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  From 
  the 
  northern 
  boundary 
  line 
  of 
  western 
  Pennsylvania 
  to 
  cen- 
  

   tral 
  West 
  Virginia 
  the 
  regularity 
  of 
  the 
  dip 
  of 
  the 
  rocks 
  is 
  modified 
  

   by 
  low, 
  broad 
  folds, 
  which 
  become 
  pronounced 
  ridges 
  in 
  the 
  south- 
  

   eastern 
  portion 
  and 
  are 
  parallel 
  to 
  the 
  general 
  Appalachian 
  folding 
  

   to 
  the 
  east; 
  but 
  the 
  general 
  topography 
  is 
  that 
  of 
  a 
  plateau 
  descend- 
  

   ing 
  gently 
  toward 
  the 
  west, 
  more 
  or 
  less 
  dissected 
  by 
  valleys, 
  accord- 
  

   ing 
  to 
  the 
  degree 
  of 
  erosion. 
  The 
  region 
  is 
  drained 
  by 
  the 
  Ohio 
  and 
  

   its 
  tributaries, 
  of 
  which 
  the 
  most 
  important 
  are 
  the 
  Allegheny 
  and 
  

   the 
  Monongahela 
  rivers. 
  

  

  The 
  coal-bearing 
  formations 
  of 
  the 
  northern 
  Appalachian 
  coal 
  

   field 
  consist 
  principally 
  of 
  shales, 
  sandstones, 
  and 
  conglomerates, 
  

   with 
  occasional 
  beds 
  of 
  limestone, 
  fire 
  clay, 
  and 
  coal. 
  The 
  chief 
  

   source 
  of 
  coal 
  in 
  Pennsylvania 
  has 
  been 
  the 
  Monongahela 
  and 
  Alle- 
  

   gheny 
  formations. 
  Statistics 
  compiled 
  in 
  1902 
  by 
  the 
  United 
  States 
  

   Geological 
  Survey 
  report 
  Pennsylvania 
  as 
  having 
  one-half 
  the 
  coal 
  

   and 
  two-thirds 
  of 
  the 
  coke 
  production 
  of 
  the 
  United 
  States. 
  

  

  The 
  room 
  and 
  pillar 
  system 
  is 
  in 
  use 
  in 
  nearly 
  all 
  mines 
  through- 
  

   out 
  the 
  region. 
  The 
  coal 
  is 
  either 
  hand 
  or 
  machine 
  mined, 
  and 
  is 
  

   removed 
  from 
  the 
  mines 
  in 
  cars 
  by 
  electric, 
  compressed-air, 
  cable, 
  

   or 
  steam 
  haulage. 
  Horses 
  or 
  mules 
  are 
  used 
  in 
  the 
  mines 
  to 
  convey 
  

   the 
  cars 
  to 
  the 
  main 
  haulages. 
  Wood 
  is 
  required 
  for 
  the 
  following 
  

   principal 
  uses: 
  

  

  1. 
  Large 
  and 
  small 
  material 
  in 
  buildings, 
  tipples, 
  etc., 
  above 
  

   ground. 
  

  

  2. 
  Mine 
  props 
  or 
  posts 
  in 
  the 
  mines. 
  By 
  law 
  the 
  minimum 
  size 
  

   at 
  the 
  top 
  end 
  must 
  be 
  16 
  square 
  inches. 
  The 
  length 
  varies 
  from 
  

   6 
  to 
  10 
  feet, 
  according 
  to 
  the 
  thickness 
  of 
  the 
  seam 
  being 
  worked. 
  

   The 
  more 
  durable 
  species, 
  such 
  as 
  oak, 
  chestnut, 
  and 
  locust, 
  are 
  

   used 
  for 
  roof 
  support 
  in 
  the 
  main 
  haulages, 
  but 
  any 
  other 
  species 
  of 
  

   wood 
  can 
  be 
  used 
  elsewhere. 
  

  

  3. 
  Ties 
  for 
  mine 
  railways. 
  The 
  ties 
  in 
  common 
  use 
  are 
  5 
  feet 
  in 
  

   length, 
  with 
  a 
  cross 
  section 
  from 
  4 
  to 
  6 
  inches 
  in 
  width 
  and 
  depth. 
  

   Durable 
  woods 
  are 
  used 
  under 
  the 
  tracks 
  of 
  the 
  main 
  haulage 
  ways, 
  

   but 
  almost 
  any 
  species 
  on 
  temporary 
  branches. 
  

  

  4. 
  Large-sized 
  timbers 
  in 
  the 
  mine. 
  These 
  require 
  durability 
  and 
  

   strength, 
  and 
  are 
  used 
  at 
  the 
  mine 
  entrance, 
  or 
  in 
  special 
  cases 
  for 
  

   roof 
  support. 
  

  

  Mining 
  frequently 
  affects 
  the 
  physical 
  character 
  of 
  the 
  surface 
  of 
  

   the 
  land. 
  With 
  the 
  final 
  removal 
  of 
  the 
  coal 
  between 
  the 
  rooms 
  

   the 
  roof 
  of 
  the 
  mine 
  tends 
  to 
  fall. 
  This 
  is 
  an 
  advantage 
  in 
  mining, 
  

   since 
  it 
  relieves 
  the 
  pressure 
  from 
  above 
  on 
  the 
  adjacent 
  unmined 
  

   portions 
  of 
  the 
  seam. 
  By 
  withdrawing 
  the 
  pit 
  posts 
  into 
  a 
  line 
  

   next 
  to 
  the 
  unmined 
  portion 
  the 
  fall 
  of 
  the 
  rock 
  above 
  the 
  mined 
  

   portion 
  can 
  usually 
  be 
  effected. 
  Wherever 
  the 
  coal 
  lies 
  close 
  to 
  the 
  

   surface 
  the 
  fall-in 
  extends 
  to 
  the 
  surface 
  and 
  a 
  sink 
  hole 
  of 
  the 
  same 
  

  

  