196 
fine  jointing  of  the  Eagle  coal  of  the  Silvertip  and  Bridger  fields 
approaches  that  of  eastern  bituminous  coals,  though  it  is  not  quite 
so  closely  spaced.  The  jointing  of  the  Laramie-Fort  Union  coals, 
on  the  other  hand,  is  much  more  imperfect  than  that  of  the  Eagle 
coal,  yet  superior  to  that  of  coals  of  the  same  age  near  Sheridan,  and 
to  that  of  the  coals  of  the  other  coal-mining  regions  of  eastern 
Wyoming  and  Montana.  As  jointing  and  cleavage  are  doubtless 
results  of  rock  pressure  and  are  promoted  by  mechanical  movement, 
the  perfection  of  the  jointing  is  to  some  extent  an  index  of  the  pres- 
sures and  movements  to  which  the  coal  has  been  subject.  The 
firmer  and  better  bituminous  and  subbituminous  coals  have  more 
highly  developed  jointing  than  poorer  coals  of  the  same  groups, 
probably  in  part  because  they  have  been  subject  to  greater  pressure 
or  more  deformation.  This  may  explain  why  jointing  is  more  per- 
fect in  the  Fort  Union-Laramie  coals  of  the  Bighorn  Basin,  where 
the  rocks  are  deformed,  than  it  is  in  coals  of  the  same  age  in  the  flat, 
undisturbed  strata  of  the  coal  fields  east  of  the  Bighorn  Mountains 
and  in  eastern  Montana.  Other  differences  in  the  coals  of  the  two 
regions  may  be  due  to  wholly  unlike  causes,  such  as  the  deposition 
of  different  vegetable  materials  or  deposition  under  different  condi- 
tions, but  the  important  difference  in  jointing  seems  to  be  the  effect 
of  subsequent  deformation  or  pressure  on  the  coal.  In  like  manner 
the  difference  in  jointing  between  the  Eagle  coal  of  the  Bridger  field 
and  the  Fort  Union  coal  of  the  Bear  Creek-Red  Lodge  fields,  a  short 
distance  to  the  west,  may  be  assigned  to  the  greater  pressure  which 
has  been  exerted  on  the  former  by  an  excess  of  load  consisting  of 
over  5,000  feet-  of  strata,  and  to  the  post-Laramie  pre-Fort  Union 
deformation,  which  affected  only  the  earlier  coal. 
The  jointing  of  the  Eagle  coal  is  rectangular.  It  tends  to  divide 
the  coal  into  small,  roughly  cubical  blocks,  with  smooth,  shiny  faces. 
The  little  cubical  blocks  are  larger  than  those  common  to  most  of  the 
bituminous  coals  of  the  Appalachian  province,  owing  to  the  wider 
spacing  of  the  cleavage.  Within  each  block  the  fracture  is  conchoidal, 
but  the  conchoidal  breaks  are  not  conspicuous,  owing  to  their  small 
size  and  to  the  prominence  of  the  rectangular  joints.  Joints  parallel 
to  the  bedding  are  not  so  prominent  as  either  of  the  two  sets  of  joints 
normal  to  the  bedding. 
The  jointing  of  the  Laramie-Fort  Union  coals  of  the  Bighorn  Basin 
is  irregular  and  poorly  developed.  At  places  cubical  jointing  is 
noticeable,  but  it  never  has  the  perfection  of  that  in  the  Eagle  coal. 
Parting  parallel  to  the  bedding  is  conspicuous.  In  the  Eogers  & 
Gapin  mine,  7  miles  southeast  of  Basin,  this  parting  parallel  to  the 
bedding  is  so  pronounced  that  the  coal  breaks  out  in  flat  slabs,  which 
