smith.]  CEMENT    RESOURCES    OF    ALABAMA.  435 
THE    SELMA    CHALK    OR    ROTTEN   LIMESTONE. 
Geologic  horizon. — The  Cretaceous  system  in  Alabama  is  susceptible 
of  classification  into  four  divisions.  These  are,  in  ascending-  order: 
(1)  The  Tuscaloosa,  a  formation  of  fresh-water  origin,  made  up  in  the 
main  of  sands  and  clay  in  many  alternations.  In  places  the  clays  occur 
in  deposits  of  sufficient  .size  and  of  such  a  degree  of  purity  as  to  make 
them  of  commercial  value.  (2)  The  Eutaw,  which  is  of  marine  origin 
and  composed  of  sands  and  clays  more  or  less  calcareous,  but  nowhere 
showing  beds  of  limestone  properly  so  called.  (3)  The  Selma  chalk, 
which  is  of  marine  origin,  and  is  composed,  in  part  at  least,  of  the 
microscopic  shells  of  Foraminifera.  This  formation,  throughout  the 
western  part  of  the  belt  covered  by  it  in  Alabama,  is  about  1,000  feet 
in  thickness,  and  is  made  up  of  beds  of  chalky  and  more  or  less  argil- 
laceous limestone.  In  a  general  way  it  may  be  said  that  the  lower  and 
upper  thirds  of  the  formation  contain  25  per  cent  or  more  of  clayey 
matters  mixed  with  the  calcareous  material,  while  the  middle  third 
will  hold  less  than  25  per  cent  of  these  clayey  impurities.  (1)  The 
Ripley.  This,  like  the  preceding,  is  a  marine  formation,  in  which, 
generally,  the  calcareous  constituents  predominate,  but  in  places  it 
contains  sandy  and  clayey  beds. 
From  this  summary  it  will  be  seen  that  the  Selma  chalk  is  the  only 
Cretaceous  formation  in  Alabama  which  offers  limestone  in  such 
quantity  and  of  such  composition  as  to  be  fit  for  Portland  cement 
material. 
General  description. — As  has  been  stated  above,  the  Selma  chalk  is 
a  calcareous  formation  throughout  its  entire  thickness  of  about  1,000 
feet.  The  rock,  however,  varies  in  composition  between  somewhat 
wide  limits,  and  taking  account  of  the  composition  we  may  readily 
distinguish  three  divisions  of  it.  The  rock  of  the  upper  or  Portland 
division  is  highly  argillaceous,  holding  25  per  cent  or  more  of  clavey 
matters;  portions  of  it  are  composed  of  calcareous  clays  or  marls 
rather  than  limestone,  and  in  these  beds  are  found  great  numbers  of 
fossils,  mainly  oysters.  Along  Tombigbee  River  these  beds  make  the 
bluffs  from  Paces  Landing  down  nearly  to  Moscow,  and  on  the  Ala- 
bama they  form  the  banks  of  the  river  from  Elm  Bluff  down  to  Old 
Lexington  Landing.  The  strata  exhibited  in  these  bluffs  consist  of 
dark-colored,  fossiliferous,  calcareous  clays  alternating  with  lighter- 
colored  and  somewhat  more  indurated  ledges  of  purer,  less  argillaceous 
rock.  At  Elm  Bluff,  which  is  about  125  feet  high,  the  upper  half  of 
the  bluff  is  of  this  character.  The  lower  half  of  the  bluff  is  composed 
of  rock  more  uniform  in  composition  and  freer  from  clay,  and  is  the 
top  of  the  middle  part  of  the  Selma  formation  (the  Demopolis  division), 
which  is  made  up  of  limestone  of  more  uniform  character,  containing 
generally  less  than  25  per  cent  of  clayey  material. 
