﻿72 
  E. 
  H. 
  Perkins 
  — 
  Dighton 
  Conglomerate. 
  

  

  granites 
  and 
  the 
  upper 
  beds. 
  The 
  rocks 
  are 
  very 
  gran- 
  

   itic 
  in 
  appearance 
  and 
  in 
  places 
  are 
  hardly 
  distinguish- 
  

   able 
  from 
  the 
  nearby 
  granites. 
  These 
  arkoses 
  pass 
  into 
  

   and 
  contain 
  layers 
  of 
  the 
  basal 
  conglomerates 
  the 
  pebbles 
  

   of 
  which 
  are 
  small 
  and 
  angular. 
  The 
  composition 
  of 
  

   these 
  beds 
  closely 
  resembles 
  the 
  arkose. 
  

  

  Following 
  the 
  basal 
  beds 
  come 
  the 
  10,000 
  feet 
  plus 
  or 
  

   minus 
  of 
  the 
  Coal 
  Measures. 
  While 
  conglomerates 
  still 
  

   occur 
  in 
  small 
  beds, 
  the 
  dominant 
  deposits 
  are 
  sand- 
  

   stones 
  and 
  shales 
  with 
  coal 
  and 
  graphite 
  beds. 
  The 
  

   sandstones 
  still 
  contain 
  feldspar, 
  and 
  the 
  shales 
  have 
  a 
  

   clayey 
  smell, 
  but 
  evidently 
  decomposition 
  was 
  more 
  im- 
  

   portant 
  than 
  it 
  had 
  previously 
  been. 
  A 
  rich 
  vegetation 
  

   developed, 
  partly 
  on 
  the 
  site 
  of 
  deposition 
  and 
  partly 
  

   nearer 
  the 
  headwaters, 
  as 
  is 
  indicated 
  by 
  the 
  drift 
  logs 
  

   found 
  in 
  the 
  gray 
  sandstone. 
  Apparently 
  the 
  climate 
  

   was 
  becoming 
  still 
  more 
  moist, 
  allowing 
  the 
  formation 
  of 
  

   coal 
  swamps 
  over 
  the 
  delta. 
  The 
  mantle 
  of 
  vegetation 
  

   in 
  the 
  headwaters 
  may 
  have 
  covered 
  the 
  ground 
  so 
  as 
  to 
  

   permit 
  the 
  erosion 
  of 
  only 
  fine 
  material. 
  

  

  The 
  Coal 
  Measures 
  pass 
  through 
  an 
  alternation 
  of 
  

   shale, 
  sandstone 
  and 
  conglomerate 
  layers 
  into 
  the 
  Digh- 
  

   ton 
  formation. 
  The 
  writer 
  found 
  no 
  indication 
  of 
  an 
  

   unconformity 
  between 
  the 
  two 
  formations. 
  The 
  zone 
  

   of 
  passage 
  seems 
  to 
  indicate 
  a 
  progressive 
  rhythmic 
  

   change 
  from 
  conditions 
  which 
  permitted 
  the 
  formation 
  

   of 
  the 
  Coal 
  Measures 
  to 
  those 
  which 
  permitted 
  the 
  forma- 
  

   tion 
  of 
  the 
  great 
  thickness 
  of 
  Dighton 
  conglomerate. 
  

  

  Summary 
  of 
  evidence. 
  

  

  I. 
  Evidence 
  favoring 
  a 
  fluviatile 
  origin. 
  

  

  A. 
  Structure. 
  

  

  1. 
  Pell-mell 
  structure 
  common. 
  

  

  2. 
  Absence 
  of 
  definite 
  though 
  considerable 
  thickness 
  

  

  of 
  conglomerate. 
  

  

  3. 
  Great 
  vertical 
  and 
  horizontal 
  variation 
  in 
  compo- 
  

  

  sition 
  of 
  beds. 
  

  

  4. 
  Lenses 
  of 
  shale, 
  sandstone 
  and 
  conglomerate 
  of 
  

  

  various 
  sizes 
  interlaced 
  in 
  different 
  directions. 
  

  

  5. 
  Extremely 
  irregular 
  cross-bedding 
  common. 
  

  

  6. 
  Pebble 
  bands 
  common. 
  

  

  7. 
  Isolated 
  pebbles 
  common 
  in 
  the 
  sandstone 
  layers. 
  

  

  8. 
  Thickness 
  1,000 
  to 
  15,000 
  feet. 
  

  

  B. 
  Matrix. 
  

  

  1. 
  Rapid 
  variations 
  in 
  amount. 
  

  

  2. 
  Grains 
  angular 
  and 
  unsorted. 
  

  

  