332  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    L906,    PART    I. 
without  cracking  when  being  burned.  Other  clays  sufficiently  frel 
from  iron  and  manganese  not  to  burn  red  must  be  mixed  with  it  to- 
give  it  cohesiveness.  Purple  clays  from  the  sand  mines  at  Mount 
Holly  Springs  and  black  clays  from  the  northeast  side  of  the  gap  are 
used  to  a  small  extent,  but  a  large  part  of  the  binder  comes  from  New 
Jersey  and  Maryland.  The  color  of  the  black  clay  is  due  to  organic 
substances,  which  burn  oil'  in  the  kiln.  Because  of  the  siliceous  char- 
acter of  the  crude  white  clay  additional  sand  is  not  needed,  but  a 
certain  per  cent  (A'  grog  (ground  brick)  i-  added. 
The  material  is  first  ground  in  a  dry  pan,  an  iron  cylinder  with  two 
large  iron  wheels  or  mulls,  which  revolve  w  hen  t  he  hot  torn  of  t  he  cylin- 
der i-  rotated.  The  powdered  material  is  conveyed  from  the  bottom 
by  pan  elevators  to  a  revolving  sieve,  from  which  the  tailings  auto- 
matically return  to  the  dry  pan.  The  screened  mixture  passes  to  the 
temperer  or  pug,  a  horizontal  rotating  mixer,  where  water  in  i  he  form 
of  -team  i-  added,  and  i-  then  delivered  by  gravity  to  the  top  of  a 
brick  machine,  in  which  2  vertical  and  2  horizontal  augers  thoroughly 
mix  the  clay  and  destroy  grain.  'The  brick  column  is  conveyed  by  a 
continuous  bell  to  the  cutter,  the  -peed  of  which  i^  automatically, 
regulated  by  the  speed  of  the  belt.  Fifteen  bricks  are  cut  at  a  time, 
and  are  separated  by  passing  onto  ,-i  belt  moving  with  greater  speed. 
They  are  U^\  by  hand  into  two  double  pressing  machines,  where  they 
are  shaped  under  light  pressure  and  stamped  "Mt.  Holly."  If  brick) 
are  delivered  too  rapidly  to  be  pressed,  t  he  surplus  are  returned  to  the 
pug  by  elevator  and  belt . 
The  brick-  are  dried  in  a  L2-track  steam  drier  128  feel  long.  The 
cars,  which  carrj  t40  bricks  each,  are  run  into  the  drier  at  one  end  and 
removed  at  the  opposite  end.  taking  about  two  days  in  the  passage! 
The  temperature  is  140°  at  the  ends.  The  bricks  are  finally  fired  in 
down-drafl  kilns  with  large  smokestacks  al  the  ends.  The  furnaces] 
with  small  smoke-tack-,  are  on  the  sides.  Five  kilns  wore  in  opera- 
tion, the  largesl  of  188,000  capacity,  with  a  total  capacity  of  600,000 
per  month  and  an  output  of  between  100,000  and  500,000  a  month. 
The  kiln  i-  fired  to  cone  8  (2,354  I  in  the  upper  part,  and  the  burn  is 
completed  in  about  a  week. 
Laurel  Forgt  and  Pirn  Grovi  Furnace.  At  Laurel  Forge,  1  mile 
above  1  Ienry  Clay,  pure  white  clay  occurs  in  abandoned  iron  pits,  w  here 
it  is  now  mined  on  a  small  scale  and  used  in  the  manufact  lire  of  brick 
by  t  he  Fuller  Brick  and  Slate  ( Jompany,  of  Pine  ( rrove  Furnace,  2  miles 
farther  up  the  valley.  It  i-  mixed  with  pale-green  sericite  schist, 
locally  called  ^soapstone,"  for  the  manufact  ure  of  light-colored  pressed 
brick.  Red  pressed  brick-  are  now  being  made  from  local  vv(\  clay 
and  black  .slate  by  the  South  Mountain  Mining  and  Iron  Company, 
Allen   Butler,    superintendent.      The   bricks    are   dry   pressed,    and 
