232  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1005. 
The  western  side  of  Twentymile  Park  is  bounded  by  ridges  of  steeply  upturned  strata. 
Fish  Creek  cuts  through  this  wall  in  a  sharp  canyon  a  short  distance  below  Dunkley  post- 
office.  In  this  canyon  the  section- of  nearly  the  whole1  coal-bearing  scries  is  well  exposed 
within  a  short  distance.  Seams  of  the  lower  and  middle  groups  have  been  worked  for  Iocs 
use.  An  old  entry  on  a  seam  of  the  middle  group  measured  12  feet,  more  or  less,  of  coal, 
overlying  8  feet  of  thin-bedded  sandstone  and  shale.  Under  this  is  another  seam  of  coal 
3  feet  thick.     The  dip  is  56°  E. 
The  coals  of  the  upper  group  are  exposed  in  many  places  in  Twentymile  Park.  The 
representation  on  the  map  of  the  outcrop  of  this  coal  group  is  only  approximate,  for  these 
coals  lie  above  the  more  massive  beds  of  sandstones  of  the  coal-bearing  series  and  are 
themselves  covered  by  beds  of  soft  and  easily  eroded  shale.  As  both  the  upper  coals  and 
the  overlying  shale  in  large1  part  occupy  an  approximately  horizontal  position,  slight  undula- 
tions  <>f  these  beds  may  bring  these  coals  to  outcrop  almost  anywhere  in  the  shale  valley! 
and.  on  the  other  hand.  ma\  also  carry  the  same  coal  seams  to  a  considerable  depth  under 
t  he  shale  cover,  without  necessarily  showing  in  the  sagebrush  prairie  any  surface  indi<  at  ion 
of  this  st  met  ure.  This  is  true  of  I  he  upper  coal  group  throughout  the  field,  with  the  excep- 
tion of  the  few  districts  in  which  the  coals  arc  tilted  at  such  high  angles  as  to  make  their1 
outcrop  a  fairly  definite  line.  The  inferior  quality  of  the  coal  in  some  of  these  seams  that 
have  been  opened  is  probably  due  to  lack  of  sufficient  cover  to  have  afforded  protection 
from  atmospheric  agencies.  The  probability  is,  however,  that  a  large  proportion  of  this 
coal  group  lies  sufficiently  buried  to  have  preserved  the  coal  from  such  deterioration.  The 
seams  most   deeply  buried  are  not   the  ones  most    readily  opened  in  prospecting. 
WII.LIAM.s    FORK    MOUNTAINS. 
This  general  district  includes  all  the  coals  on  the  south  side  of  the  main  field  and  west  of 
Twentymile  basin.  The  coals  are  exposed  in  outcrop  on  the  crest  and  flanks  of  Williams 
fork  Mountains,  which  constitute  the  divide  between  Williams  Fork  and  Yampa  River. 
This  definition  of  the  district  includes  the  Sage  Creek  coals,  although  these  extend  in  out- 
crop, following  the  Sage  Creek  anticline,  considerably  north  of  the  general  trend  of  the 
coal  groups  of  this  district.  From  Sage  Creek  westward  the  outcrop  of  the  three  coal 
groups  approximately  parallels  the  crest  of  the  Williams  Fork  divide,  but  is  complicated 
by  cross  folds  near  the  junction  of  Yampa  River  and  Williams  Fork.  The  cross  folds  are 
repeated  for  8  or  K)  miles  to  the  west  of  the  area  shown  in  the  map  (PI.  VI)  and  finally 
disappear  beneath  the  overlying  strata  in  the  vicinity  of  Fay  post-office. 
Sage  Creek  Canyon.  A  little  over  5  miles  south  of  Hayden,  Sage  Creek  emerges  from  a 
canyon  eroded  on  the  axis  of  a  minor  anticlinal  fold.  This  fold,  like  that  on  the  west  of 
Twentymile  Park,  is  not  symmetrical,  and  has  a  steep  dip  on  the  east  flank  and  a  much 
lighter  dip  to  the  west.  The  three  coal  groups  are  exposed  as  indicated  on  the  map.  On  t  he 
north  slope  a  scam  of  the  upper  group  has  been  opened  and  worked  for  the  1 1  ay  den  market. 
In  the  canyon  two  seams  of  the  middle  group  have  been  opened  by  drifts  and  are  now  being 
worked  for  domestic  use  in  and  about  Hayden.  One  of  these  seams  (N)  of  the  middle  group 
measured  6  feet  10  inches  of  clear  coal,  and  a  sample  for  analysis  representing  the  whole 
face  was  taken  at  the  breast  of  a  300-foot  drift.  This  seam  dips  28°  NE.  The  analysis 
is  as  follows: 
Analysis  of  coal  (Sample  N)  from  tin  Barnes  mine. 
[F.  M.  Stanton,  analyst.] 
M  i  listure 11. 23 
Volatile  matter :<7.(I7 
Fixed  carbon 47.. 52 
As), 4.18 
inn. on 
Sulphur 1 .  60 
Loss  ef  moisture  on  air  dry,;  . ; 4.20 
