﻿44 
  H. 
  G. 
  FORDHAM 
  ON 
  STRUCTURE 
  DEVELOPED 
  IN 
  CHALK. 
  

  

  wished 
  to 
  know 
  to 
  what 
  combination 
  with 
  lime 
  for 
  a 
  base 
  the 
  

   form 
  of 
  the 
  crystals 
  was 
  due. 
  He 
  thought 
  tbat 
  a 
  certain 
  amount 
  

   of 
  phosphate 
  of 
  lime 
  was 
  present 
  in 
  the 
  concretions, 
  but 
  was 
  absent 
  

   in 
  similar 
  specimens 
  in 
  the 
  Upper 
  Chalk; 
  so 
  that 
  it 
  appeared 
  as 
  if 
  

   the 
  same 
  cause 
  could 
  not 
  apply 
  in 
  both 
  cases. 
  The 
  striae 
  were 
  not, 
  

   he 
  thought, 
  due 
  to 
  slipping 
  or 
  to 
  organic 
  growth, 
  but 
  might 
  arise 
  

   from 
  some 
  alteration 
  in 
  the 
  character 
  of 
  the 
  chalk. 
  

  

  Mr. 
  Evans 
  observed 
  that 
  the 
  striae 
  appeared 
  to 
  be 
  due 
  to 
  two 
  

   causes 
  — 
  crystallization, 
  whether 
  incipient 
  or 
  destroyed, 
  and 
  slicken- 
  

   sides. 
  He 
  thought 
  that 
  in 
  the 
  Ashwell 
  specimens 
  much 
  was 
  owing 
  

   to 
  the 
  hard 
  nodules 
  resisting 
  pressure 
  better 
  than 
  the 
  surrounding 
  

   chalk, 
  which, 
  in 
  being 
  condensed, 
  passed 
  over 
  their 
  surfaces 
  and 
  

   produced 
  a 
  kind 
  of 
  slickensides. 
  The 
  same 
  appeared 
  to 
  have 
  been 
  

   the 
  case 
  with 
  the 
  nodule 
  of 
  pyrites. 
  

  

  Mr. 
  Forbes 
  remarked 
  that 
  an 
  inspection 
  of 
  the 
  specimens 
  on 
  the 
  

   table 
  convinced 
  him 
  that 
  in 
  several 
  instances 
  the 
  structure 
  was 
  due 
  

   to 
  slickensides, 
  but 
  that 
  in 
  others 
  traces 
  of 
  a 
  very 
  different 
  structure 
  

   were 
  visible, 
  Avhich 
  he 
  imagined 
  was 
  due 
  to 
  crystallization, 
  the 
  car- 
  

   bonate 
  of 
  lime 
  having 
  most 
  probably 
  assumed 
  the 
  form 
  of 
  Aragonite, 
  

   which, 
  owing 
  to 
  its 
  instability, 
  had 
  lost 
  its 
  crystalline 
  lustre 
  and 
  

   assumed 
  a 
  mealy 
  or 
  chalky 
  appearance. 
  

  

  Mr. 
  Jttdd 
  thought 
  that 
  the 
  difference 
  between 
  the 
  conclusions 
  of 
  

   the 
  two 
  papers 
  communicated 
  to 
  the 
  Society 
  upon 
  this 
  subject 
  was 
  

   mainly 
  due 
  to 
  the 
  difference 
  between 
  the 
  Chalk 
  of 
  Yorkshire 
  and 
  

   that 
  of 
  the 
  south 
  of 
  England. 
  The 
  abundance 
  of 
  this 
  structure 
  in 
  

   Yorkshire 
  had 
  caused 
  Mr. 
  Mortimer 
  to 
  connect 
  it 
  with 
  some 
  

   organism 
  concerned 
  in 
  the 
  formation 
  of 
  the 
  Chalk 
  ; 
  while 
  its 
  rarity 
  

   in 
  the 
  south 
  had 
  led 
  Mr. 
  Fordham 
  to 
  assign 
  another 
  and 
  a 
  chemical 
  

   cause. 
  He 
  thought 
  that 
  it 
  became 
  apparent 
  only 
  in 
  those 
  parts 
  of 
  

   the 
  Chalk 
  through 
  which 
  water 
  most 
  readily 
  passed, 
  and 
  considered 
  

   that 
  in 
  some 
  cases 
  the 
  crystallization 
  had 
  been 
  that 
  of 
  Aragonite, 
  

   in 
  others 
  that 
  of 
  Calcite. 
  He 
  commented 
  on 
  the 
  dimorphism 
  of 
  

   carbonate 
  of 
  lime, 
  which 
  under 
  slightly 
  different 
  conditions 
  assumed 
  

   different 
  crystalline 
  forms 
  ; 
  and 
  remarked 
  that, 
  as 
  was 
  the 
  case 
  with 
  

   many 
  igneous 
  rocks, 
  the 
  structure 
  in 
  the 
  Chalk 
  only 
  became 
  apparent 
  

   after 
  weathering. 
  

  

  Mr. 
  Meter 
  remarked 
  that 
  the 
  striae 
  were 
  most 
  common 
  where 
  

   nodules 
  were 
  present, 
  and 
  in 
  beds 
  in 
  which 
  the 
  fossils 
  were 
  crushed. 
  

   So 
  far 
  as 
  he 
  had 
  observed, 
  they 
  were 
  always 
  vertical 
  ; 
  and 
  he 
  attri- 
  

   buted 
  them 
  to 
  a 
  re-arrangement 
  of 
  the 
  particles 
  of 
  the 
  Chalk 
  

   under 
  pressure. 
  

  

  Mr. 
  Fordham 
  explained 
  that 
  two 
  of 
  the 
  specimens 
  had 
  been 
  taken 
  

   from 
  the 
  sides 
  of 
  a 
  fissure, 
  and 
  were 
  actually 
  slickensides. 
  He 
  

   thought 
  that 
  some 
  of 
  the 
  nodules 
  exhibit 
  traces 
  of 
  internal 
  struc- 
  

   ture 
  ; 
  but 
  it 
  was 
  true 
  that 
  the 
  general 
  direction 
  of 
  the 
  striae 
  was 
  at 
  

   right 
  angles 
  to 
  the 
  lines 
  of 
  bedding. 
  He 
  was 
  glad 
  that 
  his 
  remarks 
  

   had 
  led 
  to 
  so 
  interesting 
  a 
  discussion. 
  

  

  Mr. 
  Prestwich 
  suggested 
  that 
  an 
  analysis 
  should 
  be 
  made 
  of 
  the 
  

   specimens. 
  

  

  