﻿H. 
  G. 
  FORDHAM 
  ON 
  STRUCTURE 
  DEVELOPED 
  IN 
  CHALK. 
  43 
  

  

  8. 
  Notes 
  on 
  the 
  Structure 
  sometimes 
  developed 
  in 
  Chalk. 
  By 
  

   H. 
  George 
  Fordham, 
  Esq., 
  F.G.S. 
  (Bead 
  December 
  3, 
  1873.) 
  

  

  A 
  paper 
  on 
  this 
  subject 
  (the 
  peculiar 
  parallel 
  striae 
  found 
  in 
  

   Chalk), 
  read 
  before 
  the 
  Society 
  last 
  May, 
  brought 
  up 
  a 
  discussion 
  

   in 
  which 
  such 
  a 
  great 
  variety 
  of 
  opinion 
  as 
  to 
  the 
  origin 
  of 
  this 
  

   structure 
  was 
  shown, 
  that 
  I 
  thought 
  the 
  following 
  few 
  facts 
  might 
  

   perhaps 
  be 
  of 
  interest. 
  

  

  In 
  a 
  pit 
  one 
  mile 
  east 
  of 
  Ash 
  well, 
  on 
  the 
  borders 
  of 
  Hertfordshire 
  

   and 
  Cambridgeshire, 
  a 
  small 
  section 
  of 
  the 
  " 
  Chalk 
  without 
  flints 
  " 
  of 
  

   the 
  Geological 
  Survey, 
  about 
  40 
  or 
  50 
  feet 
  above 
  its 
  base, 
  is 
  exposed. 
  

   The 
  chalk 
  here 
  is 
  hard, 
  and 
  may 
  be 
  divided 
  into 
  layers, 
  which 
  differ 
  

   considerably 
  in 
  structure. 
  There 
  are 
  at 
  least 
  three 
  varieties: 
  — 
  (1) 
  

   layers 
  of 
  hard 
  chalk, 
  divided 
  into 
  slabs 
  about 
  an 
  inch 
  thick; 
  (2) 
  beds 
  

   much 
  thicker 
  and 
  more 
  massive 
  than 
  the 
  last 
  ; 
  and 
  (3) 
  a 
  bed 
  of 
  a 
  

   concretionary 
  nature. 
  These 
  beds 
  are 
  from 
  six 
  inches 
  to 
  several 
  

   feet 
  in 
  thickness. 
  

  

  The 
  concretions 
  are 
  marked 
  nearly 
  all 
  over 
  by 
  the 
  lines 
  of 
  

   this 
  structure. 
  I 
  find 
  that 
  the 
  lines 
  are 
  only 
  found 
  on 
  the 
  con- 
  

   cretions 
  (which 
  are 
  easily 
  removed 
  from 
  the 
  parent 
  rock) 
  and 
  in 
  

   their 
  very 
  immediate 
  neighbourhood. 
  The 
  other 
  beds 
  are 
  entirely 
  

   free 
  from 
  any 
  trace 
  of 
  the 
  structure. 
  

  

  The 
  chalk 
  at 
  this 
  particular 
  spot 
  has 
  not 
  been 
  very 
  much 
  faulted, 
  

   and 
  the 
  layers 
  can 
  be 
  traced 
  without 
  change 
  for 
  some 
  little 
  distance; 
  

   it 
  is 
  hard 
  ; 
  and 
  the 
  fossils 
  in 
  it, 
  which 
  at 
  this 
  place 
  are 
  compa- 
  

   ratively 
  numerous 
  in 
  the 
  concretionary 
  bed, 
  are 
  invariably 
  crushed, 
  

   as 
  if 
  by 
  pressure 
  : 
  the 
  structure 
  seems 
  to 
  have 
  no 
  connexion 
  with 
  

   them. 
  

  

  These 
  facts 
  seem 
  to 
  point 
  to 
  a 
  relationship 
  between 
  the 
  concre- 
  

   tions 
  and 
  this 
  structure. 
  I 
  believe 
  the 
  peculiar 
  striae 
  in 
  this 
  case 
  

   are 
  due 
  to 
  an 
  incipient 
  crystallization 
  arising 
  from 
  the 
  formation 
  of 
  

   the 
  concretions. 
  In 
  support 
  of 
  the 
  view 
  that 
  they 
  are 
  due 
  to 
  crys- 
  

   tallization, 
  I 
  exhibit 
  a 
  specimen 
  of 
  iron 
  pyrites 
  from 
  the 
  Chalk 
  

   of 
  Beachy 
  Head, 
  attached 
  to 
  which 
  is 
  a 
  small 
  portion 
  of 
  chalk, 
  very 
  

   hard 
  and 
  striated. 
  Now 
  the 
  chalk 
  there 
  is 
  in 
  very 
  ]arge 
  masses, 
  as 
  

   much 
  as 
  6 
  feet 
  each 
  way, 
  and 
  does 
  not 
  appear 
  to 
  be 
  in 
  any 
  way 
  

   anomalous 
  in 
  structure. 
  It 
  would 
  seem 
  that 
  in 
  this 
  specimen 
  the 
  

   crystallization 
  of 
  the 
  pyrites 
  had 
  induced 
  a 
  crystallization 
  in 
  the 
  

   chalk. 
  

  

  I 
  believe 
  that 
  in 
  some 
  places, 
  however, 
  an 
  almost 
  identical 
  

   structure 
  is 
  due 
  to 
  slickensides, 
  but 
  only 
  in 
  very 
  broken 
  and 
  faulted 
  

   strata. 
  

  

  Discussion. 
  

  

  Mr. 
  Seelet 
  observed 
  that 
  the 
  structure 
  was 
  familiar 
  to 
  all. 
  If 
  

   it 
  were 
  due 
  to 
  crystallization, 
  whether 
  incipient 
  or 
  otherwise, 
  he 
  

  

  