﻿264 
  Bucher 
  — 
  Origin 
  of 
  Ripples, 
  and 
  

  

  b. 
  Large 
  current-ripples 
  are 
  found 
  only 
  on 
  rocks 
  of 
  

   relatively 
  coarse 
  grain, 
  as 
  conglomeratic 
  sands 
  (Cuya- 
  

   hoga 
  formation) 
  or 
  fragmental 
  limestones, 
  never 
  on 
  fine- 
  

   grained 
  sediments, 
  that 
  is, 
  on 
  dense 
  blue 
  argillaceous 
  

   limestones. 
  These 
  are, 
  however, 
  frequently 
  covered 
  

   with 
  small 
  current-ripples, 
  ranging 
  in 
  wave-length 
  from 
  

   1 
  to 
  30 
  cm., 
  and 
  are 
  often 
  inter 
  stratified 
  with 
  fragmental 
  

   limestones 
  of 
  coarse 
  grain 
  covered 
  with 
  large 
  ripples 
  and 
  

   separated 
  from 
  them 
  only 
  by 
  thin 
  layers 
  of 
  shale. 
  The 
  

   calcareous 
  layers 
  show 
  delicate 
  tracks 
  of 
  gastropods 
  or 
  

   trilobites 
  well 
  preserved 
  which 
  practically 
  exclude 
  any 
  

   current 
  action. 
  

  

  This 
  seems 
  to 
  indicate 
  that 
  the 
  current 
  in 
  question 
  

   varied 
  in 
  intensity 
  from 
  a 
  maximum 
  to 
  nil, 
  in 
  relatively 
  

   short 
  intervals. 
  The 
  finer 
  sediments 
  could 
  record 
  only 
  

   the 
  weaker 
  movements, 
  as 
  stronger 
  currents 
  would 
  have 
  

   thrown 
  them 
  into 
  suspension. 
  

  

  c. 
  In 
  the 
  Ordovician, 
  I 
  have 
  repeatedly 
  found 
  large 
  

   asymmetrical 
  ripples 
  on 
  two 
  successive 
  limestones, 
  not 
  

   more 
  than 
  a 
  foot 
  apart, 
  with 
  nearly 
  the 
  same 
  strike, 
  but 
  

   with 
  their 
  lee 
  sides 
  facing 
  in 
  opposite 
  directions. 
  The 
  

   current, 
  therefore, 
  reversed 
  its 
  direction 
  in 
  relatively 
  

   short 
  intervals. 
  

  

  Observations 
  b 
  and 
  c 
  exclude 
  ocean 
  currents 
  of 
  larger 
  

   dimensions, 
  while 
  they 
  point 
  consistently 
  to 
  tidal 
  cur- 
  

   rents. 
  These, 
  too, 
  are 
  the 
  only 
  marine 
  currents 
  flowing 
  

   parallel 
  to 
  the 
  shore-line 
  in 
  which 
  velocities 
  of 
  at 
  least 
  

   1 
  m. 
  sec, 
  which 
  seem 
  necessary 
  to 
  produce 
  the 
  effects 
  

   observed, 
  are 
  found 
  over 
  wide 
  areas. 
  

  

  The 
  great 
  similarity 
  of 
  form 
  existing 
  between 
  these 
  

   large 
  fossil 
  ripples 
  and 
  some 
  of 
  the 
  tidal 
  para-ripples 
  

   referred 
  to 
  above 
  goes 
  far 
  to 
  confirm 
  these 
  conclusions. 
  

   From 
  Cornish's 
  paper 
  on 
  these 
  tidal 
  ripples 
  62 
  we 
  know 
  

   that 
  on 
  open 
  shores, 
  such 
  as 
  at 
  Mundsley 
  (Norfolk, 
  p. 
  183) 
  

   above 
  the 
  mouth 
  of 
  Barmouth 
  Estuary 
  (p. 
  173), 
  or 
  espe- 
  

   cially 
  on 
  the 
  Goodwin 
  Sands 
  (p. 
  189), 
  about 
  six 
  miles 
  off 
  

   the 
  shore 
  of 
  Kent, 
  these 
  tidal 
  ripples 
  invariably 
  trend 
  at 
  

   right 
  angles 
  to 
  the 
  shore, 
  often 
  at 
  right 
  angles 
  to 
  the 
  

   waves. 
  On 
  the 
  open 
  shore, 
  too, 
  their 
  wave-length 
  is 
  the 
  

   same 
  as 
  that 
  of 
  most 
  large 
  Paleozoic 
  ripples, 
  while 
  those 
  

   observed 
  in 
  estuaries, 
  where 
  the 
  velocity 
  of 
  the 
  tidal 
  cur- 
  

   rent 
  is 
  greatly 
  increased, 
  have 
  a 
  greater 
  wave-length. 
  

  

  The 
  inference, 
  therefore, 
  seems 
  justified 
  that 
  the 
  large 
  

  

  ■"Cornish, 
  Sand 
  Waves 
  in 
  Tidal 
  Currents, 
  Geogr. 
  Jour., 
  vol. 
  18, 
  pp. 
  170- 
  

   202. 
  1901. 
  

  

  