228  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    LG05. 
Each  group  of  coal  seams  ranges  through  a  v(  rtical  distance  of  from  200  to  I(K)  fe<  t  and 
the  several  groups  arc  separated  by  from  500  to  1,000  feet  of  barren  shale  and  sandstone. 
The  number  and  thickness  of  coal  beds  constituting  each  group  vary  from  place  to  place. 
Usually  there  are  in  each  group  at  least  three  seams  of  workable  thickness  and  a  number  of 
smaller  seams.     Single  beds  of  coal  occur  up  to  a  thickness  of  about  20  feet. 
At  or  near  the  base  of  each  coal  group  are  found  conspicuous  sandstone  beds.  These 
are  characteristically  massive  and  white  and  frequently  more  than  100  feet  thick.  While 
generally  ridge  or  ledge  makers,  the  exposed  surfaces  readily  weather  to  a  friable  condition. 
The  broad  exposures  are  frequently  marked  by  cracks  making  polygonal  figures  a  foot  or 
more  in  diameter.     Great  concretions  and  stains  of  iion  oxide  are  also  common. 
Below  the  three1  coal  groups  is  a  great  series  of  very  massive  sandstone  beds,  sometimes 
designated  as  the  "barren  sandstones."  Where  exposed  these  are  usually  prominent  as  a 
series  of  hogbacks.  As  stated  in  the  generalized  section,  a  few  small  seams  of  coal  are; 
found  low  down  in  this  series. 
The  Laramie,  overlying  the  Montana,  is  composed  of  alternating  sandstones  and  clay 
shales.  At  least  iXM)  feet  of  such  sediments  are  embraced  in  the  steep  scarp  on  the  north1 
bank  of  Yanipa  River  at  llayden.  In  this  blull'  indications  of  lignite  were  found  at  twenty 
horizons,  distributed  somewhat  regularly  throughout  the  upper  two-thirds  of  this  thickness. 
COAL. 
For  convenience  in  describing  the  various  mines  and  outcrops  the  held  has  been  divided 
into  luiii-  main  structural  disl  ricts,  as  follows:  ( 1 )  Twentymile  basin,  including  the  extreme 
southeast  point  <>i  the  syncline  and  bounded  on  the  northwest  by  the  cross  anticlinal  fold 
mentioned  on  a  previous  page;  (2)  Williams  Fork  Mountains,  including  the  southern 
rim  of  the  main  basin  west  of  the  cross  anticline  and  Twentymile  Park,  and  the  outcrops  in 
Yanipa  Yalle\  in  the  vicinity  of  Craig:  (3)  Wolf  Creek  and  Anthracite  Range,  including 
the  coal  beds  exposed  oh  the  eastern  rim  of  the  main  basin  from  Yampa  River  as  far  north; 
as  the  coals  have  been  traced:  |  [)  Lower  Llkhead  and  Fortification  creeks,  including  the 
lignite  coals  of  the  Laramie  formation  and  lying  in  the  structural  center  of  the  main  basin 
of  the  field. 
TWENT1  MILE    BASIN. 
This  structural  basin  includes  the  coal  beds  exposed  on  Oak,  Trout,  Middle,  and  Fish 
creeks  and  a  small  area  on  and  near  Yampa  River  about  2  miles  w,  st  of  Fool  post-odiee. 
In  this  basin  all  the  coal-bearing  rocks  dip  in  general  toward  its  center,  which  is  Twenty- 
mile  Lark,  and  thus  they  form  a  single  structural  basin.  Each  creek  valley,  however, 
forms  a  distinct  district  or  commercial  unit,  for  the  developments  in  a  single  valley  are  in 
the  main  subject  to  similar  condit  ions  of  accessibility  and  convenience  to  market ,  and  these 
conditions  are  usually  different  from  those  of  adjacent  valleys  in  th.    same  structural  basin. 
Oak  Creek  district.  Coal  seams  outcrop  along  the  upper  valley  of  Oak  Creek  for  a  dis- 
tance of  at  least  '•>  miles.  The  valley  of  this  creek  practically  outlines  the  extreme  south- 
eastern limit  of  the  coal  field.  Coal  beds  occur  in  the  high  blufT  north  of  Pinnacle  post- 
office  and  probably  they  are  continuous  along  the  blufF  on  the  north  side  of  the  creek  as  far 
as  the  Yanipa  and  Eddy  road.  By  the  side  (^i  this  road  in  a  small  tributary  valley  of  Oak 
Creek  there  are  a  number  of  openings,  which  are  generally  known  as  the  James  mine  (W  OD 
map,  LI.  VI).  Then1  is  some  doubt  as  to  the  position  of  the  beds  opened  in  these  mines, 
but  they  probably  belong  to  the  lower  coal  group.  Openings  have  been  made  on  three 
seams,  separated  by  intervals  of  about  35  feet  each.  The  coal  dips  about  10°  XW.  The 
principal  mine  is  on  the  uppermost  bed  that  is  exposed.  An  entry  has  been  driven  in  on 
the  coal  bed  for  some  distance,  and  considerable  coal  has  been  mined  for  local  use  and  to 
supply  the  growing  town  of  Yanipa.  The  mine  was  not  in  operation  at  the  time  it  was 
visited,  and  a  complete  section  could  not  be  obtained.  The  expos  d  part  of  the  bed  is  as 
follows: 
