﻿TO 
  E. 
  H. 
  Perkins 
  — 
  Dighton 
  Conglomerate. 
  

  

  blunted 
  and 
  pointed. 
  The 
  idea 
  that 
  rhombic 
  pebbles 
  

   might 
  be 
  a 
  criterion 
  of 
  glacial 
  origin 
  was 
  suggested 
  by 
  

   Professor 
  Charles 
  Brown 
  of 
  Brown 
  University, 
  while 
  

   forms 
  blunted 
  at 
  one 
  end 
  and 
  pointed 
  at 
  the 
  other 
  were 
  

   first 
  found 
  in 
  the 
  Dighton 
  conglomerate 
  and 
  then 
  looked 
  

   for 
  in 
  other 
  deposits. 
  Table 
  II 
  shows 
  the 
  high 
  percent- 
  

   ages 
  of 
  both 
  these 
  forms 
  in 
  glacial 
  and 
  aqueoglacial 
  

   deposits 
  as 
  well 
  as 
  the 
  low 
  percentages 
  for 
  fluviatile 
  

   gravels. 
  The 
  percentage 
  of 
  these 
  special 
  shaped 
  pebbles 
  

   in 
  marine 
  beach 
  deposits 
  is 
  due 
  to 
  the 
  fact 
  that 
  most 
  of 
  

   the 
  beaches 
  studied 
  were 
  made 
  up 
  of 
  glacial 
  pebbles. 
  It 
  

   could 
  be 
  seen, 
  however, 
  that 
  the 
  waves 
  were 
  destroying 
  

   the 
  rhombic 
  and 
  blunted 
  forms 
  instead 
  of 
  producing 
  

   them. 
  

  

  Neither 
  of 
  the 
  forms 
  under 
  consideration 
  is 
  dependent 
  

   on 
  the 
  composition 
  of 
  the 
  pebbles. 
  They 
  have 
  been 
  

   found 
  in 
  both 
  granite 
  and 
  quartzite. 
  In 
  some 
  cases 
  

   these 
  forms 
  may 
  be 
  produced 
  by 
  breaking 
  along 
  planes 
  

   of 
  weakness. 
  The 
  small 
  number 
  found 
  in 
  fluviatile 
  

   deposits 
  may 
  be 
  explained 
  in 
  this 
  way. 
  The 
  percentages 
  

   for 
  the 
  Dighton 
  conglomerate 
  are 
  much 
  higher 
  than 
  

   those 
  of 
  fluviatile 
  deposits 
  and 
  lower 
  than 
  those 
  of 
  gla- 
  

   cial 
  or 
  aqueoglacial 
  origin. 
  In 
  fact, 
  the 
  averages 
  for 
  

   the 
  Dighton, 
  blunted 
  and 
  pointed 
  7-6 
  and 
  rhombic 
  6-2 
  per 
  

   cent, 
  are 
  not 
  far 
  from 
  the 
  averages 
  for 
  the 
  marine 
  

   beaches 
  formed 
  from 
  Pleistocene 
  deposits, 
  blunted 
  and 
  

   pointed 
  8-3 
  and 
  rhombic 
  5-5 
  per 
  cent. 
  The 
  forms 
  of 
  the 
  

   pebbles 
  in 
  the 
  Dighton 
  may, 
  like 
  those 
  of 
  the 
  marine 
  

   deposits, 
  be 
  an 
  inheritance 
  from 
  former 
  glacial 
  action. 
  

  

  Concave 
  fractures 
  caused 
  by 
  the 
  pressure 
  of 
  one 
  peb- 
  

   ble 
  on 
  another 
  have 
  been 
  held 
  to 
  be 
  a 
  mark 
  of 
  glacial 
  

   origin. 
  These 
  fractures 
  may 
  occur, 
  however, 
  during 
  

   dynamic 
  metamorphism. 
  Several 
  instances 
  were 
  noted 
  

   in 
  the 
  Dighton 
  formation 
  where 
  the 
  corner 
  of 
  one 
  pebble 
  

   was 
  lying 
  imbedded 
  in 
  the 
  side 
  of 
  another. 
  For 
  this 
  

   reason 
  the 
  presence 
  of 
  these 
  fractures 
  in 
  the 
  Dighton 
  

   cannot 
  be 
  considered 
  as 
  a 
  good 
  criterion 
  of 
  origin. 
  

  

  Color. 
  

  

  Over 
  the 
  greater 
  part 
  of 
  the 
  basin 
  the 
  color 
  of 
  the 
  con- 
  

   glomerates 
  and 
  sandstones 
  is 
  gray 
  or 
  blue-gray. 
  The 
  

   shales 
  are 
  either 
  the 
  same 
  color 
  or 
  black 
  with 
  organic 
  

   matter. 
  Many 
  of 
  the 
  outcrops 
  are 
  iron-stained, 
  showing 
  

  

  