128         CONTRIBUTIONS   TO    ECONOMIC   GEOLOGY,   1907,   PART  II. 
they  take  on  yellow  to  brown  rusty  hues  and  break  into  angular  lumps 
of  small  size.  These  segregations  of  ferruginous,  sandy  clay  are 
more  abundant  in  the  upper  part  than  elsewhere  in  the  member. 
Bluish  shale  of  varied  texture,  interspersed  with  a  great  number  of 
beds  of  carbonaceous  shale,  makes  up  the  greater  part  of  the  rock 
section.  The  carbonaceous  strata  range  from  thin  seams  to  beds  15 
to  20  feet  thick,  and  locally  contain  thin  bands  of  coal.  At  one 
locality  near  the  State  line,  west  **  the  mapped  area,  800  to  1,000 
feet  below  the  top  of  the  member,  e  is  a  variable  and  local  bed  of 
coal  separated  into  benches  by  seve  ,1  bands  of  blue  clay  and  brown 
carbonaceous  shale.  The  coal  layers  aggregate  5  feet  in  thickness  at 
one  place.  At  another  place  near  by  the  total  thickness  of  coal  is  4 
feet  10  inches.  Near  the  top  of  the  member,  as  exposed  on  the  north 
side  of  the  Tongue  River  vt^ley,  there  are  two  other  coal  beds  that 
appear  to  be  of  local  extent.  One  is  nea^y  60  feet  above  the  other, 
and  they  are  separated  by  dark  shale.  <  The  upper  of  the'  two  beds  is 
about  75  feet  below  the  Carnry  coal  T  vd,  from  which  it  is  separated 
by  bluish  shale  and  thin  beds  of  1  'Wn  sandstone.  These  two  coal 
beds  are  each  about  6  feet  thick  oi  north  side  of  the  Tongue  River 
valley,  in  T.  57  N.,  R.  85  W.,  and  are  locally  known  as  the  Masters 
coal  beds. 
The  total  thickness  of  the  lower  member  north  of  Tongue  River  is 
estimated  to  be  2,500  to  2,800  feet.  The  exposures  are  so  obscure 
south  of  Tongue  River  that  reliable  estimates  of  the  thickness  there 
could  not  be  made. 
UPPER    MEMBER. 
General  character. — The  rocks  from  the  lower  member  upward  to 
the  top  of  the  section  exposed  in  the  Sheridan  district  consist  of  shale, 
sandstone,  and  coal,  in  many  beds  inters tratified.  It  is  estimated 
that  the  aggregate  thickness  of  the  shale  is  approximately  twice  that 
of  the  sandstone.  The  shales  range  from  clay  of  bluish  tints  to  brown 
carbonaceous  clay  and  bony  coal,  on  the  one  hand,  and  from  clay 
through  grades  of  sandy  clay  to  shaly  sandstone,  on  the  other.  In 
weathered  surfaces  the  shale  and  sandstone,  aside  from  the  carbo- 
naceous beds,  present  various  hues  of  light  yellow,  brown,  and  white, 
except  where  the  burning  of  the  coal  has  transformed  them  to  hues  of 
red  and  pink.  The  sandstones  range  in  color  from  drab  or  brown  to 
white.  The  carbonaceous  strata,  being  originally  black  to  dark 
brown,  retain  a  dark  shade  until  weathered  to  soil.  As  a  whole  the 
surface  coloring  is  yellow. 
The  thickness  of  the  upper  member  exposed  in  the  Sheridan  dis- 
trict is  estimated  to  be  not  less  than  2,200  feet.  The  rocks  are  very 
slightly  tilted  or  lie  in  a  horizontal  position,  and  the  edges  of  the  beds 
are  spread  over  a  wide  area.     As  ele     tions  were  obtained  by  means 
