MINERAL    RESOURCES    OF    THE   BIGHORN    MOUNTAIN 
REGION. 
By  N.  H.  Darton. 
GENERAL  STATEMENT. 
Although  in  the  Bighorn  uplift  there  is  an  extensive  area  of  the  old  crystalline  rocks, 
they  appear  to  give  but  little  promise  of  yielding  valuable  mineral  deposits.  Much  of  the 
area  has  been  prospected,  but  only  a  few  claims  have  been  worked  to  any  notable  extent, 
and  as  yet  they  have  not  paid  expenses.  Gold  and  copper  have  been  found  in  small  amounts 
and  it  is  claimed,  perhaps  erroneously,  that  platinum  was  detected.  The  Red  Beds  surround- 
ng  the  uplift  contain  large  deposits  of  gypsum.  Beds  of  bentonite  and  nodules  of  phos- 
phate of  lime  occur  in  the  Benton  shales  and  inexhaustible  supplies  of  limestone  and  build- 
ing stones  are  obtainable  from  the  mountains.  Extensive  beds  of  lignite  coal  underlie  por- 
tions of  the  adjacent  plains  on  both  sides  of  the  mountains,  and  these  are  mined  profitably. 
Clays  for  brick  making  and  other  uses  are  available  at  many  localities,  but  are  in  limited 
demand,  a 
COAL. 
The  "  Laramie  "  formation  is  extensively  developed  in  the  wide  synclinal  basins  on  both 
sides  of  the  Bighorn  Mountains,  and,  as  in  most  other  regions,  this  formation  contains  large 
deposits  of  lignite.  In  portions  of  the  area  these  deposits  are  thick  and  to  some  extent 
utilized  commercially.  The  formation  is  several  thousand  feet  thick,  and  on  the  eastern 
side  of  the  mountains  comprises  three  members,  of  which  the  upper  one  may  be,  in  part 
at  least,  younger  than  Laramie  in  age.  The  basal  member,  the  Piney  formation,  consists 
of  2,000  feet  of  sandstone  and  carbonaceous  shale  lying  on  the  Parkman  sandstone,  which 
carries  marine  fossils  and  is  supposed  to  represent  the  Fox  Hills,  for  it  overlies  typical  Pierre 
shale.  The  medial  member  is  a  thick  mass  of  conglomerate  of  considerable  extent,  which 
contains  materials  from  the  mountains  to  the  west.  It  has  been  designated  the  Kingsbury 
conglomerate.  The  coal  measures  occur  next  above  and  have  a  thickness  of  4,000  feet  or 
more.  They  have  been  designated  the  De  Smet  formation.  In  places,  especially  in  the 
upper  measures,  the  coals  and  coaly  shales  are  widely  burned  and  the  resulting  clinker,  or 
clay  baked  and  reddened  by  the  heat,  is  a  conspicuous  and  characteristic  feature  in  the 
outcrop  region.  At  some  localities  the  coal  is  still  burning.  On  the  east  side  of  the  Big- 
horn Mountains  the  principal  mines  are  near  Sheridan  and  Buffalo,  but  the  lignite  could  be 
worked  at  many  other  localities.  The  largest  mines  are  at  and  near  Dietz,  on  Big  Goose 
Creek,  7  miles  below  Sheridan.     The  succession  at  this  place  is  as  follows: 
Section  at  Dietz  coal  mine  near  Sheridan,  Wyo. 
Feet. 
Rusty  slabby  sandstone 2 
Light-gray  soft  sandstone 18 
Dark  coaly  shale  with  fragmentary  leaf  impressions 2 
Blue  clay 6 
a  A  detailed  description  of  the  Geology  of  the  Bighorn  Mountains,  by  N.  H.  Darton,  is  now  in  press 
as  Professional  Paper  Mo.  51,  U.  S.  Geol.  Survey. 
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