CLAYS    AND    SHALES    IN    CAMBRIA    COUNTY,  PA.  345 
In  the  Allegheny  formation  flint  clay  was  observed  in  the  north- 
west section  of  the  area,  in  the  valley  of  Mardis  Run.  It  occurs 
about  25  feet  below  what  is  presumably  the  Upper  Freeport  coal, 
and  hence  may  correspond  with  the  Bolivar  fire  clay  of  the  region 
farther  southwest. 
The  most  important  flint  clay  is  that  occurring  at  the  Mercer  hori- 
zon, in  the  Pottsville  formation,  in  the  South  Fork  district. 
Plastic  clays  have  been  observed  at  many  horizons  in  this  region. 
At  only  a  few  places,  however,  is  the  position  of  the  beds  with  refer- 
ence to  transportation  and  their  thickness  such  as  to  make  them 
of  great  economic  importance.  A  few  unimportant  plastic  clay 
beds  have  been  observed  in  the  Conemaugh  formation.  Many  of 
the  coals  in  the  Allegheny  formation  are  underlain  by  plastic  clays. 
The  clay  below  the  "E"  coal  is  at  some  places  of  workable  thickness, 
though  it  is  not  comparable  with  that  below  the  aB,"  or  Lower 
Kittanning,  or  Miller  seam.  Below  the  limestone  bed  associated  with 
the  Upper  Kittanning  coal  (known  also  as  the  cement  or  "C"  coal)  a 
deposit  of  clay  was  noted  at  some  points  about  Johnstown.  The 
plastic  clay  tying  below  the  Miller  coal  is  the  most  valuable  in  the 
region.  It  varies  from  4  to  12  feet  in  thickness,  ^his  fire  clay  has 
been  extensively  exploited  about  Johnstown  and  to  a  less  extent 
about  South  Fork.  Along  Blacklick  Creek,  in  the  northern  part  of 
the  area,  the  clay  at  this  horizon  is  of  workable  thickness,  but,  so 
far  as  known,  has  never  been  utilized. 
The  lowest  plastic  clay  in  this  region  is  found  at  the  Mercer  horizon, 
though,  as  indicated  above,  a  flint  clay  lies  at  this  horizon  at  some 
places. 
The  valuable  shales  in  this  area  appear  to  be  confined  chiefly  to 
the  Conemaugh  formation,  though  important  beds  may  occur  in  the 
Allegheny  and  Pottsville.  In  the  Johnstown  district  valuable  shale 
beds  occur  in  the  lower  300  feet  of  the  Conemaugh  formation  and  are 
worked  near  the  city.  The  higher  beds  of  this  formation  are  exposed 
in  the  railroad  cuts  west  of  Wilmore,  and  the  many  beds  of  shale 
observed  there  indicate  the  presence  of  good  brick-making  material 
in  the  hills  between  Wilmore  and  Summerhill  along  the  Pennsylvania 
Railroad.  At  the  Mercer  horizon  also  shales  are  associated  with  the 
coal  and  fire  clay. 
At  the  Bruce  H.  Campbell  quarry,  north  of  Sheridan,  a  clay  that 
is  associated  with  bowlders  and  is  presumably  of  Pleistocene  age  is 
worked.  Residual  clays  are  so  widely  distributed  in  this  region  as 
hardly  to  merit  detailed  mention. 
The  clays  and  shales  that  are  now  worked  will  be  described  and  the 
deposits  not  yet  exploited  will  be  noted,  especially  those  that  are 
situated  near  markets  and  lines  of  transportation. 
