﻿156 
  Bucher 
  — 
  Origin 
  of 
  Ripples, 
  and 
  

  

  therefore, 
  must 
  be 
  due 
  to 
  special 
  conditions, 
  which, 
  under 
  

   circumstances, 
  may 
  be 
  of 
  value 
  to 
  the 
  paleogeographer. 
  

   In 
  general 
  there 
  are 
  two 
  possibilities 
  : 
  

  

  1st. 
  The 
  sediment 
  is 
  carried 
  in 
  suspension 
  by 
  a 
  strong 
  

   surface 
  current, 
  from 
  which 
  it 
  settles 
  into 
  the 
  quiet 
  bot- 
  

   tom 
  water, 
  for 
  instance, 
  the 
  strong 
  surface 
  current 
  of 
  a 
  

   river 
  emptying 
  into 
  an 
  ocean 
  12 
  or 
  into 
  another 
  river, 
  or 
  

   flowing 
  over 
  deeper 
  retarded 
  portions 
  of 
  its 
  own 
  course 
  ; 
  

   or 
  a 
  surficial 
  wind-drift 
  current 
  carrying 
  sediment 
  in 
  

   suspension 
  in 
  the 
  upper 
  layers 
  of 
  the 
  water 
  from 
  shal- 
  

   lower 
  or 
  more 
  disturbed 
  regions 
  into 
  quieter 
  or 
  deeper 
  

   water. 
  

  

  2d. 
  The 
  sediment 
  settles 
  into 
  the 
  water 
  from 
  the 
  air, 
  

   as 
  e. 
  g., 
  volcanic 
  ashes 
  or 
  windblown 
  sand. 
  The 
  green 
  

   slates 
  of 
  Langdale, 
  as 
  described 
  by 
  Sorby 
  (1908, 
  p. 
  197, 
  

   pi. 
  15), 
  are 
  a 
  good 
  example 
  of 
  the 
  former, 
  since 
  they 
  con- 
  

   sist 
  practically 
  of 
  fine-grained 
  volcanic 
  ash 
  and 
  show 
  this 
  

   structure 
  beautifully. 
  In 
  case 
  of 
  ripple-drift, 
  the 
  bottom 
  

   water 
  is 
  in 
  gentle 
  motion. 
  

  

  5. 
  Wave-length 
  and 
  amplitude. 
  — 
  In 
  addition 
  to 
  the 
  

   quantitative 
  data 
  compiled 
  in 
  tables 
  2 
  and 
  3, 
  the 
  follow- 
  

   ing 
  observations 
  are 
  of 
  importance 
  : 
  

  

  (a.) 
  When 
  first 
  appearing, 
  current-ripples 
  show 
  a 
  

   wave-length 
  which 
  is 
  half 
  that 
  of 
  their 
  full 
  development 
  

   (Blasius, 
  1910, 
  p. 
  468). 
  

  

  (b.) 
  The 
  wave-length 
  increases 
  with 
  the 
  velocity 
  of 
  

   the 
  current. 
  Hahmann's 
  experiments 
  (1912, 
  p. 
  646; 
  cf. 
  

   table 
  2) 
  seem 
  to 
  prove 
  that 
  the 
  increment 
  of 
  wave-length 
  

   is 
  proportional 
  to 
  the 
  increment 
  of 
  velocity. 
  This 
  law, 
  

   of 
  course, 
  holds 
  good 
  only 
  between 
  the 
  two 
  critical 
  points 
  

   of 
  velocity, 
  between 
  which 
  current-ripples 
  can 
  exist. 
  It 
  

   cannot 
  be 
  reversed; 
  i. 
  e., 
  the 
  wave-length 
  does 
  not 
  

   decrease 
  in 
  proportion 
  to 
  the 
  velocity. 
  

  

  (c.) 
  Hahmann 
  made 
  an 
  attempt 
  to 
  determine 
  whether 
  

   depth 
  of 
  water 
  aside 
  from 
  its 
  controlling 
  influence 
  on 
  the 
  

   distribution 
  of 
  velocities 
  within 
  the 
  current 
  influences 
  

   the 
  wave-length 
  of 
  current-ripples. 
  His 
  determinations 
  

   were 
  made 
  with 
  a 
  disk, 
  covered 
  with 
  sand, 
  rotating 
  in 
  

   water. 
  Within 
  the 
  narrow 
  limits 
  of 
  depth 
  at 
  his 
  disposal 
  

   no 
  influence 
  of 
  the 
  depth 
  of 
  water 
  on 
  the 
  wave-length 
  was 
  

   found 
  (1912, 
  p. 
  655). 
  

  

  u 
  Ia 
  this 
  case 
  the 
  ucik 
  bottom 
  current 
  may 
  flow 
  in 
  opposite 
  direction 
  to 
  

   the 
  surface 
  current.- 
  For 
  ;i 
  very 
  interesting 
  ease 
  of 
  the 
  checking 
  of 
  a 
  

   strong 
  surflcia] 
  tidal 
  current 
  in 
  the 
  Golden 
  Gate 
  at 
  periods 
  of 
  great 
  freshets 
  

   in 
  the 
  Sacramento 
  and 
  San 
  Joaquin 
  rivers, 
  see 
  Kindle, 
  1917, 
  p. 
  46. 
  

  

  