156         CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1907,    PAKT   II. 
white  to  brown  sandstone,  already  described,  which  is  best  exposed 
near  Glenrock  and  on  Cole  Creek  near  Big  Muddy;  a  middle  member, 
which  underlies  a  large  area  north  and  northwest  of  Douglas  and  is 
made  up  of  dark  shale  and  sandstone ;  and  an  upper  member  of  bluish 
shale  which  weathers  white  and  outcrops  in  a  small  area  on  the  north 
side  of  the  Glenrock  field. 
The  central  part  of  the  Fort  Union  (?)  is  a  great  mass  of  dark  shale, 
much  of  it  carbonaceous,  but  it  contains  also  light  shale,  sandstone, 
and  some  coal  beds.  Near  the  top  is  a  zone  of  loglike  concretionary 
masses,  the  origin  of  which  is  not  well  understood.  They  are 'found 
in  greatest  abundance  6  to  10  miles  north  of  Douglas. 
Near  the  middle  of  the  north  side  of  the  field  there  are  outcrops  of 
very  light  beds  consisting  of  shale  and  some  sandstone,  which  do  not 
appear  elsewhere  in  this  field.  The  weathered  surface  of  the  shale  is 
white,  but  the  unweathered  parts  are  blue.  No  coal  was  found  in 
this  member,  and  it  is  thought  to  constitute  the  top  of  the  Fort 
Union  (?)  section. 
FORMATIONS  ABOVE  THE  COAL. 
The  coal-bearing  rocks  described  above  are  unconformably  over- 
lain by  Tertiary  and  Quaternary  formations.  The  White  River,  a 
Tertiary  formation  of  white  sandy  clay  and  conglomerate,  covers  a 
considerable  area  in  the  southeastern  part  of  the  field.  It  is  spread 
over  parts  of  all  the  older  formations,  and  is  even  found  high  up  in  the 
mountains.  The  so-called  Chalk  Buttes,  3  miles  southwest  of  Doug- 
las, are  also  composed  of  Tertiary  rocks,  and  the  outcrop  of  similar 
rocks  extends  westward  for  25  miles  along  the  foot  of  the  mountains. 
The  irregular  areas  of  sand  dunes  which  cover  a  large  part  of  the 
field  obscure  the  geology  of  the  rocks  below.  Some  whole  townships 
are  covered  to  a  depth  of  1  to  200  feet,  the  average  thickness  of  the 
sand  being  about  25  feet.  Here  and  there  among  the  dunes  are  out- 
crops of  coarse  sandstone,  and  it  seems  probable  that  this  rock  is  of 
Tertiary  age,  and  that  the  sand  is  the  result  of  its  disintegration. 
Red  conglomerate  of  Tertiary  age  occurs  on  the  mesa  15  miles 
north  of  Douglas.  This  is  an  isolated  deposit  and  can  not  be  cor- 
related with  other  rocks  in  this  field  of  the  same  general  age. 
A  gray  Tertiary  conglomerate  occurs  in  the  valley  of  the  North 
Platte  10  to  150  feet  above  the  river,  but  it  is  not  extensive  and  hence 
is  not  an  important  consideration  in  coal  mining. 
The  Quaternary  formations  consist  of  the  sand  just  mentioned, 
gravel  deposits  on  mesas  and  terraces,  rain  wash,  and  stream  deposits. 
Rain  wash  is  of  much  importance  in  this  field,  for  it  obscures  the  bed 
rock  and  reduces  the  number  of  exposures.  There  are  large  areas 
of  gently  rolling  country  where  the  products  of  weathering  completely 
cover  the  underlying  rocks.     In  such  places,  as  in  the  sand  areas,  the 
