280         CONTRIBUTIONS    TO    ECONOMIC   GEOLOGY,   1907,   PART   II. 
68  per  cent  of  the  coal  yield  of  the  State  and  employed  67  per  cent  of 
the  miners.  In  the  same  year  (1906)  the  Rock  Springs  field  produced 
34  per  cent  of  the  State's  entire  coal  output  and  employed  30  per  cent 
of  the  miners.  Practically  all  the  coal  in  this  field  is  mined  under 
contract  at  a  definite  rate  per  ton,  established  by  an  agreement  be- 
tween the  operators  and  the  miners'  union. 
Most  of  the  mining  in  this  field  will  no  doubt  be  carried  on  by  slopes 
from  the  outcrop,  although  in  places  it  may  be  found  advantageous  to 
sink  shafts,  particularly  along  the  valleys  some  distance  back  from 
the  outcrop,  where  several  hundred  feet  of  overlying  Tertiary  beds 
have  been  removed.  The  room  and  pillar  system  prevails  throughout 
the  field,  and  as  a  rule  the  pillars  are  not  pulled  at  present.  Mining 
of  the  coal  is  generally  done  by  shooting  off  the  solid  face  of  the  bed. 
Undercutting  by  hand  and  machine  is  employed  in  some  of  the  mines. 
Haulage  from  the  rooms  is  effected  by  horse  and  mule,  and  in  the  main 
slope  or  gangway  either  by  mules  or  cable.  Some  of  the  mines  at 
Rock  Springs  are  equipped  with  electric  motors,  and  all  are  supplied 
with  a  complete  mine  telephone  system.  Ventilation  in  the  mines  is 
accomplished  by  means  of  fans  or  furnaces.  There  is  comparatively 
little  gas,  and  accidents  from  this  cause  are  relatively  rare. 
MINING  CONDITIONS. 
In  opening  new  mines  in  this  locality  or  in  contemplating  any  future 
economic  development  among  the  factors  that  must  be  taken  into 
consideration  are  transportation,  timber  supply,  and  water  supply. 
TRANSPORTATION. 
As  previously  stated,  all  the  producing  mines,  except  those  at 
Superior,  are  on  the  main  line  of  the  Union  Pacific  Railroad.  This 
railroad  cuts  the  Rock  Springs  coal  field  approximately  in  halves  and 
is  destined  to  control  the  development  of  the  field.  The  grade  along 
this  line  is  moderate,  and  by  short  spur  lines,  also  with  moderate 
grade,  similar  to  the  Superior  branch,  it  will  be  comparatively  easy  to 
reach  all  points  at  which  mines  may  be  opened  to  advantage. 
TIMBER    SUPPLY. 
As  far  as  the  mines  have  been  developed  at  present  it  appears  that 
the  roof  and  floor  of  the  coal  beds  are  firm  and  give  but  little  trouble 
in  mining.  The  mines  are  not  dangerously  gaseous  and  water  has 
not  proved  particularly  troublesome.  In  places  part  of  the  coal  is 
left  as  roof  in  order  to  prevent  caving  of  the  shale.  Timber  is  used 
extensively  for  props  to  support  the  roof  until  the  pillars  are  drawn, 
and  considerably  increases  the  cost  of  mining.  All  timbers  used  in 
this  field  must  be  shipped  in,  as  there  is  no  timber  in  the  region.  The 
nearest  sources  of  timber  supply  for  this  area  are  (1)  the  Uinta  Forest 
