PHOSPHATE    DEPOSITS    IN    NORTHERN    ARKANSAS.  465 
but  he  did  not  recognize  them  as  phosphate  rocks.  I  saw  and  made  notes  of  several 
of  the  phosphate  localities  in  1890,  but  the  rocks  were  not  recognized  to  be  phosphate 
beds  until  1895,  when  I  had  analyses  made  here  at  Stanford  University.  That  same 
year  I  went  to  Arkansas  *  *  *  and  gathered  the  bulk  of  the  information  that  ena- 
bled me  to  publish  the  paper  brought  out  by  the  American  Institute  of  Mining  Engi- 
neers in  vol.  26  of  their  Transactions. 
What  has  been  written  concerning  the  phosphate  deposits  of  north- 
ern Arkansas  as  such  is  to  be  found  in  the  papers  by  Professor  Branner 
and  by  Professors  Branner  and  Newsom. a  Inasmuch  as  the  present 
paper  deals  with  only  a  limited  portion  of  the  area  over  which  the  phos- 
phates extend,  those  desiring  a  wider  knowledge  of  the  extent  and 
nature  of  the  beds  are  referred  to  these  two  papers. 
GEOLOGY  OF  THE  REGION. 
In  the  area  over  which  the  phosphate  deposits  of  northern  Arkansas 
occur  nothing  but  sedimentary  rock  is  exposed  at  the  surface.  The 
ages,  relations,  and  names  of  the  formations  in  the  eastern  part  of  the 
area  are  given  in  the  following  section: 
General  section  in  phosphate  region  of  northern  Arkansas. 
Carboniferous: 
Boone  chert,  including  St.  Joe  marble. 
Devonian : 
Chattanooga  shale  and  Sylamore  sandstone. 
Silurian : 
St.  Clair  limestone. 
Ordovician: 
Cason  shale. 
Polk  Bayou  limestone. 
Izard  limestone. 
The  above  rocks  will  be  briefly  described,  for  the  purpose  of  assist- 
ing those  who  may  desire  to  prospect  for  phosphate  rock. 
Izard  limestone. — This  limestone,  as  described  by  Dr.  T.  C.  Hop- 
kins,6 uis  a  smooth,  fine-grained,  compact,  homogeneous,  nonfossil- 
iferous,  evenly  bedded  limestone,  breaking  with  a  conchoidal  fracture, 
and  is  mostly  of  a  dark  blue  color,  varying  locally  to  buff,  light  and 
dark  gray,  and  almost  black." 
This  limestone  occupies  the  lower  part  of  the  valleys  in  the  region 
of  the  developed  deposits.  It  constitutes  the  lower  part  of  Penters 
Bluff  and  occurs  along  Lafferty  Creek  and  its  tributaries  and  at  Phelps 
Spring,  a  half  mile  north  of  Cushman.  This  limestone  varies  con- 
siderably in  thickness.  According  to  Mr.  Hopkins,  280  feet c  at  the 
base  of  Penters  Bluff  is  Izard  limestone,  which  reaches  down  to  the 
a  Branner,  J.  C,  The  phosphate  deposits  of  Arkansas:  Trans.  Am.  Inst.  Min.  E'ng.,  vol.  26,  1896,  pp. 
580-598.  See  also  Branner,  J.  C,  and  Newsom,  J.  F.,  The  phosphate  rocks  of  Arkansas:  Bull.  Arkansas 
Agr.  Exp.  Sta.  No.  74,  1902. 
b  Ann.  Kept.  Geol.  Survey  Arkansas  for  1890,  vol.  4,  p.  109. 
cThis  measurement  is  probably  too  great,  according  to  Mr.  E.  O.  Ulrich. 
