264         CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1907,    PART   II. 
Near  Black  Buttes  station  the  Union  Pacific  Railroad  Company 
opened  a  mine  in  1868,  which  was  worked  for  some  time  and  then 
abandoned.  The  old  Hall  mine,  2  miles  south  of  Hallville  station, 
was  opened  in  these  beds.  After  working  a  few  years  they  were 
abandoned,  as  the  coal  was  not  so  good  as  that  mined  at  Rock  Springs. 
During  the  summer  of  1907  a  mine  was  opened  in  these  beds  1|  miles 
south  of  Black  Buttes  station,  but  up  to  the  close  of  the  field  season 
no  shipments  had  been  made.  The  following  sections  illustrate  the 
thickness  and  character  of  the  coal  beds  in  the  Black  Buttes  coal 
group : 
Sections  of  coal  beds  in  the  Black  Buttes  coal  group. 
Location 
Section  of  coal  bed. 
Location. 
Section  of  coal  bed. 
Sec. 
T.  j   R. 
Sec.                 T. 
R. 
100 
SE.iNE.i. 
9 
18 
100 
Ft.  in. 
Coal 5 
NE.iSW.i. 
34     19 
Ft.  in. 
Sandstone  ...     2 
Coal                  0 
10 
18 
100 
100 
Shale 4 
Coal 2 
Shale 3 
Coal 2 
Shale 3 
Coal 3 
Shale 4 
Coal 1    5 
G  10 
8 
SW.fSW.i 
SW.iNW.i. 
29  j  18 
100 
Shale 5 
Coal 3    4 
Shale 7  10 
Coal 3 
Shale 2 
Coal 2 
Shale 9 
SE.iNE.i. 
31 
19 
Coal 6 
Coal 5    8 
SE.iNE.i. 
31 
19 
100 
Coal 6    8 
36  10 
Black  Rock  coal  group.- — The  Black  Rock  coal  group  is  thought  to 
belong  to  the  Fort  Union  formation.  At  the  base  is  a  thin  band  of 
conglomerate,  ranging  in  thickness  from  2  to  6  feet.  The  pebbles 
are  very  fine,  consisting  mostly  of  quartz,  although  in  many  places 
other  pebbles  are  present.  This  conglomerate  marks  an  unconform- 
able contact  between  this  coal  group  and  the  Black  Buttes.  Lith- 
ologically,  this  group  resembles  the  upper  part  of  the  Black  Buttes 
coal  group.  The  sandstone  and  shale,  however,  are  more  highly 
colored  and  more  poorly  cemented  and  contain  a  large  number  of 
spherical  and  irregular  concretions.  The  formation  is  prolifically 
coal  bearing,  some  of  the  coal  beds  reaching  a  thickness  of  25  feet. 
The  coal  beds  lie  nearly  horizontal,  and  not  uncommonly  the  out- 
crops of  the  coal  beds  are  concealed  by  a  large  amount  of  burnt  mate- 
rial. Along  the  south  line  of  sec.  18,  T.  21  N.,  R.  100  W.,  as  well  as 
at  various  other  places  throughout  the  field,  burning  is  going  on  at 
the  present  time.  In  sec.  16  a  considerable  area  of  the  surface  is 
broken  by  depressions  resembling  sink  holes,  some  of  which  are  10 
to  15  feet  deep.  In  some  places  a  hollow  sound  is  produced  when 
a  horse  walks  over  the  surface.  In  the  southeast  corner  of  sec.  18 
there  is  a  large  sink  about  100  feet  wide,  200  feet  long,  and  10  feet 
