CLINTON    ORES    OF    BIRMINGHAM    DISTRICT,  ALABAMA.  149 
mine  is  remarkably  regular.  The  facts  brought  out  by  this  series  of 
analyses  show  that  the  content  of  metallic  iron  increases  about  1  per 
cent  for  each  1,000  feet  away  from  the  outcrop,  that  the  lime  (CaO) 
decreases  about  1  per  cent  in  the  same  distance,  and  that  the  silica 
content  slightly  increases. 
Studies  by  members  of  the  Alabama  Geological  Survey  extending 
over  many  years  have  shown  that  the  Rockwood  formation  tends  to 
thin  out  and  become  sandier  toward  the  southeast.  There  is  no  rea- 
son why  this  change  should  not  be  shared  proportionately  by  the  in- 
closed ore  beds,  and  it  is  believed  that  the  drill  records  just  referred 
to  indicate  that  such  is  the  case.  However,  the  complete  drill  records 
available  from  the  valley  east  of  Red  Mountain  are  so  few  that 
reliable  conclusions  can  be  based  on  them  regarding  the  ore  basin 
only  in  the  southern  third  of  the  district.  Ore  can,  perhaps,  be 
expected  to  underlie  the  valley  southeast  of  Red  Mountain,  probably 
as  far  as  Shades  Mountain.  The  width  of  Shades  Valley  is  a  rough 
indication  of  the  relative  extent  of  the  Red  Mountain  ore  toward  the 
southeast,  and  the  width  of  the  valley  is  sensibly  greater  southwest 
than  it  is  northeast  of  Reeder  Gap. 
In  reference  to  the  theories  heretofore  advanced  regarding  the  ori- 
gin of  the  Clinton  ores  it  may  be  stated  that  all  the  new  facts  observed 
in  the  course  of  the  work  in  the  Birmingham  district  are  in  accord- 
ance with  the  hypothesis  that  the  ore  is  the  result  of  original  deposi- 
tion of  ferruginous  sediments.  The  transition,  vertically,  between 
sandstone  and  ore  or  between  shale  and  ore,  is  as  sharp  as  that  be- 
tween coal  and  its  inclosing  rocks.  The  variation  in  composition  of 
an  ore  bed  from  place  to  place  is  not  unlike  the  local  changes  in  com- 
position and  character  of  a  coal  bed.  The  lenslike  form  of  the  beds 
is  common  to  both  coal  and  ore.  Finally,  as  the  lens  thins,  whether 
of  coal  or  of  ore,  it  tends  to  become  shaly  and  siliceous.  That  the  ore 
is  due  to  the  replacement  of  limestone  seems  hardly  possible  when 
it  is  considered  that  instead  of  a  decrease  in  percentage  of  iron  and  an 
increase  in  that  of  lime,  with  depth,  until  the  bed  becomes  a  limestone, 
almost  the  reverse  has  been  noted.  The  lime  in  the  bed  is  evidently 
an  accessory  deposit,  as  is  the  silica.  The  term  " depth"  in  this  con- 
nection is  subject  to  misconception,  for  the  sediments  were  deposited 
in  a  horizontal  position,  or  nearly  so,  and  their  present  attitudes  are 
the  result  of  subsequent  foldings.  The  depth  to  which  the  beds  now 
extend  is  therefore  incidental,  and  in  no  wise  affects  their  charac- 
ter beyond  the  soft-ore  limit.  Indeed,  the  best  criterion  for  judg- 
ing the  character  of  the  imexploited  ore  beds  in  the  basin  southeast 
of  Red  Mountain,  is  the  strike  section  of  the  same  beds  that  has  been 
afforded  by  the  mine  workings.  From  northwest  to  southeast  there 
are  likely  to  occur  changes  similar  to  those  that  are  known  to  take 
place  from  northeast  to  southwest.     With  all  these  possibilities  kept 
