466  CONTRIBUTIONS   TO    ECONOMIC    GEOLOGY,  1906,  PART    I. 
water's  edge  of  White  River.  A  mile  and  a  half  west  of  Cushman,  on 
the  point  of  the  hill  in  the  northern  part  of  sec.  7,  T.  14  N.,  R.  7  W. 
the  limestone  is  110  feet  thick.  On  the  south  slope  of  Pumpkin 
Branch  it  is  130  feet  thick;  in  the  northwestern  part  of  sec.  1,  T.  14 
N.,  R.  8  W.,  it  is  210  feet  thick.  At  least  50  feet  of  it  is  exposed 
along  the  lower  part  of  West  Lafferty  Creek. 
The  Polk  Bayou  limestone. — This  limestone  immediately  overlies 
the  Izard  limestone.  It  is  everywhere  present  in  the  vicinity  of  the 
developed  deposits,  but  is  not  so  widespread  over  northern  Arkansas 
as  the  Izard  limestone.  At  Phelps  Spring,  just  north  of  Cushman, 
it  is  120  feet  thick.  Both  limestones  arc  exposed  here,  the  spring- 
issuing  from  the  Izard.  At  the  cave  1'.  miles  west  of  Cushman  it  is 
130  feel  t  hick.  ( )ne-half  mile  north  of  the  cave  and  across  the  hollow 
from  it,  in  the  northern  part  of  see.  7,  T.  14  N.,  R.  7  W.,  it  is  75  feet 
thick. 
This  limestone  occurs  in  massive  beds.  In  color  it  varies  from  a 
rather  light  gray  at  the  basal  portion  to  a  brown,  in  some  places  a 
chocolate  color,  at  the  top.  The  texture  is  very  coarse,  the  rock 
being  made  up  of  crinoid  steins  and  other  fossil  fragments,  bound 
together  by  crystals  of  calcite.  Much-weathered  specimens  of  this 
limestone  present  a  spongy  resistance  to  the  blows  of  the  hammer, 
and  break  up  into  coarse,  sandlike  material. 
Tin    ('as, ni    shah.      Within    the   area   examined   by  the  writer   the 
rocks  at  the  horizon  of  the  Cason  shale  are  of  variable  nature.     It  is 
at    this   horizon    that    the    phosphate    rocks    herein   considered   occur. 
Usually  somewhat  more  than  1  feet  at  the  top  consist  of  shale,  or,  in 
the  outer  part,  of  clay  derived  from  the  disintegration  of  the  shale. 
The  upper  part  of  the  shah'  is  yellow  to  brown  in  color;   the  lower  por-  I 
tion,  which  ranges  in  thickness  from  2  to   14  inches,  is  green.     In   j 
some  places,  a-  at   the  phosphate  quarry  later  to  be  described,  the 
shale  contains  a  thin  bed  of  low-grade  iron  ore.      In  other  places  it  is    I 
manganiferous.     Such  samples  as  were  tested  were  found  to  be  phos- 
phatic,  and  the  rock  is  probably  so  everywhere. 
The  phosphate  rocks  of  the  region  are  associated  with  this  bed  of 
shale.  All  of  the  developed  deposits  here  are  below  it,  but  in  other 
places  there  are  phosphatic  beds  above  shale,  which  is  presumably  the  I 
same  as  this.  The  phosphate  beds  vary  in  character,  ranging  from 
those  that  are  brown  and  sandy  and  of  low  grade  to  those  that  on 
fresh  surfaces  are  bluish  gray,  apparently  without  sand,  and  of  uni-  j 
form  texture  and  color.  It  is  the  latter  that  are  worked.  They  will 
be  more  fully  discussed  later  in  the  paper. 
Manganese  ore  is  at  many  places  closely  associated  with  the  phos- 
phate beds.  So  common,  indeed,  is  the  association  of  the  two  that 
debris  of  the  manganese,  which  is  always  plainly  perceptible  where 
present,  is  a  good  guide  in  prospecting  for  phosphate  rock. 
