PHOSPHATE    DEPOSITS    IN    NORTHERN    ARKANSAS.  473 
J  limestone,  that  breaks  easily  under  the  blows  of  the  hammer.     This 
lis  the  Izard  limestone.     Above  this  is  a  coarsely  crystalline  lime- 
I stone,  light  gray  at  the  bottom,  but  growing  darker  toward  the  top, 
I  until  the  upper  portion  is  at  some  places  almost  chocolate  colored. 
J  This  is  the  Polk  Bayou  limestone.     It  is  at  the  top  of  this  limestone 
I  that   the  phosphate  of  the  locality  occurs.     In  case  the  St.   Clair 
limestone  is  present  it  will  be  found  above  the  phosphate;  if  not, 
the  Boone  chert  (possibly  the  St.  Joe  marble)  will  be  found  above  it. 
It  is  useless  to  look  for  phosphate  above  the  base  of  the  Boone  chert 
|or  below  the  top  of  the  Polk  Bayou  limestone.     In  case  the  rocks 
on  the  hillsides  are  all  hidden,  the  position  of  the  phosphate  beds 
may  be  determined  approximately  by  examining  the  debris  of  the 
surface.     At   many  places   there   are  fragments   of   dull-gray  rocks 
'  that  look  like  sandstone,  but  these  on  being  broken  are  yellow  on 
the   fresh   surfaces.     These   are  fragments   of   the   phosphate   rock. 
Fragments  of  manganese  ore,  which  are  easily  recognized  and  always 
conspicuous  when  present,   are  good  indications   of  the  phosphate 
horizon,   as  the  two  are  closely  associated.     Of  course  it  must  be 
remembered  that  loose  material  works  its  way  downhill,  so  that  only 
the  upper  limit  of  the  material  here  described  marks  the  position  of 
the  phosphate  beds. 
SUMMARY  AND   CONCLUSION. 
While  it  is  known  that  there  is  a  phosphatic  horizon  of  wide  extent 
in  northern  Arkansas,  the  deposits  have  been  developed  at  only  one 
place,  viz,  on  Lafferty  Creek,  in  the  western  part  of  Independence 
County. 
The  geological  formations  of  the  vicinity  of  the  developed  deposits 
from  below  upward  are:  The  Izard  limestone,  the  Polk  Bayou  lime- 
stone, the  Cason  shale,  the  St.  Clair  limestone,  the  St.  Joe  marble, 
and  the  Boone  chert.  The  developed  deposits  occur  between  the 
Polk  Bayou  limestone  and  the  St.  Clair  marble,  consequently  at  the 
horizon  of  the  Cason  shale,  which  is  thought  to  be  of  Ordovician  age. 
The  phosphate  rock  is  of  sedimentary  origin,  and  where  developed 
is  light-gray,  homogeneous,  and  conglomeratic,  the  pebbles  being  the 
size  of  peas  and  smaller. 
The  beds  probably  were  laid  down  near  shore  as  the  sea  advanced 
landward.  Their  phosphatic  nature  is  thought  to  be  due  mainly  to 
the  fragments  of  organic  matter  that  constitute  so  large  a  portion  of 
their  mass,  though  it  may  be  due  in  part  to  the  droppings  of  marine 
animals. 
