268         CONTRIBUTIONS    TO    ECONOMIC   GEOLOGY,    1907,    PART   II. 
many  of  them  there  is  an  actual  displacement  of  one-half  inch  to  a 
foot  or  more.  Much  sHckensided  coal  is  present  along  such  surfaces. 
As  a  rule  these  small  faults  do  not  interfere  with  mining,  but  rather 
assist  in  breaking  or  parting  the  coal,  thus  making  it  easier  to  mine. 
The  larger  faults,  however,  greatly  increase  the  difficulties  in  mining 
and  tend  to  retard  development  work.  In  regions  of  much  faulting 
the  offsetting  of  the  coal  beds  may  so  increase  the  cost  of  mining 
that  the  mines  will  be  abandoned  or  development  work  stopped. 
IGNEOUS   ROCKS. 
At  the  north  end  of  the  dome  numerous  intrusive  and  extrusive 
masses  have  been  forced  up  through  the  Cretaceous  and  Tertiary 
rocks  and  cap  these  rocks  in  several  localities.  The  exposures  of 
leucite  range  from  talus-covered  hills,  isolated  volcanic  necks,  and 
associated  dikes  to  lava  flows  with  cones,  intruded  sheets,  and 
dikes.  Many  of  the  lava  sheets  present  abrupt  walls  from  50  to  150 
feet  in  height.  Some  of  the  lava  flows  and  volcanic  necks  he  along 
fault  lines,  through  which  the  molten  mass  may  have  found  an  out- 
let; elsewhere  the  lava  seems  to  have  eaten  its  way  through  the 
rocks  without  causing  any  disturbance,  as  the  sedimentary  beds 
lie  practically  horizontal  all  around  the  igneous  masses,  which  con- 
tain many  fragmentary  inclusions  of  sedimentary  rock.  No  bulging 
or  distortion  was  noted  in  the  vicinity  of  the  leucite.  Although  the 
lavas  cut  many  coal  beds  and  rocks  of  the  coal-bearing  formations, 
in  no  place  were  the  coal  and  igneous  rock  seen  in  actual  contact. 
Coal  samples  were  collected  from  two  prospect  pits  in  sec.  10,  T.  21 
N.,  R.  102  W.,  a  few  rods  below  the  overlying  lava  sheet  and  about 
a  mile  from  several  volcanic  cones,  but  no  apparent  difference  was 
observed  in  the  physical  and  chemical  properties  of  the  coal. 
QUALITY   OF   COAL. 
PHYSICAL  PROPERTIES. 
The  Rock  Springs  coal  is  jet-black  as  it  comes  from  the  mine,  has  a 
bright  or  even  glassy  luster,  and  in  places  shows  beautiful  iridescent 
colors.  The  structure  of  the  bedding  planes  is  as  a  rule  well  pre- 
served, but  jointing  is  not  strongly  developed.  The  coal  is  dense  in 
texture  and  somewhat  brittle.  The  streak  ranges  in  color  from 
brownish  black  to  black.  Many  slickensided  surfaces  are  present  in 
this  coal,  as  well  as  numerous  small  faults  displacing  the  bedding 
planes  from  a  fraction  of  an  inch  to  several  inches.  Considerable 
deposits  of  salts  occur  on  the  faces  of  the  coal  and  on  the  sandstones 
along  the  entries  in  the  mines.  The  coal  on  exposure  to  the  air 
remains  firm  and  compact  and  stands  shipping  without  breaking 
down.     It  produces  no  clinker  and  leaves  on  burning  a  small  bulk 
