﻿Vol. 
  53-] 
  THE 
  CRETACEOUS 
  STRATA 
  OF 
  COUNTY 
  ANTRIM. 
  579 
  

  

  original 
  nature 
  of 
  the 
  test 
  thus 
  altered. 
  Apart 
  from 
  these 
  fragments, 
  

   the 
  detrital 
  minerals 
  are 
  more 
  numerous 
  than 
  in 
  the 
  other 
  cases 
  

   analysed, 
  quartz-grains 
  being 
  abundant. 
  These 
  are 
  mostly 
  very- 
  

   angular, 
  and 
  are 
  either 
  colourless 
  or 
  coloured 
  yellow 
  by 
  ferruginous 
  

   coatings. 
  In 
  addition 
  to 
  these, 
  zircon-crystals 
  and 
  tourmaline 
  have 
  

   been 
  noted, 
  though 
  these 
  are 
  very 
  rare. 
  Glauconite-grains 
  are 
  also 
  

   present, 
  and 
  show 
  clearly 
  that 
  they 
  have 
  originated 
  as 
  the 
  internal 
  

   casts 
  of 
  foraminifera. 
  

  

  Speaking 
  generally, 
  the 
  Belemnitella 
  mucronata-zone 
  is 
  distin- 
  

   guished 
  by 
  the 
  absence 
  of 
  detrital 
  minerals. 
  

  

  The 
  reasons 
  guiding 
  the 
  selection 
  of 
  the 
  following 
  specimens 
  

   obtained 
  at 
  Murlough 
  Bay 
  have 
  been 
  stated 
  on 
  p. 
  565. 
  

  

  Pebble-Conglomerate. 
  (No. 
  12 
  in 
  Table 
  II.) 
  

  

  The 
  specimens 
  obtained 
  consisted 
  of 
  a 
  pink 
  limestone, 
  spotted 
  

   with 
  glauconitic 
  grains, 
  and 
  enclosing 
  rounded 
  and 
  subangular 
  

   pebbles 
  of 
  quartzite 
  and 
  vein-quartz. 
  The 
  analysis 
  is 
  that 
  of 
  the 
  

   chalk 
  itself, 
  the 
  pebbles 
  having 
  been 
  previously 
  removed. 
  

  

  The 
  original 
  weight 
  after 
  drying 
  == 
  53*67 
  grammes. 
  After 
  solution 
  

   in 
  20°/ 
  o 
  hydrochloric 
  acid, 
  there 
  remained 
  : 
  Clay, 
  etc. 
  =2 
  *22 
  grammes; 
  

   heavy 
  residue 
  = 
  38*56 
  grammes. 
  

  

  Total 
  percentage 
  composition 
  : 
  — 
  

  

  Carbonate 
  of 
  lime 
  28*16% 
  

  

  Clay, 
  etc 
  , 
  4*14% 
  

  

  Heavy 
  residue 
  67*70% 
  

  

  100 
  

  

  The 
  main 
  residue 
  calls 
  for 
  little 
  comment, 
  consisting 
  only 
  of 
  fine 
  

   clayey 
  material, 
  enclosing 
  minute 
  grains 
  of 
  quartz 
  and 
  glauconite. 
  

  

  The 
  heavy 
  residue 
  has 
  been 
  further 
  subdivided, 
  by 
  means 
  of 
  

   sifting, 
  into 
  four 
  parts 
  : 
  — 
  

  

  A 
  (material 
  that 
  does 
  not 
  pass 
  through 
  a 
  sieve 
  with 
  30 
  meshes 
  

   to 
  the 
  inch). 
  Mainly 
  composed 
  of 
  pebbles 
  of 
  quartzite, 
  some 
  

   altogether 
  colourless, 
  others 
  coloured 
  green 
  owing 
  to 
  the 
  presence 
  

   of 
  a 
  thin 
  chloritic 
  coating, 
  yellow-green 
  masses 
  of 
  glauconite 
  showing 
  

   no 
  traces 
  of 
  organic 
  structure, 
  red 
  orthoclase 
  -felspar, 
  felspathic 
  

   micaceous 
  sandstone, 
  finely 
  micaceous 
  golden-yellow 
  sand-rock, 
  and 
  

   among 
  the 
  smaller 
  examples 
  colourless 
  subangular 
  grains 
  of 
  quartz. 
  

  

  B 
  (not 
  traversing 
  a 
  sieve 
  of 
  60 
  meshes 
  to 
  the 
  inch). 
  In 
  this 
  

   residue 
  the 
  glauconite 
  has 
  more 
  definite 
  outlines, 
  but 
  its 
  organic 
  

   origin 
  is 
  very 
  much 
  obscured. 
  Colourless 
  and 
  brown 
  grains 
  of 
  

   quartz, 
  both 
  rounded 
  and 
  angular, 
  are 
  abundant. 
  In 
  many 
  cases 
  

   the 
  glauconite-grains 
  are 
  distinctly 
  tubular, 
  having 
  apparently 
  been 
  

   the 
  internal 
  casts 
  of 
  sponge-spicules. 
  The 
  light 
  yellow-green 
  

   glauconite-grains 
  are 
  clearly 
  the 
  first 
  stages 
  in 
  the 
  alteration 
  of 
  the 
  

   deep-green 
  varieties, 
  most 
  of 
  which 
  are 
  undoubted 
  internal 
  casts 
  of 
  

   organisms. 
  The 
  latter 
  are 
  often 
  coated 
  with 
  a 
  lighter 
  layer, 
  or, 
  

   if 
  a 
  dark-green 
  grain 
  be 
  broken, 
  it 
  is 
  often 
  seen 
  to 
  have 
  a 
  distinct 
  

   whitish 
  to 
  yellow 
  border, 
  penetrating 
  to 
  some 
  depth. 
  A 
  few 
  

   mica-flakes 
  and 
  silicified 
  tests 
  of 
  foraminifera 
  are 
  also 
  present. 
  

  

  2q2 
  

  

  