COAL  OF  THE  BIGHORN  BASIN,  IN  NORTHWEST  WYOMING. 
By  Cassius  A.  Fisher. 
GEOLOGIC  RELATIONS. 
The  coal  of  the  Bighorn  Basin  is  confined  mainly  to  the  Laramie 
formation,  but  it  occurs  also  in  the  basal  sandstone  of  the  Dakota  on 
No  Wood  Creek,  near  Tensleep,  and  in  the  Benton  in  the  vicinity  of 
Clarks  Fork  Canyon.  The  Laramie  coal  beds  are  distributed  through 
the  formation  from  base  to  summit.  Those  of  workable  character, 
however,  are  confined  principally  to  the  basal  sandy  series,  where  coal 
is  characteristic  of  one  horizon  throughout  a  considerable  area. 
In  the  Bighorn  Basin  the  Laramie  formation  consists  of  a  series  of 
alternations  of  sandstones,  shales,  and  clays,  with  beds  of  coal  and 
coaty  shale  of  variable  thickness.  The  shales  are  chiefly  gray  or  drab 
in  color,  and  often  contain  a  sufficient  amount  of  carbonized  wood 
particles  to  impart  a  lignitic  character.  The  shales  vary  greatly 
in  composition,  often  becoming  very  sandy,  and  occur  in  irregular 
bodies  at  various  horizons.  Concretionary  inclusions  are  common, 
consisting  of  sand  locally  silicified  to  increased  hardness,  and  darkened 
by  oxidation  of  iron.  In  the  vicinity  of  Red  Lodge,  Mont.,  limestone 
concretions  are  reported  as  occurring  in  the  shale. 
The  sandstones  are  principally  gra}^,  but  a  variable  amount  of  iron 
gives  rise  to  local  coloring.  They  occur  in  massive  beds  from  1  to  15 
feet  thick,  those  of  25  to  30  feet  being  most  common.  The  sandstones 
exhibit  considerable  variation  in  character,  particularly  where  shale 
intercalations  predominate.  At  the  base  of  the  formation,  however, 
they  show  a  fair  degree  of  persistence.  Sandstone  concretions  are  not 
uncommon,  varying  in  size  from  1  to  10  feet.  The  form  is  usually 
elongated,  with  rounded  edges,  although  spherical  and  lens-shaped 
concretions  abound. 
The  thickness  of  the  Laramie  formation  in  the  Bighorn  Basin 
varies,  by  estimate,  from  1,000  to  1,500  feet,  but  the  maximum 
probably  was  greater  originally,  the  thickness  having  been  greatly 
reduced  by  denudation  of  the  upper  members  prior  to  the  deposition 
of  the  Wasatch. 
The  exposed  area  of  the  Laramie  formation  is  extensive,  comprising 
approximately  1,500  square  miles.     It  outcrops  as  a  continuous  zone 
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