COAL   ON   SOUTHWEST   SIDE   OF   BIGHOKN    BASIN,    WYO.  203 
evidence,  differences  in  lithology  alone  were  accepted  as  a  basis  for 
the  subdivision  into  formations.  A  subsequent  study  of  the  fossils, 
both  in  the  field  and  in  the  laboratory,  by  T.  W.  Stanton  and  F.  H. 
Knowlton,  has  shown  that  the  lithologic  units  do  not  coincide  in  all 
particulars  with  the  formations  as  they  are  recognized  farther  north 
and  elsewhere.  Fossils  collected  at  various  localities  in  the  fields 
point  to  the  presence  of  the  Eagle  and  Laramie  formations,  though 
the  evidence  is  not  conclusive,  as  many  of  the  species  are  either  new  or 
heretofore  unknown  to  the  formations,  and  strictly  characteristic 
specimens  are  lacking.  A  fresh-water  fauna  which  contains  forms 
suggestive  of  upper  Montana  occurs  in  beds  between  the  Eagle  and 
Laramie  (?).  There  is  abundant  fossil  evidence  to  prove  the  presence 
of  the  Fort  Union  formation.  A  suggestive  break  occurs  at  the  base 
of  the  conglomerate  beds  noted  in  the  table,  which  was  thought  in 
the  field  to  be  an  unconformity  at  the  base  of  the  Fort  Union,  but 
later  study  of  the  plant  collections  shows'  that  Fort  Union  forms  occur 
a  few  feet  below  this  horizon.  As  the  final  determinative  evidence 
has  not  been  obtained,  the  divisions  shown  in  the  table  are  only 
provisionally  introduced. 
Of  the  seven  formations  shown  in  the  table  only  four,  the  Colorado, 
Eagle,  Laramie (?),  and  Fort  Union,  are  coal  bearing;  and  theCloverly, 
which  seems  to  include  at  least  part  of  the  coal-bearing  Kootenai  for- 
mation of  Montana/  is  barren  in  the  field  here  discussed. 
COLORADO    SHALE. 
The  Colorado  shale  consists  mainly  of  gray  to  black  shale  3,375 
feet  thick,  as  measured  at  Cody,  with  massive  rusty  sandstones  in 
the  lower  part  and  a  mass  of  tan-colored  sandy  shale  200  feet  thick 
forming  the  upper  part.6  Broad  strike  valleys  bordered  by  sharp 
ridges  constitute  the  characteristic  topographic  expression  of  this 
formation  where  it  is  exposed  along  the  west  side  of  the  area.  Thin 
beds  of  coal  interbedded  with  massive  sandstone  occur  in  the  loWer 
part  of  the  formation.  Abed  8  inches  thick  outcrops  on  a  branch  of 
Pat  O'Hara  Creek  near  Allison's  ranch,  and  another  6  inches  thick 
occurs  in  the  bluffs  along  Shoshone  River  a  short  distance  above  the 
Cody,  bridge.  Exposures  of  coal  in  the  Colorado  were  noted  at 
many  points  in  the  field,  but  none  approaches  workable  thickness. 
MONTANA    GROUP. 
In  the  northern  part  of  Montana  this  group  is  subdivided  into 
Eagle,  Claggett,  Judith  River,  and  Bearpaw.  Of  these  subdivisions 
the  Eagle  is  the  only  one  recognized  in  the  area  under  discussion. 
a  Fisher,  C.  A.,  Econ.  Geology,  vol.  3,  L908,  pp.  77-99. 
b  Fossils  were  collected  in  i(.x>8  from  the  upper  sandy  shale  in  the  southeastern  part  of  the  Bighorn 
Basin.    T.  W.  Stanton  states  that  "These  fossils  are  more  closely  related  to  the  Montana  fauna  than  to 
the  Colorado."  It  is  probable,  therefore,  that  a  part  if  not  all  of  this  shale  belongs  to  the  Montana 
group. 
