318  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906,  PART    I. 
Beneath  the  good  fire  clay  are  several  feet  of  green  or  iron-stained 
clay  not  mined.  Beneath  this  is  pyritiferous  sandstone  of  unknown 
thickness,  probably  at  least  6  feet.  At  an  outcrop  one-half  mile  to 
the  south  this  sandstone  is  underlain  by  limestone  containing  large 
red  cherts. 
Section  at  Krilmmel  and  Buchner  mine,  Columbia  avenue  and  Woods  street. 
Ft.  in. 
Soil,  loess,  etc 38  0 
Limestone 4  0 
Coal 2  6 
Very  hard  '•blue  rock" 8  0 
Clay,  not  refractory L0  0 
Blue  shale  and  "keel" 15  0 
Dark  shaly  sandstone  "roof " 2  6 
Fire  clay,  Cheltenham  scam 7  0 
Light  pyritiferous  sand  floor. 
Depth  to  floor 87     0 
THICKNESS  OF  FIRE  CLAY. 
The  thickness  of  the  main  or  Cheltenham  seam  may  vary  from  1  to 
12  feet  within  a  small  fraction  of  a  mile;  it  rarely  if  ever  pinches  out 
entirely.  Most  of  the  mining  is  done  where  the  thickness  ranges  from 
3  to  8  feet ;  where  it  exceeds  that  amount,  as  at  the  northeast  corner 
of  Cheltenham,  the  mining  is  generally  limited  to  about  8  feet  from 
the  best  part  of  the  seam.  Near  the  center  of  the  area,  which  is  near 
the  Laclede  mine  or  within  1  mile  to  the  south  of  it,  the  seam  gener- 
ally ranges  about  7  feet  in  thickness,  all  of  this  being  good  clay  and 
taken  out.  The  seam  is  mined  at  places  where  it  is  only  2\  feet  thick. 
The  common  belief  is  that  where  the  seam  outcrops  it  is  increased  in 
t  hickness  by  weathering.  Along  the  Des  Peres  River  through  Chelten- 
ham its  thickness  is  perhaps  1  foot,  being  from  12  to  15  per  cent  of  the 
original  thickness. 
ROOF. 
The  variable  thickness  and  changeable  character  of  most  of  the  beds 
above  the  clay  arc  unimportant,  but  the  character  of  the  roof  is  a  matter 
of  much  concern.  Bed  No.  8  in  the  above  general  section  is  the  bed 
which  in  all  mines  supports  the  overburden  and  under  which  all  posts 
must  be  set.  This  rock  is  generally  sufficiently  strong  to  span  all 
entries  and  rooms,  but  breaks  off  short  and  falls,  allowing  the  over- 
burden to  fall  in  behind  the  line  of  supporting  posts  when  the  pillars 
have  been  mined  out.  The  mining  is  planned  with  this  fact  in  view. 
The  roof  is  more  apt  to  be  too  strong  than  too  weak.  In  that  case  it 
fails  to  break  short  just  behind  the  row  of  posts,  but  sinks  down  over 
a  large  area,  squeezing  the  unmined  clay  and  probably  breaking  the 
posts. 
