buechaed.]      TRO^    ORES    IN    BROOK  WOOD    QUADRANGLE,    ALA.  333 
Possible  origin  of  the  brown  ore. — As  this  ore  occurs  near  the 
contact  of  limestone  or  dolomite  \\  i  1  li  later  unconsolidated  deposits, 
and  apparently  requires  the  presence  of  both  classes  of  rock  for  its 
deposition,  the  question  arises  as  to  what  part  each  class  of  rock 
has  played  in  its  origin.  It  has  been  generally  considered  that  the 
iron  was  derived  from  the  Cambrian  or  Ordovician  limestone  and 
that  it  has  been  segregated  in  residual  clays  resulting  from  the 
decomposition  of  the  limestone.  This  is  perhaps  the  case  with  strati- 
fied limonite  that  is  the  result  of  the  alteration  of  pyrites,  or  of 
carbonate  ores  originally  disseminated  through  the  limestone,  but 
this  class  of  ore  is  not  represented  among  the  deposits  under  con- 
sideration. There  are  two  hypotheses  which  may  contain  elements 
of  a  possible  explanation  of  the  origin  of  the  ore.  The  first  is  that 
the  ore  may  have  been  derived  from  the  limestone.  That  the  lime- 
stone or  dolomite  was  essential  to  the  formation  of  the  ore  is  indi- 
cated by  the  fact  that  where  the  later  unconsolidated  deposits  lie 
over  other  rocks — as,  for  instance.  Coal  Measures  sandstone  and 
shale — no  such  bodies  of  ore  are  present  near  the  contact.  Analyses 
showT  that  the  limestone  and  the  dolomite  of  the  valley  both  contain 
small  percentages  of  iron.  If  the  ore  has  come  from  the  limestone 
and  dolomite,  an  enormous  mass  of  rock  must  have  been  involved, 
and,  likewise,  a  long  geologic  period  would  have  been  required  for 
the  collection  of  the  iron.  This,  it  seems,  would  preclude  the  possi- 
bility of  the  ore  being  of  post-Cretaceous  age. 
The  second  hypothesis  is  that  the  ore  has  been  leached  from  post- 
Cretaceous  beds  of  sand  ami  loam  by  percolating  water,  aided  by 
organic  acids,  and  carried  downward  into  the  beds  of  sandy  clay  and 
precipitated  near  the  clay-limestone  contact.  Observations  have 
shown  that  all  the  extensive  workings  in  Roups  Valley  northeast- 
ward from  Woodstock  to  beyond  Goethite  are  at  the  base  of  uncon- 
solidated Cretaceous  clays  and  later  loams  overlying  Paleozoic  lime- 
stone or  dolomite.  When  the  northeastern  limit  of  post-Paleozoic 
^deposits  is  passed  no  further  ore  deposits  of  the  same  class  are  found 
pn  the  quadrangle,  although  the  limestone  and  dolomite  still  floor  the 
valley.  From  this  it  may  be  construed  that  the  unconsolidated  de- 
posits are  also  essential  factors  in  the  formation  of  the  ore.  The  <>re 
deposits  themselves  are  situated  in  the  clay,  in  cavities,  openings,  arid 
[channels,  which  have  apparently  afforded  passageways  for  waters 
from  the  overlying  beds.  The  nature  of  the  ore  bodies  and  their 
[relations  to  the  clays  suggest  that  they  may  be  post-Cretaceous  and, 
indeed,  of  rather  recent  origin;  also  that  they  have  grown  by  accre- 
tions added  by  waters  entering  the  clay  from  above. 
It  is  believed  that  the  iron  could  have  come  from  post-Cretaceous 
deposits,    for   in   all   the   unconsolidated   deposits   lying   above   the 
