BROWN    ORES    OF    VIRGINIA.  191 
Occurrence  and  character  of  the  ores. — The  residue  from  the  decay  of  the  limestone  shows 
considerable  variation  in  depth.  In  places  it  is  so  thin  that  the  limestone  is  exposed  every 
'ew  feet,  while  in  other  places  mining  has  shown  it  to  have  a  depth  of  80  feet.  Where  the 
•esidue  is  protected  by  overlying  strata  from  the  weight  of  other  material,  it  can  frequently 
)e  traced,  stratum  for  stratum,  into  the  limestone.  It  is  perfectly  certain  that  this  material 
las  its  origin  in  the  limestone.  Solution  of  the  limestone  takes  place  very  irregularly,  and 
n  consequence  the  surface  of  the  unaltered  rock  is  very  uneven.  Commonly  these  irregu- 
arities  take  the  form  of  pinnacles  or  domes,  varying  from  those  which  are  small  and  closely 
spaced  to  larger  ones  20  feet  in  diameter  and  50  feet  in  height.  The  residue  lies  between 
/hese  limestone  "horses"  and  forms  a  cover  over  them  from  5  to  20  feet  thick. 
In  this  iron-ore  region  the  residue  contains  a  large  percentage  of  iron  oxide.  Most  of 
his  is  in  the  form  of  fine  particles,  which  give  to  the  residue  its  deep-red  color.  Where  the 
ron  oxide  takes  the  form  of  granules  of  such  size  that  they  will  not  pass  a  screen  which 
las  12  meshes  to  the  inch  and  the  particles  are  present  in  an  appreciable  quantity,  it 
jecomes  an  iron  ore.  The  individual  pieces  of  ore  may  vary  from  the  minimum  size  up  to 
umps  several  feet  in  diameter.  Much  of  the  ore  consists  of  angular  fragments  which  have 
)een  broken  and  moved  from  the  position  in  which  they  were  deposited  from  solution.  A 
smaller  portion  of  the  pieces  of  ore  have  the  form  and  position  which  they  assumed  when 
hey  were  precipitated.  The  form  of  the  pieces  varies  with  the  environment  in  which  the 
)recipitation  occurred.  The  clay  derived  from  the  limestone  is  relatively  pervious.  Ores 
leposited  in  this  material  assume  irregular  shapes  and,  where  the  aggregates  are  of  sufficient 
iize,  are  generally  cellular,  frequently  inclosing  clay  in  the  cavities.  In  a  few  instances 
;he  ore  has  been  precipitated  on  the  surface  of  the  limestone  itself.  In  such  cases  the  ore 
amime  are  one-half  inch  or  less  in  thickness  and  sometimes  form  closed  spaces,  including 
ectangular  blocks  of  limestone  an  inch  or  so  on  a  side.  The  residue  from  shale  is  relatively 
mpervious.  Iron  oxide  precipitated  in  or  above  this  material,  instead  of  having  cavernous 
>r  honeycomb  structure,  is  apt  to  occur  in  sheets  of  compact  material  2  inches  or  less  in 
liickness  or  in  irregular,  compact  masses  of  greater  size.  Not  only  do  these  ores  have  a 
lifferent  physical  appearance,  but  the  impurities  which  they  hold  give  them  a  somewhat 
lifferent  chemical  composition.  The  ores  occurring  in  the  limestone  residue  are  locally 
mown  as  "limonites;"  the  shale  and  sandstone  ores  as  "mountain"  ores.  There  are  a 
ew  mines  whose  ores  have  characteristics  intermediate  between  these  two  classes  and  such 
>res  are  known  as  "semilimonites."  Analyses  furnished  by  the  companies  a  operating 
hese  ores  show  the  following  content: 
Analyses  of  iron  ores  from  New  River-Cripple  Creek  district,  Virginia. 
Limonites.a 
Semi- 
limonites.b 
Mountain 
ores.c 
43.76 
13.52 
1.79 
.17 
.58 
40. 72 
20. 12 
1.81 
.31 
.83 
37.86 
18.60 
Uumina 
2.32 
.40 
Manganese 
1.49 
a  Average  of  56  analyses  from  12  mines. 
b  Average  of  15  analyses  from  5  mines. 
c  Average  of  17  analyses  from  4  mines. 
The  "limonites"  are  relatively  high  in  iron  and  low  in  silica,  phosphorus,  and  manganese, 
rhe  three  first  named  are  fairly  constant  from  mine  to  mine,  while  the  manganese  varies 
)etween  1  and  0.1  per  cent.  The  alumina  values  are  bases  on  few  determinations  These 
values  will  vary  according  to  the  thoroughness  with  which  these  ores  are  washed.  In  the 
'mountain"  ores  there  are  lower  values  in  the  iron  content,  while  the  silica,  phosphorus, 
md  manganese  are  higher.     The  manganese  here  varies  between  wider  limits  than  in  the 
Virginia  Iron,  Coal  and  Coke  Company;  Pulaski  Iron  Company;  New  River  Mineral  Company. 
Bull.  285—06 13 
