallic  iron,  0.68  per  cent  :  alumina,  0.50  per  cent . 
The  following  table  shows  a  comparison  between  these  analyses  and  that  of  the  residue 
in  the  mine,  including  the  ore.  The  percentages  of  the  limestone  analyses  are  reduced  to 
the  basis  of  100. 
( 'om  parativi  analyses  of  limestom  and  residm  /rout  X<ir  Rlr< -r-Crijtjih  Creek  mines,  Virginia. 
Ferric  oxide. 
Silica 
Alumina 
Insoluble 
matter  in 
600  feet  of 
limestone. 
Residua 
The  above  comparison  shows  that,  with  the  per  cent  of  iron  in  the  limestone  as  indicated 
by  these  analyses,  a  thickness  of  600  feet  of  the  limestone  will  contain  a  quantity  of  iron 
equal  to  thai  in  the  mine,  the  latter  being  estimated  from  the  above  analyses  of  the  residue 
and  from  its  thickness.  Asint  his  case  several  times  600  feet  of  limestone  has  been  weathered, 
it  is  unnecessary  to  look  farther  for  the  source  of  the  iron.  Systematic  sampling  of  the 
limestone  has  not  been  done  over  the  region,  hut  scattered  analyses  indicate  similar  condi- 
tions in  regard  to  the  iron  content.  In  older  to  make  a  comparison  wit  h  a  region  not  ion- 
bearing  samples  were  taken  through  about  300  feet  of  limestone  at  Staunton,  \  a.  The 
analysis  of  this  material  shows  that  ferric  oxide  here  constitutes  7  per  cent  of  (he  insoluble 
portion  of  the  limestone.  This  corresponds  to  t  he  30  per  cent  given  for  the  Poster  Fall 
region,  which  is  iron  hearing.  The  average  content  in  ferric  iron  of  twelve  "limonite! 
mines  is  found  to  be  39  per  cent.     This  is  somewhat  higher  than  at  Foster  Falls. 
It  is  evident  that  there  has  been  concent  i at  i<  n  of  the  ii<  n  in  the  residue  by  at  least  two 
methods.  The  first  is  essentially  chemical.  The  body  of  the  residue  has  a  deep-red  color, 
while  the  upper  two  feet  are  usually  dark  colored,  but  not  red.  This  difference  in  color  is 
believed  to  be  due  to  the  loss  of  iron  by  the  upper  portion.  The  reducing  action  of  plant 
roots  changed  the  ferric  oxide  to  the  ferrous  condition  and  tendered  it  soluble.  Percolating 
waters  carried  this  down  a  few  feet  to  a  point  where  it  was  oxidized  and  precipitated! 
Erosion  removed  the  leached  portion  until  the  ferric  oxide  was  again  within  reach  of  the 
influence  of  vegetation.     The  process  was  repeated  many  tunes.      Where  iron  was  especially 
a  Analyses  by  .1.  II.  Gibboney. 
'■  McCreath,  A.  S.,  and  d'lnvilliers,  E.  V.,  The  New  River-Cripple  Creek  Mineral  Region;  a  report  to 
the  president  of  the  Norfolk  and  Western  Railway,  1887. 
c  Analyses  by  J.  Jl.  Gibboney. 
