334  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  260. 
Cretaceous  clays  there  is  a  vast  amount  of  iron  distributed  in  the 
ferruginous  sandstone,  siliceous  hematite,  and  beds  of  deep-red  sand 
and  loam.  As  these  deposits  are  porous,  it  is  entirely  possible  that 
they  may  have  undergone  comparatively  rapid  leaching  by  waters 
entering  at  the  surface  and  passing  downward  through  them.  As 
the  upper  beds  are  extremely  ferruginous,  it  seems  possible  that  they 
could  have  furnished  sufficient  quantities  of  iron  to  form  the  richer 
deposits  below  without  showing  much  evidence  of  having  beei 
leached. 
If  such  a  circulation  should  take  place,  deposition  of  the  iroi 
would  occur  wherever  favorable  conditions  were  encountered.     The 
solution    evidently   could    pass   through   the   Cretaceous   sands   and 
clays  along  channels,  cracks,  and  other  openings,  but  on   reaching 
the  limestone  or  dolomite  it  would  meet  there  a  relatively  impervious! 
bed  which  would  retard  its  circulation,  and,  if  in  a  nearly  saturated 
condition,  the  solution  would  tend  to  deposit  its  dissolved  material; 
but  the  most  important  factor  influencing  the  deposition  of  the  iron 
salt  probably  would  be  the  limestone  itself.     If  the  iron  traveled  as] 
a  carbonate  in  solution,  contact  with  the  limestone  would  tend  to  pre-] 
cipitate  the  iron  carbonate  while  calcium  carbonate  would  pass  into] 
solution.     This,   in   principle,   would   be   a   process   of   replacement, 
but  as  the  iron  ore  does  not  impregnate  the  limestone  to  a  noticeable 
extent  it  is  possible  that  reaction  would  take  place  by  the  mixing 
of  the  iron  solutions  with  waters  flowing  along  the  limestone  and  con- 
taining  calcium   and    magnesium   carbonates   in   solution.     At   aril 
rate,  a  nucleus  of  iron  ore  being  formed,  it  would  grow  by  accretion. 
The  nuclei  of  the  ore  masses  have  been  deposited  in  the  opening 
in  the  clay,  and  from  these  nuclei  ramifying  masses  have  branchej 
into  the  surrounding  clay.  The  richest  deposits  of  ore  are  fre- 
quently found  directly  on  the  limestone  contact,  and  in  places  where 
the  limestone  surface  is  an  uneven  one  ore  masses  usually  fill  depres- 
sions or  surround  bowlders  of  the  limestone  that  project  above  the  ol( 
ledges.  Good  examples  of  these  latter  conditions  are  found  in  th| 
workings  of  the  Central  Iron  and  Coal  Company  at  Giles. 
The   second   method   of  origin  suggested   for   the  ore  necessarib 
assumes  a  subsequent  alteration  of  iron  carbonate  to  limonite,  but 
this  is  a  characteristic  change  of  the  first  mineral  and  would  perhaps 
take  place  through  the  action  of  oxidizing  solutions  very  soon  after 
precipitation  of  the  carbonate. 
The  evidence  afforded  by  the  deposits  herein  described  points  mosl 
strongly  to  the  second  process  outlined,  and  this  process  appears  to 
contain  the  greatest  number  of  elements  of  possibility;    but  as  only 
a  small  part  of  the  whole  field  has  been  studied  this  is  offered  merely 
as  a  suggestion  at  present. 
