282         CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,    1907,    PART   II. 
to  Oregon  and  California.  However,  as  stated  before,  most  of  the 
coal  has  been  used  by  the  railroad  companies  and  only  a  small  amount 
of  the  Rock  Springs  coal  has  found  its  way  into  commercial  channels 
to  compete  successfully  with  coals  from  other  localities;  for  example, 
with  Interior  Province  Carboniferous  coal  at  Omaha,  with  the  New- 
castle and  Sheridan  coal  in  the  Black  Hills,  with  the  Colorado  coal  at 
Denver,  and  with  Utah  coal  at  Salt  Lake  and  other  western  cities.  At 
present  all  of  the  commercial  Rock  Springs  coal  goes  west,  the  greater 
part  of  it  to  Salt  Lake. 
AMOUNT  OF  AVAILABLE  COAL. 
In  computing  the  approximate  amount  of  available  coal  in  the 
portion  of  the  Rock  Springs  field  mapped  last  summer  and  discussed 
in  this  report,  only  those  coal  beds  which  are  2  feet  6  inches  or  more 
in  thickness  have  been  taken  into  consideration,  although  it  is  well 
known  that  bituminous  coals  2  feet  and  less  in  thickness  are  being 
mined  at  present  in  Arkansas  and  other  States.  The  average  thick- 
ness of  the  beds  may  be  obtained  from  the  representative  sections 
given  under  the  discussion  of  the  four  coal  groups.  It  is  assumed 
that  the  average  thickness  of  the  coal  bed  as  computed  from  the 
measurements  made  along  the  outcrop  will  hold  equally  well  for  the 
bed  down  the  dip,  as  the  present  outcrop  at  an  earlier  stage  of  erosion 
represented  the  conditions  along  the  dip  at  that  place.  From  the 
analyses  of  the  coal  it  appears  that  all  the  coal  of  the  Rock  Springs 
coal  group  belongs  to  the  bituminous  class,  and  that  all  the  coal  of  the 
Almond,  Black  Buttes,  and  Black  Rock  groups  belong  to  the  sub- 
bituminous  class.  The  density  of  these  classes  of  coal  is  about  1.3. 
A  bed  of  coal  with  a  specific  gravity  of  1.3  and  1  foot  thick  will  run 
approximately  1,123,000  tons  per  square  mile,  or,  in  round  numbers, 
1,800  tons  per  acre.  Using  this  factor  in  the  calculation  and  taking 
all  coal  beds  2  feet  6  inches  or  more  in  thickness  to  a  depth  of  1,500 
feet  below  the  surface,  or  approximately  200  feet  loss  than  the  deepest 
mine  worked  at  present  at  Rock  Springs,  gives  for  the  Rock  Springs 
field  the  following  tonnage: 
Bituminous  coal 4,  843,  000,  000 
Subbituminous  coal 41, 477,  873,  000 
46,  320,  873,  000 
Using  the  same  factors  but  extending  the  computation  from  the 
surface  to  a  depth  of  4,000  feet  gives  the  following  figures: 
Bituminous  coal .* 12,  649,  569, 000 
Subbituminous  coal 58,  688,  926,  000 
1,338,495,000 
It  should  be  borne  in  mind  that  these  figures  are  only  approxi- 
mations, and  the  total  amount  may  exceed  these  figures  by  as  much 
as  50  per  cent. 
