FISHER.] 
BENTONITE    DEPOSITS    OF    WYOMING. 
561 
formation."  The  Graneros  is  composed  chiefly  of  shale,  but  con- 
tains occasional  layers  of  sandstone,  and  about  halfway  from  the 
base  beds  of  harder  gray  shale  and  sandstone,  which  weather  light 
gray  and  form  bare  ridges  of  considerable  prominence.  Mr.  N.  H. 
Darton  has  called  this  Series  the  "  Mowrie  beds,"  from  Mowrie 
Creek,  near  Buffalo,  Wyo.,  where  they  attain  their  greatest  develop- 
ment. The  Greenhorn  is  an  impure  limestone  near  the  middle  of 
the  Benton,  which  forms  low  but  distinct  escarpments,  while  the 
Carlile  consists  of  a  shaly  series,  with  sandstone  layers  near  the  base 
and  a  band  of  large  fossiliferous  concretions  at  the  top,  characterized 
by  the  occurrence  of  Prionotropis  woolgari  and  Prionocyclas  wyo- 
mingensis. 
Fig.  25. — Sketch  map  showing  distribution  of  the  Benton  formation  in  Wyoming. 
The  above  subdivisions,  so  obvious  in  the  Black  Hills  and  Laramie 
Mountains,  are  not  all  recognizable  farther  west  in  the  region  of  the 
Bighorn  Mountains,  owing  mainly  to  the  absence  of  the  Greenhorn 
limestone,  while  the  basal  member  of  the  Graneros  is  here  repre- 
sented by  a  much  greater  thickness  of  beds.  It  is  in  this  region  of 
greater  development  of  the  Graneros  shales  that  the  thickest  deposits 
of  bentonite  have  been  found. 
Occurrence  and  development. — Bentonite  occurs  at  various  horizons 
in  the  Benton  group.     In  one  locality,  near  Newcastle,  Wyo.,  this  clay 
occurs  in  passage  beds  at  the  top  of  the  Niobrara. 
Bull.  260—05  M 36 
