CLAY    RESOURCES    OF    ST.  LOUIS    DISTRICT,  MISSOURI.  319 
FLOOR. 
The  floor  of  the  mine  is  generally  the  light-colored  pyritiferous  sand, 
No.  12,  above.  Locally  the  pyrite  is  oxidized  and  the  sand  is  yellow. 
At  some  places,  especially  where  the  clay  seam  is  very  thick,  its  basal 
portion  is  sandy  and  abounds  in  pyrite,  and  is  poorly  distinguished 
from  the  underlying  bed.  At  places,  as  in  the  Christy  mine  and  the 
Evans  and  Howard  No.  11,  this  basal  clay  is  green  and  very  sandy, 
locally  oxidized  to  yellow  and  brown  colors.  At  the  latter  mine  and 
at  the  Parker  and  Russell  mine,  west  of  Kingshighway,  this  basal 
clayey  mass  contains  large  red  cherts,  which  elsewhere  (as  at  the 
Laclede,  Christy,  and  Jamieson-French  mines)  are  found  only  in  the 
underlying  limestone. 
This  limestone  is  reached  in  a  few  of  the  mines  in  digging  sumps, 
into  which  the  mine  is  drained  and  from  which  the  water  is  pumped. 
On  account  of  the  imperviousness  of  the  overlying  beds,  most  mines 
are  practically  dry  until  the  pillars  have  been  drawn  from  some  por- 
tion, thus  allowing  settling  and  cracking  of  the  higher  beds. 
INTERRUPTIONS  OF  THE  CLAY  SEAM. 
The  surface  of  the  floor  is  locally  affected  by  gentle  ridges  or  rolls 
that  rise  from  a  few  inches  to  1  or  2  feet  in  a  width  of  a  few  yards. 
One  such  ridge  in  the  floor  of  the  Jamieson-French  mine  reaches  the 
roof,  cutting  out  the  clay  entirely.  It  is  known  as  a  "whaleback." 
Its  trend  is  northwest-southeast. 
In  several  mines  the  roof  thickens,  at  some  places  its  surface  being 
depressed  almost  or  quite  to  the  floor,  as  in  the  Hume  mine,  south  of 
the  Insane  Asylum,  where  a  band  about  200  feet  wide  trending  north- 
east-southwest is  thus  affected.  In  this  mine,  as  in  a  group  of  mines 
near  the  southeast  corner  of  Forest  Park,  the  lower  surface  of  the 
" horse7'  is  longitudinally  grooved  or  fluted  on  a  large  scale. 
GRADES  OF  CLAY. 
The  clay  seam  is  rarely  of  uniform  quality  throughout  its  thickness. 
Its  usual  variations  are  due  to  varying  proportions  of  silica  and  car- 
bonaceous matter  and  to  the  presence  or  absence  of  pyrite  and  other 
forms  of  iron.  In  the  western  part  of  the  field  pyrite,  the  chief 
impurity,  is  found  mainly  in  the  upper  part  of  the  seam.  The  lower 
clay  is  of  the  highest  grade.  It  is  probable,  however,  that  several 
feet  remaining  as  floor  are  equivalent  to  the  basal  portion  of  the  clay 
bed  in  Cheltenham.  In  that  district  it  is  common  to  regard  the  middle 
of  the  seam  asj^est,  the  top  being  liable  to  contain  carbonaceous  mat- 
ter and  the  basal  portion  being  siliceous.  In  the  southern  part  of  the 
field  it  is  customary  to  speak  of  the  clay  as  upper  and  lower,  the  for- 
mer being  of  the  highest  grade,  the  latter  being  lighter  colored  and 
more  siliceous. 
