140         CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1906,   PART    T. 
the  partly  open  outer  shucks.  The  ore  is  of  too  low  grade  to  be 
worked  at  present.  The  seam  is  developed  principally  in  the  dis- 
trict between  Birmingham  and  Bessemer,  and  where  recognized  it 
lies  about  12  to  20  feet  above  the  next  lower  sen  in. 
Ida  seam. — This  seam  consists  of  2  to  6  feet  of  rat  her  siliceous  ore 
associated  with  1  1  to  16  feet  of  ferruginous  sandstone.  Ore  at  this 
horizon  is  more  continuous  and  extensive  than  at  the  horizon  of  the 
Hickory  Nut  seam.  It  has  been  recognized  at  many  of  the  working! 
from  Bald  Eagle  Gap  beyond  Clear  Branch  Gap.  Where  worked  the 
seam  is  in  general  from  3  to  5  feel  thick  and  soft  ore  only  has  been 
obtained  from  it  in  surface*  workings.  .Such  ore  carries  35  to  II  pel 
cent  of  metallic  iron,  with  a  corresponding  range  in  silica  of  45  to  31 
pel- cent.  The  Ida  seam  occurs  35  to  50  feel  above  the  top  of  the  Bij 
seam. 
Big  and  Irondali  seams.  These  two  ore  beds  are  considered  togethej 
since  they  are  very  closely  associated  in  space.  The  ore,  however,  is 
no  mew  hat  different  in  quality,  and  the  sea  ins  are  so  sharply  separate! 
by  thin  sandstone  or  shale  that  they  may  be  mined  independently. 
The  thickness  of  the  Big  seam  is  variously  estimated  at  1(>  to  40  l 
feet.  It  extends  as  a  traceable  unit  on  Red  Mountain  practically  thl 
w  hole  lengl  h  of  t  be  mining  dist  rid  .  Not  wit  hstanding  t  he  great  1  hickl 
ness  there  are  rarely  more  than  l(>  to  L2  feel  of  good  ore  in  a  singll 
bench,  and  at  mosl  places  only  7  to  It)  feel  are  mined.  Probably 
the  maximum  thickness  is  attained  between  Red  Gap  (near  Iron- 
dale)  and  Bald  Eagle,  although  for  a  mile  southwest  of  Red  Gaj 
the  bed  remains  nearly  as  thick.  From  northeast  to  south- 
west the  total  thickness  of  the  ore-bearing  sediments  gradually 
decreases,  without,  however,  altering  greatly  the  thickness  of  the 
workable  port  ion.  About  t  he  middle  of  t  he  district  the  seam  become! 
separate!  into  two  benches,  either  bj  a  well-defined  parting  along  thl 
bedding  plane  or  by  a  shale  bed,  t  bin  at  first ,  hut  t  hickening  gradualrl 
to  the  southwest.  The  middle  of  the  Big  seam  is  the  workable  part 
in  the  northeast  end  of  the  district,  but  the  upper  bench  is  of  most 
importance  throughout  the  rest  of  the  area.  In  the  southwest  por- 
tion of  the  district  the  lower  bench,  which  farther  northeast  is  coml 
posed  of  ore  t  hat  will  in  later  years  be  mined,  becomes  a  series  of  thin 
strata  of  lean  ore  and  shale,  and  is  consequently  of  no  possible  value! 
and  finally  the  upper  bench  itself  becomes  shaly  and  carries  only  a 
wry  low-grade  ore. 
The  Irondale  seam  is  best  developed  on  Red  Mountain  between 
Pilot  Knob  on  the  northeast  and  Lone  Pine  Gap  on  the  southwest! 
Soutlrwest  of  Lone  Bine  (lap  the  seam  either  consists  of  interbedded 
Low-grade  iron  ore  and  shale  or  else  its  identity  is  completely  lost. 
Its  soft  ore,  now  nearly  all  mined  out  either  by  surface  trenches  or 
