﻿w 
  2 
  54 
  Buclier 
  — 
  Origin 
  of 
  Ripples, 
  and 
  

  

  pies 
  and 
  the 
  testimony 
  of 
  the 
  later 
  deposits 
  of 
  the 
  Mauch 
  

   Chunk 
  shale, 
  render 
  it 
  probable 
  that 
  they 
  were 
  monsoon 
  

   winds 
  blowing 
  to 
  and 
  from 
  the 
  large 
  North 
  Atlantic 
  Con- 
  

   tinent 
  in 
  the 
  northeast. 
  

  

  II. 
  Current-ripples. 
  

  

  1. 
  Description. 
  

  

  Current-ripples 
  are 
  much 
  more 
  common 
  in 
  all 
  kinds 
  of 
  

   sediments 
  than 
  might 
  be 
  judged 
  from 
  the 
  scarcity 
  of 
  the 
  

   name 
  or 
  its 
  equivalent 
  in 
  geological 
  literature. 
  It 
  is 
  

   essential 
  that 
  they 
  should, 
  in 
  all 
  cases, 
  be 
  clearly 
  sep- 
  

   arated 
  in 
  geological 
  reports 
  from 
  oscillation-ripples. 
  

  

  The 
  current 
  origin 
  of 
  some 
  limestone 
  ripples, 
  especially 
  

   small 
  one§, 
  is 
  not 
  always 
  easily 
  recognized, 
  as 
  their 
  

   asymmetry 
  often 
  is 
  not 
  pronounced. 
  A 
  study 
  of 
  a 
  cross- 
  

   section 
  or 
  a 
  careful 
  determination 
  of 
  the 
  position 
  of 
  the 
  

   line 
  of 
  greatest 
  depth 
  will, 
  however, 
  reveal 
  their 
  true 
  

   nature. 
  In 
  most 
  cases, 
  broadly 
  rounded 
  crests 
  lacking 
  

   any 
  indication 
  of 
  a 
  definite 
  crestline, 
  may 
  be 
  taken 
  as 
  

   evidence 
  of 
  current 
  origin. 
  

  

  Fossil 
  current-ripples 
  cover 
  the 
  same 
  range 
  of 
  sizes 
  

   as 
  recent 
  ones. 
  In 
  contrast 
  to 
  oscillation-ripples, 
  they 
  

   are 
  unfit 
  for 
  the 
  discrimination 
  of 
  upper 
  and 
  lower 
  sur- 
  

   faces 
  in 
  structurally 
  complicated 
  regions, 
  owing 
  to 
  the 
  

   similarity 
  of 
  crests 
  and 
  troughs. 
  

  

  2. 
  Interpretation. 
  

  

  The 
  following 
  points 
  are 
  of 
  value 
  to 
  the 
  Paleogeog- 
  

   rapher 
  : 
  

  

  1. 
  Eolian 
  versus 
  subaqueous 
  ripples. 
  — 
  If 
  the 
  observa- 
  

   tions 
  of 
  Cornish 
  and 
  of 
  Kindle 
  (p. 
  197) 
  prove 
  to 
  hold 
  

   true 
  in 
  a 
  great 
  majority 
  of 
  cases, 
  at 
  least, 
  ripples 
  of 
  eolian 
  

   and 
  subaqueous 
  nature 
  may 
  be 
  distinguished 
  by 
  the 
  ratios 
  

   of 
  their 
  wave-lengths 
  and 
  amplitudes. 
  The 
  absence 
  of 
  

   the 
  subaqueous 
  type 
  from 
  arenaceous 
  sediments, 
  espe- 
  

   cially 
  when 
  combined 
  with 
  the 
  typical 
  rounding 
  of 
  grains 
  

   and 
  the 
  peculiar 
  cross-bedding 
  characteristic 
  of 
  dunes, 
  

   should 
  be 
  ^ 
  good 
  evidence 
  of 
  their 
  eolian 
  origin. 
  It 
  

   appears 
  quite 
  doubtful, 
  however, 
  if 
  eolian 
  current-ripples 
  

   ordinarily 
  can 
  be 
  preserved 
  at 
  all. 
  36 
  

  

  2. 
  Current-ripples 
  in 
  marine 
  sediments. 
  — 
  An 
  indica- 
  

   tion 
  of 
  currents 
  of 
  any 
  kind 
  in 
  marine 
  sediments 
  is 
  always 
  

   a 
  matter 
  of 
  interest. 
  I 
  have 
  no 
  doubt 
  that, 
  once 
  atten- 
  

  

  *Cf. 
  p. 
  269 
  (Part 
  II). 
  

  

  