226         CONTRIBUTIONS   TO   ECONOMIC    GEOLOGY,    1907,   PART   II. 
STRUCTURE. 
A  line  trending  20°  west  of  north  from  Knobs,  on  the  Union  Pacific 
Railroad,  divides  the  region  into  two  structural  units.  The  part  to 
the  west  is  a  broad,  flat  synclinal  basin  and  is  filled  with  nearly  hori- 
zontal Tertiary  strata.  Beds  of  undifferentiated  Tertiary  outcrop  in 
small  isolated  patches  across  the  southeastern  corner  of  this  area, 
with  a  strike  of  about  N.  30°  E.  and  a  dip  of  15°  to  18°  NW.  which 
within  a  short  distance  flattens  to  between  5°  and  7°.  The  coal 
beds  in  this  part  are  very  nearly  horizontal  and  can  be  easily  worked 
some  distance  back  from  the  outcrop,  or  they  may  be  reached  by 
shafts  at  moderate  depths.  In  the  area  east  of  the  line  mentioned 
above  the  formations  are  more  or  less  highly  tilted.  From  the  Union 
Pacific  Railroad  between  Ferris  and  Knobs  the  coal-bearing  forma- 
tions strike  northwestward  to  a  point  near  Lost  Soldier,  in  T.  26  N., 
R.  90  W.,  having  a  dip  of  25°  to  80°  W.;  thence  they  swing  eastward, 
dipping  to  the  north  under  Whisky  Peak.  The  lower  formations  con- 
tinue southeastward  in  a  narrow  syncline  parallel  to  and  about  6 
miles  from  the  crest  of  the  Ferris  Mountains,  until  they  disappear 
beneath  sand  dunes  in  T.  26  N.,  R.  88  W.  The  upper  formations 
turn  around  a  low  anticline  in  the  large  syncline  and  join  the  north 
limb  of  the  syncline,  whence  they  continue  northwestward  until  they 
disappear  under  the  terraces  and  against  the  granite  of  the  Sweet- 
water Valley.  Throughout  this  area  the  coal  beds  are  highly  inclined 
and  can  in  only  a  few  places  be  developed  more  than  a  mile  from  the 
outcrop.  Whisky  Peak,  the  Green  Mountains,  and  the  ridge  trending 
southeastward  from  the  vicinity  of  Lost  Soldier  are  composed  of 
massive  Wasatch  conglomerate  capped  by  waterworn  granite  bowl- 
ders, and  these  rocks  conceal  the  coal-bearing  formations  over  a  large 
area. 
THE  COAL. 
COAL-BEARING  FORMATIONS. 
COLORADO    GROUP. 
The  oldest  beds  containing  coal  in  this  region  are  probably  of  lower 
Colorado  (Benton)  age.  They  are  the  shale  and  sandstone  members 
which  lie  between  the  heavy  Dakota  sandstone  and  conglomerate  and 
the  overlying  Mowry  shale  member.  Although  no  fossils  have  been 
found  in  these  beds,  from  their  lithologic  resemblance  to  the  overlying 
rocks  they  have  been  tentatively  referred  to  the  lower  portion  of  the 
Benton.  They  contain  coal  in  a  few  places  along  the  southern  flank 
of  the  Ferris  Mountains.  The  beds  are  thin  and  the  coal  is  very 
impure;  consequently  they  are  of  no  economic  value,  and  are  there- 
fore not  shown  on  the  accompanying  map.  The  Frontier  sandstone 
member,  which  contains  the  high-grade  Kemmerer  coals  of  western 
Wyoming,  was  not  observed  to  be  coal  bearing  in  this  region. 
