﻿Belated 
  Sedimentary 
  Surface 
  Forms. 
  181 
  

  

  derived 
  for 
  the 
  simplest 
  case 
  only, 
  namely, 
  waves 
  formed 
  

   on 
  the 
  contact 
  of 
  two 
  liquids 
  of 
  infinite 
  extent. 
  

  

  If 
  this 
  interpretation 
  of 
  the 
  nature 
  of 
  sand-waves 
  is 
  

   correct, 
  it 
  is 
  evident, 
  as 
  was 
  indicated 
  already, 
  that 
  they 
  

   can 
  not 
  exist 
  at 
  velocities 
  other 
  than 
  those 
  creating 
  them. 
  

   When 
  the 
  velocity 
  decreases 
  with 
  the 
  fall 
  of 
  the 
  river 
  or 
  

   the 
  tide, 
  the 
  sediment 
  from 
  the 
  water 
  above 
  the 
  waves 
  

   fills 
  up 
  the 
  troughs, 
  forming 
  smaller 
  waves, 
  correspond- 
  

   ing 
  to 
  the 
  smaller 
  depth 
  and 
  velocity, 
  on 
  the 
  newly 
  

   created 
  surface. 
  As 
  the 
  velocity 
  drops 
  below 
  the 
  third 
  

   critical 
  point, 
  the 
  bodies 
  of 
  the 
  waves 
  settle 
  and 
  become 
  

   rigid, 
  a 
  vortex 
  forms 
  on 
  the 
  lee 
  side 
  and 
  now 
  the 
  weak 
  

   current, 
  like 
  wind 
  on 
  a 
  dune, 
  moves 
  but 
  the 
  grains 
  of 
  the 
  

   surface 
  layer, 
  rolling 
  them 
  up 
  the 
  weather 
  slope 
  and 
  

   dropping 
  them 
  on 
  the 
  lee 
  side. 
  Since 
  the 
  angle 
  of 
  rest 
  

   of 
  ordinary 
  materials 
  differs 
  greatly 
  from 
  the 
  gentle 
  

   slope 
  of 
  the 
  sand-wave, 
  this 
  must 
  undergo 
  a 
  fundamental 
  

   change 
  in 
  form, 
  from 
  more 
  or 
  less 
  symmetrical 
  waves 
  to 
  

   strongly 
  asymmetrical 
  " 
  dunes." 
  

  

  It 
  appears 
  to 
  be 
  advisable 
  to 
  use 
  different 
  terms 
  to 
  dis- 
  

   tinguish 
  between 
  the 
  friction- 
  waves 
  and 
  the 
  secondary 
  

   forms 
  derived 
  from 
  them. 
  I, 
  therefore, 
  suggest 
  to 
  limit 
  

   the 
  term 
  " 
  sand- 
  waves" 
  to 
  the 
  former 
  and 
  to 
  use 
  the 
  

   term 
  "meta-ripples" 
  for 
  the 
  latter. 
  

  

  Where 
  the 
  meta-ripple 
  consists 
  of 
  uniform 
  material, 
  

   this 
  secondary 
  origin 
  of 
  its 
  outline 
  can 
  not 
  be 
  demon- 
  

   strated. 
  But 
  where 
  the 
  material 
  is 
  heterogeneous, 
  it 
  

   becomes 
  at 
  once 
  evident 
  by 
  the 
  fact 
  that 
  the 
  coarse 
  

   material 
  forms 
  a 
  core 
  with 
  a 
  "hood" 
  of 
  fine 
  material 
  

   placed 
  asymmetrically 
  on 
  one 
  side. 
  

  

  The 
  secondary 
  nature 
  of 
  the 
  meta-ripples 
  explains 
  a 
  

   well-known 
  feature 
  of 
  cross-bedded 
  river 
  deposits. 
  They 
  

   show 
  the 
  steeply 
  inclined 
  layers 
  which 
  formed 
  on 
  the 
  lee- 
  

   side 
  of 
  meta-ripples 
  and 
  associated 
  complex 
  forms 
  during 
  

   the 
  subsiding 
  stages 
  of 
  the 
  water, 
  facing 
  in 
  different 
  

   directions 
  and 
  therefore 
  cut 
  at 
  different 
  angles 
  by 
  the 
  

   plane 
  of 
  the 
  accidental 
  exposure.. 
  They 
  show 
  the 
  pat- 
  

   tern, 
  but 
  never 
  the 
  surface 
  forms 
  to 
  which 
  it 
  is 
  due, 
  as 
  

   should 
  be 
  expected, 
  if 
  these 
  forms 
  existed 
  on 
  the 
  bottom* 
  

   of 
  the 
  river 
  during 
  high 
  water, 
  after 
  which 
  they 
  could 
  be 
  

   preserved 
  under 
  the 
  sediment 
  dropped 
  from 
  suspension. 
  

   The 
  inclined 
  layers 
  are 
  invariably 
  cut 
  off 
  by 
  a 
  "true 
  

   bedding 
  plane," 
  wherever 
  followed 
  by 
  sediment 
  of 
  similar 
  

   grain. 
  This 
  plane 
  indicates 
  the 
  depth 
  to 
  which 
  the 
  sedi- 
  

  

  Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Vol. 
  XLVII, 
  No. 
  279.— 
  March, 
  1919. 
  

   13 
  

  

  