SHERIDAN    COAL   FIELD,    WYOMING.  127 
The  formation  that  immediately  underlies  the  coal-bearing  rocks 
consists  of  a  group  of  thick  beds  of  drab  to  brown  sandstone  that  are 
separated  by  blue  clay  and  brownish  or  black  carbonaceous  shale 
layers.  The  carbonaceous  shale  contains  a  few  thin  seams  of  coal,  but 
no  coal  of  any  commercial  value  is  known  to  occur  in  the  formation. 
This  has  been  named  the  Piney  formation  in  the  Bald  Mountain-Day- 
ton folio.  Southward  from  Tongue  River  the  outcrops  become  gradu- 
ally obscure,  until  but  little  can  be  seen  of  them  on  Goose  Creek  near 
the  southwest  corner  of  the  area  mapped.  The  obscure  exposures 
are  evidently  due  to  the  gravel  that  has  been  spread  over  much  of 
the  upland. 
This  formation  has  been  correlated  a  locally  with  the  Fox  Hills 
sandstone,  of  known  Cretaceous  age,  and  referred  to  as  a  definite 
marker  below  the  coal.  Its  exact  age  is  not  yet  known,  and  it  is  too 
far  beneath  the  coal  beds  to  be  of  any  economic  value  as  a  datum  for 
reference. 
COAL-BEARING   ROCKS. 
The  rocks  that  contain  the  coal  in  the  Sheridan  field  consist  of 
comparatively  soft  shale  and  sandstone,  alternately  stratified  and  in 
apparently  conformable  succession.  The  shales  vary  widely  in  com- 
position from  clay  through  carbonaceous  clay  to  coaly  shale  or  bone, 
on  the  one  hand,  and  from  clay  to  sandy  clay  and  shaly  sand  on  the 
other.  The  more  clayey  shale  is,  as  a  rule,  very  slightly,  if  at  all, 
indurated,  except  where  the  burning  of  coal  beds  has  changed  its  tex- 
ture. The  sandstone  and  shaly  sandstone  strata  are  generally  friable 
or  consist  of  only  partially  consolidated  sand  that  breaks  down  readily 
into  loose  sand  on  exposure.  Here  and  there  in  the  section,  however, 
there  are  certain  sandstone  beds  that  have  been  changed  to  stony 
hardness,  and  weather  out  in  projecting  ledges.  Such  indurated  beds 
are  more  or  less  local,  and  no  individual  sandstone  formation  is  trace- 
able by  its  exposures  for  a  great  distance. 
The  coal-bearing  rocks  are  divisible  into  two  parts,  which  may  be 
called  the  lower  and  upper  members,  distinguished  by  the  relative 
quantities  of  sandstone  and  shale  and  by  the  general  color  of  the  rocks. 
The  dividing  line  is  near  the  middle  of  the  rock  section  as  exposed  in 
the  Sheridan  field  and  is  marked  approximately  by  the  Carney  coal 
bed,  as  it  is  known  on  Tongue  River. 
LOWER    MEMBER. 
The  rocks  below  the  Carney  coal  are  essentially  all  shale  or  are  shalj 
in  character  and  prevailingly  dull  drab,  bluish,  and  brown  in  color. 
They  contain  numerous  segregations  of  ferruginous,  globular  concre- 
tions, many  of  which  are  several  feet  in  diameter.     On  weathering 
a  Kennedy,  Stewart,  The  lignites  of  northeastern  Wyoming:  Mines   and   Minerals,  vol.   27,  1907, 
pp.  294-297. 
