﻿280 
  Galloway 
  — 
  Rounding 
  of 
  Sand 
  by 
  Solution. 
  

  

  dolomite 
  and 
  feldspars, 
  which 
  are 
  not 
  usually 
  simple 
  

   crystals 
  but 
  are 
  twinned 
  and 
  vary 
  in 
  composition 
  in 
  the 
  

   same 
  crystal, 
  dissolve 
  according 
  to 
  the 
  solubility 
  of 
  each 
  

   constituent, 
  producing 
  somewhat 
  rounded 
  forms 
  with 
  

   numerous 
  irregularities. 
  Included 
  gas 
  bubbles 
  and 
  crys- 
  

   tals 
  of 
  other 
  minerals, 
  and 
  distorted 
  crystals 
  also 
  inter- 
  

   fere 
  with 
  regularity 
  of 
  surface 
  produced 
  by 
  solution. 
  In 
  

   the 
  case 
  of 
  complex 
  grains, 
  such 
  as 
  those 
  composed 
  of 
  

   interlocking 
  crystals 
  or 
  other 
  lack 
  of 
  homogeneity, 
  the 
  

   attempt 
  to 
  form 
  rounded 
  grains 
  experimentally 
  by 
  solu- 
  

   tion 
  is 
  not 
  usually 
  successful. 
  

  

  Conclusions 
  

  

  Grains 
  of 
  sand 
  of 
  any 
  mineral 
  makeup 
  can 
  be 
  rounded 
  

   by 
  solution, 
  and 
  the 
  process 
  is 
  especially 
  competent 
  on 
  

   those 
  of 
  very 
  small 
  size, 
  i. 
  e., 
  grains 
  below 
  one-tenth 
  milli- 
  

   meter 
  in 
  diameter. 
  

  

  Four 
  types 
  of 
  sand 
  grains 
  are 
  produced 
  by 
  solution, 
  

   depending 
  upon 
  the 
  factors 
  discussed 
  above. 
  They 
  are 
  : 
  

  

  (1) 
  sharply 
  angular 
  grains 
  with 
  brilliant 
  surfaces, 
  as 
  in 
  

   rock 
  flour 
  from 
  glaciers 
  with 
  practically 
  no 
  solution; 
  

  

  (2) 
  angular 
  grains 
  with 
  corroded 
  surfaces, 
  as 
  in 
  Pleis- 
  

   tocene 
  glacial 
  silt 
  and 
  Cretaceous 
  clays; 
  (3) 
  round 
  grains 
  

   with 
  corroded 
  surfaces, 
  as 
  in 
  the 
  Rockaway 
  Beach 
  sand 
  

   and 
  the 
  loess 
  from 
  Iowa; 
  and 
  (4) 
  round 
  grains 
  with 
  

   brilliant 
  surfaces, 
  as 
  in 
  the 
  spring 
  sand 
  described 
  above 
  

   and 
  in 
  fuller's 
  earth 
  from 
  Georgia. 
  These 
  four 
  types 
  

   usually 
  occur 
  together, 
  one 
  or 
  the 
  other 
  predominating. 
  

  

  While 
  it 
  is 
  probably 
  always 
  true 
  that 
  a 
  sandstone 
  con- 
  

   sisting 
  wholly 
  of 
  well-rounded 
  grains, 
  many 
  of 
  which 
  are 
  

   less 
  than 
  -1 
  mm. 
  in 
  diameter, 
  was 
  deposited 
  by 
  and 
  owes 
  

   its 
  roundness 
  of 
  grain 
  to 
  the 
  wind, 
  the 
  presence 
  of 
  a 
  small 
  

   percentage 
  of 
  minute, 
  round 
  grains 
  is 
  of 
  itself 
  not 
  a 
  safe 
  

   criterion 
  for 
  assigning 
  the 
  origin 
  of 
  a 
  sandstone 
  to 
  wind 
  

   action. 
  It 
  rather 
  points 
  to 
  water 
  action. 
  

  

  Any 
  reliable 
  set 
  of 
  criteria 
  for 
  the 
  genesis 
  of 
  a 
  sedi- 
  

   mentary 
  rock 
  are 
  more 
  complex 
  than 
  is 
  usually 
  assumed. 
  

   All 
  available 
  data, 
  the 
  physical 
  form 
  and 
  mineralogical 
  

   character 
  of 
  grains, 
  the 
  structure 
  of 
  the 
  rock, 
  the 
  strati- 
  

   graphy 
  and 
  field 
  relations, 
  the 
  fossil 
  evidence 
  and 
  paleo- 
  

   geography, 
  must 
  be 
  weighed 
  before 
  the 
  origin 
  of 
  doubtful 
  

   rocks 
  can 
  be 
  correctly 
  determined. 
  

  

  Columbia 
  University, 
  

   New 
  York 
  City. 
  

  

  