134  CONTRIBUTIONS    TO    ECONOMIC1    GEOLOGY,   1906,  PART    I. 
The  questions  of  deepest  importance  to  the  district  are  (1)  whethel 
the  present  quality  of  the  ore  will  be  maintained  to  great  depth,  (2) 
whether  the  present  workable  thickness  will  continue  to  whatever 
depth  it  may  be  possible  to  mine  the  ore,  and  (3)  whether  the  structure1 
of  the  rocks  underlying  Shades  Valley — below  which  the  ore  beds  nor- 
mally lie — will  be  favorable  for  mining  the  ore.  Certain  facts  have 
been  brought  out  during  this  survey  which  have  a  bearing  on  these 
questions.  These  will  be  noted  and  discussed  in  the  complete  paper, 
but  the  nature  of  the  present  paper  precludes  more  than  a  reference 
to  the  general  conclusions. 
CHARACTER    OF    THE    ORES. 
The  ores  consist  essentially  of  red  hematite,  intimately  mixed  witl 
varying  percentages  of  lime  and  silica.     The  hematite  occurs  in  beds- 
interstratified  with  shale  and  sandstone,  and  the  strata  mostly  dip  ar 
angles  varying  from   10°  to  50°.      In  places,  more  particularly   to  tin 
norl  h  of  the  Birmingham  disl  rid  ,  the  Clinton  ore  is  oolil  ic.     'Hi rough 
out  its  extent  some  beds  are  very  fossiliferous,  and  some  of  the  faunJ 
found    in   the  Birmingham  district   are   typical  Clinton   forms  of  til 
New  York  section.      Near  Birmingham  the  ore  beds  are  largely  com 
posed   of   fine  to    coarse    silica    pebbles,    coated    and    cemented    witl 
ferric  oxide.      According  as  t  he  ore  is  high  or  low  in  lime  it    is  terme< 
"hard"  or  "soft  "  ore.      The  distinction  between   the  two   varieties  i 
based  on  differences    in   their  chemical   composition   rather  than  d 
differences    in   hardness,  although    the   terms  "hard"  and  "soft  "a 
originally  applied  to  the  ores  probably  had  reference   to  their  physic* 
condit  ion.  since  on  t  he  outcrop  t  he  soft  ore   is  in  general  rather  porou 
and  friable.      The  unaltered  ore  is  of  the  hard  variety.      The  soft   or 
has  resulted  from  t  he  leaching  by  percolal  ing  waters  of  t  he  soluble  lim 
carbonate  contained  in  the    hard  ore.     This  alteration  occurs  at   th 
outcrop  of  the  ore  beds  and  dow  n  t  he  dip  to  varying  distances,  depenc 
ing  on  t  he  1  hickness  and  permeability  of  t  he  cover.      Where  t  he  st  rat 
dip  at   fairly  high  angles  and  are  underlain   by  impervious  shale,  tl 
conditions  are  favorable  for  the  passage  of  water  through  the  beds  t 
considerable  depths.     In  a  few  places  pockets  of  sof i  ore,  surrounde 
by  u n leached  ore,  are  encountered  at  relatively  long  distances  from  il 
outcrop.      Such  an  occurrence  of  soft  ore  is  usually  due  to  the  preseni 
of  fissures  or  brecciated  rock,  through  which  surface  water  has  reachf 
the  ore  bed.     Where  the  overlying  cover  is  heavy  at   the  mouth  of 
slope  or  tunnel,  the  soft  ore  rarely  extends  more  than  50  feet,  but  he; 
and  there  the  ore  has  been  well  leached  to  a  distance  of  400  feet  fro 
the  outcrop.     With  the  removal  of  the  lime  carbonate  from  the  ori 
inal  ore  the  relative  percentages  of  the  remaining  less  soluble  consti 
uents,  mainly  iron  oxide   and   silica,  are  increased.     The  followii 
