472  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   J903.  [bull. 225. 
Beaver  River  and  west  on  Ohio  River.  The  coal  above  this  clay 
is  variously  known  by  local  names,  such  as  ''sulphur  vein"  and 
"blacksmith  vein." 
Darlington  clay. — This  occurs  from  25  to  35  feet  above  the  Lower 
Kittanning  coal  and  is  persistent  throughout  the  region.  The  clay, 
however,  is  rarely  worked,  containing,  it  is  said,  too  many  iron  nodules 
for  most  wares.  The  clay  bed  is  from  4  to  6  feet  thick,  and  in  1902 
was  being  opened  by  Dando  &  McLain  at  Merrill,  by  E.  Smith,  and 
by  the  American  Sewer  Pipe  Company,  on  Blockhouse  Run.  No 
analyses  are  available.  This  clay  does  not  seem  to  be  in  general  use 
and  its  quality  can  only  be  inferred. 
Lower  Freeport  clay. — The  coal  of  this  horizon  is  usually  absent  in 
Beaver  County  and  the  underlying  clay  is  equally  variable  in  occur- 
rence. When  present  the  clay  has  a  thickness  of  from  3  to  5  feet. 
At  one  place  on  Blockhouse  Run,  where  it  has  been  developed  for 
sewer-pipe,  it  is  reported  to  be  locally  12  to  14  feet  thick.  With  this 
exception  it  has  not  been  exploited  within  this  region,  perhaps  because 
it  contains,  as  it  is  said,  too  much  iron  to  be  of  high  grade.  Wherever 
it  has  been  observed  by  the  writer,  it  is  a  light-colored,  plastic  clay  of 
good  character.  Occurring  about  120  feet  above  the  Lower  Kittan- 
ning coal,  the  Lower  Freeport  clay  has,  of  course,  an  exposure  of 
much  greater  extent  than  the  former  along  the  streams  noted  under 
Lower  Kittanning  clay. 
Upper  Freeport  clay. — This  horizon  is  about  180  feet  above  the 
Lower  Kittanning  clay,  and  therefore  has  a  very  much  greater  line  of 
exposure  near  the  top  of  the  irregular  river  bluffs  and  along  all  the 
lateral  streams.  For  example,  it  dips  under  Raccoon  Creek  near  Inde- 
pendence, under  Mill  Creek  at  Hookstown,  under  Dry  Run  and  Island 
Run  near  Ohioville,  and  under  Bradys  Run  near  Blackhawk.  The  high 
elevation  of  the  Lower  and  Upper  Freeport  clays  is  a  drawback  to 
their  economical  exploitation.  As  a  clay  bed  of  greater  thickness  and 
superior  quality,  the  Lower  Kittanning  is  much  more  accessible.  The 
Upper  Freeport  clay  bed  is  more  persistent  than  the  coal  above  it. 
Very  often  the  clay  is  present  as  a  pale-blue  clay  of  excellent  appear- 
ance with  thin,  papery  layers  of  bituminous  matter  at  the  top  repre- 
senting the  coal.  In  many  places,  however,  the  clay  contains  nodules 
of  iron,  which  stain  it  considerably  and  which  must  be  removed  before 
using.  Usually  the  clay  is  3  to  5  feet  thick,  but  opposite  Fallston,  on 
the  south  side  of  Bradys  Run,  it  thickens  locally  to  10  feet.  Here  it 
is  mined  by  the  Fallston  Fire  Brick  Company,  and  mixed  with  the 
Lower  Kittanning  clay  to  make  different  grades  of  brick.  Aside  from 
this  instance,  the  Upper  Freeport  clay  has  not  been  exploited  as  far  as 
is  known  along  Ohio  or  Beaver  rivers  in  this  county,  but  in  Big  Beaver 
Township  it  has  been  extensively  mined  for  fire  brick.     There  it  ranges 
