294  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1903.  [bull.  225. 
frequent  intervals,  and  there  are  minor  folds  which  probably  would 
often  render  the  bed  too  irregular  for  profitable  working.  Usually 
where  an  igneous  dike  has  penetrated  a  coal  bed  in  this  general  region 
(as  is  illustrated  at  the  Capitan  mines)  the  material  on  either  side  of 
the  dike  for  a  distance  of  15  feet  or  more  is  converted  into  a  so-called 
coke  or  slag,  which  is  noncombustible.  In  mining  such  a  deposit  it 
is  necessary  to  drive  the  tunnel  through  unproductive  material  for  a 
distance  of  40  or  50  feet,  the  distance  dependent  on  the  size  of  the 
dike,  which  materially  increases  the  cost  of  the  operation.  The  out> 
crops  of  the  above-described  deposits  occur  high  in  the  hillsides  and 
in  rather  inaccessible  places.  This  district  probably  will  furnish  a 
small  amount  of  coal  to  supply  local  demand,  but,  owing  to  the  dis- 
torted and  broken  condition  of  the  strata  in  which  the  coal  occurs  and 
the  presence  of  innumerable  dikes  which  cut  the  deposits  at  various 
angles,  the  field  can  not  be  regarded  as  one  deserving  of  extensive 
development. 
At  Capitan,  N.  Mex.,  there  are  coal  fields  in  which  coal  is  mined  to 
some  extent,  but  with  great  difficulty,  owing  to  the  broken  condition 
of  the  strata.  Here,  however,  the  beds  are  relatively  uniform  in 
structure  and  contain  but  few  dikes  as  compared  with  the  White 
Mountain  region. 
