KOCK   SPRINGS    COAL   FIELD,    WYOMING.  267 
The  third  or  Black  Buttes  coal  group  rests  conformably  upon  the 
underlying  formations  and  occupies  a  belt  extending  along  the  east 
side  of  the  dome,  but  not  concentric  with  the  overlying  formations. 
Along  the  west  side  of  the  dome  the  beds  comprising  the  Black 
Buttes  coal  group  are  covered  by  the  Black  Rock  group,  which  rests 
directly  upon  the  beds  that  make  up  the  second  or  Almond  zone,  but 
along  the  east  side  of  the  dome  the  Black  Buttes  zone  is  separated 
from  the  Almond  zone  by  a  belt  of  non-coal-bearing  Lewis  shale. 
The  fourth  or  Black  Rock  coal  group  lies  unconformably  upon  the 
older  (Black  Buttes)  beds,  and  unlike  them  occupies  a  concentric  belt 
around  the  dome.  The  Black  Rock  coal  zone  spreads  over  a  large 
area  in  the  northeastern  part  of  the  field  and  occupies  a  large  portion 
of  the  Great  Divide  Basin.  The  same  group  contains  the  coals  in 
the  low  synclinal  trough  between  the  Rock  Springs  dome  and  the 
Rawlins  anticline.  For  the  eastern  boundary  of  this  coal  zone  the 
reader  is  referred  to  the  reports  on  the  Great  Divide  Basin  coal  field 
(pp.  220-242)  and  the  Little  Snake  River  coal  field  (pp.  243-255) 
in  this  volume. 
FAULTS. 
The  simple  dome  structure  above  referred  to  is  somewhat  compli- 
cated by  many  normal  faults  of  considerable  throw.  Here  and  there 
the  horizontal  displacement  amounts  to  nearly  3  miles,  but  the  ver- 
tical displacement  is  usually  less  than  100  feet  and  in  few  localities 
reaches  several  hundred  feet.  Northeast  of  Salt  Wells,  in  T.  20  N., 
R.  103  W.,  nine  large  faults  cut  the  rocks  along  the  east  township 
line  within  a  distance  of  less  than  5  miles.  Not  uncommonly  three 
separate  and  distinct  faults  occur  within  less  than  1  mile. 
Some  of  the  faults  extend  across  the  entire  arch  of  the  dome,  others 
extend  only  across  one  limb  or  part  of  one  limb,  and  still  others 
extend  for  a  few  hundred  feet  or  a  mile  or  two  and  then  die  out. 
Some  of  the  larger  faults  have  been  traced  for  a  distance  of  more  than 
20  miles.  The  trend  of  the  faults  is  nearly  at  right  angles  to  the  strike 
of  the  rocks,  or  across  the  limbs  of  the  anticline.  This  is  particu- 
larly true  near  the  north  end  of  the  dome,  where  the  faults  cut  some 
of  the  rocks  at  right  angles  to  their  strike  and,  before  dying  out, 
continue  approximately  along  the  strike  of  the  underlying  beds. 
The  position  of  the  larger  faults  is  shown  on  Pis.  XIV  and  XV. 
In  addition  to  the  larger  faults,  readily  detected  on  the  surface, 
numerous  small  faults  are  encountered  in  mine  workings.  In  the 
Rock  Springs  coal  group,  from  the  Van  Dyke  bed  upward,  there  is 
at  many  places  a  system  of  characteristic  joints  or  slips  that  part 
the  coal  at  short  intervals  from  roof  to  floor.  These  slips  incline 
toward  the  south  and  present  every  peculiarity  of  a  fault.     Along 
