SHERIDAN    COAL   FIELD,    WYOMING.  131 
there  is  an  estimated  thickness  of  200  feet  of  soft  shale  and  whitish 
to  light-yellow  sandstone,  making  the  topmost  part  of  the  section  of 
rocks  exposed  in  this  field. 
VARIATIONS   IN   THE    COAL-BEARING    ROCKS.  „ 
A  considerable  part  of  the  coal-bearing  rocks  change  in  character 
in  certain  respects  toward  the  south,  in  the  general  direction  of  the 
strike  of  the  beds.  Near  the  State  line,  in  T.  58  N.,  R.  86  W.,  certain 
light-colored  sandstone  strata  in  the  upper  part  of  the  lower  member 
thin  out  and  disappear  southward  along  the  strike.  Some  brownish 
and  yellow  sandy  strata  that  lie  still  higher,  near  the  top  of  the  same 
member  on  Tongue  River,  seem  to  thin  out  toward  the  south  and  give 
place  to  dull-colored  shale  or  sandy  strata  on  Goose  and  Beaver 
creeks  in  T.  55  N.,  R.  85  W.  These  conditions  seem  to  indicate  that 
the  parting  between  the  two  members  rises  in  the  rock  section 
toward  the  south. 
The  upper  member  also  changes  in  character  southward  along  the 
strike  of  the  rocks.  From  the  central  part  of  the  field  southward 
the  differentiation  between  the  sandstone  and  shale  strata  becomes 
less  distinct.  The  sandstone  on  the  whole  is  duller  in  color,  and  near 
the  southern  boundary  of  the  mapped  area  the  sandstone  beds  con- 
tain pebbles  of  limestone,  quartz,  and  chert.  In  the  southeastern 
part  of  T.  54  N.,  R.  83  W.,  and  in  T.  54  N.,  R.  84  W.,  many  hundred 
feet  of  strata  in  the  central  part  of  the  upper  member  merge  into 
conglomerate.  The  constituent  parts  of  the  conglomerate  become 
coarser  rather  abruptly  on  the  approach  to  the  Paleozoic  rocks  of 
the  Bighorn  Mountains,  upon  which  the  conglomerates  overlap 
unconformably.  The  exposed  section  of  conglomerate  strata  is  more 
than  1,000  feet  thick  between  Little  Goose  and  Sandy  Creek  valleys, 
at  the  base  of  the  Bighorn  Mountains,  on  the  southern  border  of  the 
Sheridan  field.  The  gradation  from  the  conglomerate  into  the  sandy 
and  shaly  strata  takes  place  toward  the  east  and  north,  and  involves 
almost  the  whole  section  of  this  member  from  the  Tongue  River  coal 
group  upward  nearly  to  the  top  of  the  rock  section.  The  economic 
bearing  of  the  conglomerate  is  a  negative  one,  for  the  coal  beds  thin 
out  and  disappear  near  its  outer  fringe. 
\<;k  of  coal-beaiu\<;   hocks. 
Many  collections  of  fossil  plants  were  made  at  various  places 
throughout  the  section  of  the  coal-bearing  rocks,  and  fossil  shells  wire 
collected  at  various  places  in  the  section  from  the  Tongue  River 
coal  group  upward.  Reports  on  the  plants  by  F.  II.  Knowlton  and 
on  the  shells  by  T.  W.  Stanton  agree  in  stating  thai  the  rocks  which 
included  the  fossil  collections  are  of  Fort  Union  age.  At  the  present 
time  the  tendency  is  to  class  the  Fort  Union  as  lower  Eocene  (basal 
Tertiary) . 
