472  (  oXTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   L906,   PART    1. 
rial,  a  character  that  was  probably  produced  by  wave  action  alon^ 
shore,  which  ground  up  the  shells  or  the  bones,  as  they  may  hav< 
been.  It  would  seem  that  the  droppings  of  marine  animals,  as  abovj 
suggested,  might  account  in  part  for  the  phosphatic  nature  of  th< 
beds,  but  the  presence  of  so  large  an  amount  of  organic  fragment] 
suggests  that  wave  action  was  probably  its  chief  cause.  The  exact 
character  of  these  organic  fragments  can  not  yet  be  stated.  Although 
they  appear  to  be  fragments  of  bones'  the  probability  that  they  are 
such  is  much  reduced  by  the  fact  that  beds  are  placed  (at  least  ten- 
tatively) in  the  Ordovieian,  and  the  probability  that  they  are  Ordo- 
vician  forces  one  to  the  conclusion  thai  the  fragments  are  more  likely 
those  of  the  tests  of  Crustacea,  which  are  known  to  be  phosphatic. 
AGE    OF    THE    PHOSPHATES. 
The  phosphates  of  northern  Arkansas  have  heretofore  been  con- 
sidered as  <>|"  Devonian  age,  though  Professors  Brainier  and  Newsom 
suggest  that  phosphates  of  earlier  age  may  occur.a  The  lithologic 
character  of  the  limestone  above  the  phosphate  beds  on  LaflVrty 
Creek  and  its  tributaries  led  to  the  supposition  that  this  is  the  St. 
Clair  limestone3  which  i^  of  Silurian  age.  Fossils  collected  from  this 
bed  and  sent  to  Mr.  E.  ().  Ulrich,  of  the  Tinted  States  Geologica 
Survey,  confirmed  this  supposition.  Fortunately,  Mr.  Ulrich  had! 
already  visited  the  locality  himself,  which  makes  his  determination 
of  the  beds  all  the  more  reliable.  Two  lots  of  fossils  from  the  lime- 
stone above  the  phosphate  beds  at  two  different  places  were  sent  to 
Mi-.  Ulrich,  and  in  a  letter  concerning  them  he  says: 
Both  l"i-  of  fossils  are  unquestionably  indicative  of  the  Si.  ('lair  limestone.  The 
-(Hii,-  are  mostl)  differenl  in  the  two  lots,  bul  all  are  of  Silurian  types  known  to 
occur  in  the  Si .  <  'lair. 
The  phosphate  along  Lafferty  Creek  occurs,  judging  from  your  evidence  and  my 
own  observations,  in  the  equivalent  of  the  Cason  -hale,  which  elsewhere  in  the 
vicinity  of  Batesville  contains  the  mangam  h  was  only  last  year  thai 
I  had  an  opportunity  to  satisfy  myself  that  the  phosphatic  deposits  being  worked 
along  the  east  bank  of  Lafferty  Creek  were  undoubtedly  in  shale  intervening  between 
the  Polk  Bayou  and  the  St.  Clair  limestone. 
The  Cason  -hale  is  considered  by  Mr.  [Jlrich  as  of  Ordovieian  age. 
Certainly  the  phosphate  beds  are  not  younger  than  Silurian. 
PROSPECTING    FOR    PHOSPHATES. 
As  the  phosphate  rocks  of  northern  Arkansas  are  usually  covered 
by  soil  where  they  outcrop  on  the  hillsides,  a  few  suggestions  to 
prospectors  in  search  of  these  beds  may  be  of  advantage.  As  one 
passes  up  the  hillsides  of  the  deeper  valleys  in  western  Independence 
County  he  goes  first  over  a  compact,  gray  to  dove  colored,   brittle 
a  The  phosphate  rocks  of  Arkansas:  Bull.  Arkansas  Exp.  Station  No.  74,  pp.  66,  67,  69. 
