﻿Yol. 
  53.] 
  THE 
  CRETACEOUS 
  STRATA 
  OF 
  COUNTY 
  ANTRIM. 
  605 
  

  

  the 
  Glauconitic 
  Sands. 
  It 
  might 
  be 
  argued 
  that 
  this 
  is 
  due 
  to 
  the 
  

   fact 
  that 
  the 
  sponges 
  yielding 
  the 
  necessary 
  silica 
  had 
  entirely 
  dis- 
  

   appeared, 
  but 
  this 
  reply 
  is 
  unsatisfactory, 
  in 
  that 
  it 
  is 
  based 
  entirely 
  on 
  

   negative 
  evidence. 
  The 
  points 
  on 
  which 
  I 
  would 
  lay 
  stress 
  are.: 
  — 
  

   (a) 
  That 
  glauconite 
  is 
  an 
  extremely 
  variable 
  mineral, 
  casts 
  of 
  light 
  

   yellow 
  to 
  dark 
  green 
  colour 
  being 
  indiscriminately 
  ranged 
  under 
  this 
  

   name. 
  (/3) 
  There 
  does 
  appear 
  to 
  be 
  a 
  definite 
  relation 
  between 
  glauco- 
  

   nitic 
  formation 
  and 
  depression, 
  the 
  glauconitic 
  grains 
  diminishing 
  in 
  

   size 
  and 
  quantity 
  as 
  the 
  calcareous 
  element 
  increases 
  ; 
  but 
  we 
  are 
  still 
  

   very 
  much 
  in 
  the 
  dark 
  as 
  to 
  the 
  precise 
  relation 
  of 
  glauconite 
  to 
  the 
  

   detrital 
  materials 
  on 
  the 
  one 
  hand, 
  and 
  the 
  organic 
  tests 
  enclosing 
  

   or 
  replaced 
  by 
  it 
  on 
  the 
  other. 
  

  

  7. 
  The 
  reappearance 
  of 
  species 
  in 
  higher 
  zones 
  after 
  having 
  

   seemingly 
  disappeared 
  has 
  been 
  frequently 
  noted 
  by 
  students 
  of 
  

   Cretaceous 
  geology. 
  Thus 
  Mr. 
  Jukes-Browne, 
  speaking 
  of 
  the 
  

   Chalk 
  Eock 
  of 
  England, 
  remarks 
  that 
  some 
  of 
  the 
  species 
  in 
  it 
  are 
  

   indistinguishable 
  from 
  those 
  of 
  the 
  Chalk 
  Marl 
  nearly 
  400 
  feet 
  

   below, 
  1 
  and 
  Mr. 
  H. 
  Woods, 
  dealing 
  with 
  the 
  mollusca 
  of 
  the 
  Chalk 
  

   Rock, 
  says 
  : 
  — 
  ' 
  As 
  a 
  whole, 
  the 
  fauna 
  presents 
  a 
  much 
  greater 
  

   resemblance 
  to 
  that 
  of 
  the 
  Lower 
  Cenomanian 
  than 
  to 
  any 
  

   which 
  occur 
  in 
  the 
  divisions 
  of 
  the 
  Senonian 
  and 
  Turonian 
  above 
  

   and 
  below 
  it 
  ; 
  and 
  whereas 
  the 
  latter 
  are 
  of 
  a 
  deep-water 
  type, 
  that 
  

   found 
  in 
  the 
  Chalk 
  Rock 
  is 
  certainly 
  of 
  a 
  comparatively 
  shallow- 
  water 
  

   character.' 
  2 
  In 
  Ireland 
  the 
  fauna, 
  which 
  has 
  thus 
  been 
  repeated 
  at 
  

   these 
  widely 
  differing 
  horizons, 
  is 
  carried 
  a 
  stage 
  higher, 
  Ammo* 
  

   nites 
  {Pachy 
  discus, 
  including 
  P. 
  peramplus, 
  a 
  common 
  Middle 
  Chalk 
  

   species), 
  Samites, 
  Anisoceras, 
  Baculites, 
  Nautilus, 
  and 
  examples 
  of 
  

   Holaster 
  planus, 
  together 
  with 
  forms 
  of 
  Turbo, 
  Trochus, 
  Emarginu- 
  

   Una, 
  etc., 
  having 
  been 
  found 
  in 
  some 
  abundance 
  at 
  the 
  base 
  of 
  the 
  

   Belemnitella 
  mucronata-zone. 
  The 
  general 
  aspect 
  is 
  undoubtedly 
  

   similar 
  in 
  many 
  respects 
  to 
  that 
  of 
  the 
  Chalk 
  Rock, 
  and 
  this 
  fauna 
  

   must 
  have 
  continued 
  to 
  exist 
  throughout 
  the 
  Upper 
  Cretaceous 
  age 
  in 
  

   portions 
  of 
  the 
  Chalk 
  sea, 
  being 
  enabled 
  from 
  time 
  to 
  time 
  to 
  extend 
  

   its 
  range 
  as 
  the 
  result 
  of 
  movements 
  of 
  elevation 
  or 
  depression 
  and 
  

   current-action. 
  

  

  8. 
  The 
  general 
  distribution 
  of 
  the 
  Rhynchonellce 
  and 
  Terebratulce 
  

   is 
  also 
  not 
  uninteresting, 
  as 
  the 
  former 
  seem, 
  in 
  the 
  Irish 
  strata 
  at 
  

   any 
  rate, 
  to 
  have 
  been 
  capable 
  of 
  existing 
  under 
  conditions 
  and 
  

   circumstances 
  which 
  are 
  not 
  so 
  favourable 
  to 
  the 
  growth 
  and 
  

   development 
  of 
  the 
  latter. 
  Thus, 
  while 
  in 
  the 
  Glauconitic 
  Sands 
  

   both 
  families 
  are 
  found 
  together, 
  in 
  the 
  beds 
  formed 
  largely 
  of 
  detrital 
  

   materials, 
  such 
  as 
  the 
  Yellow 
  Sandstones 
  and 
  lower 
  beds 
  of 
  the 
  

   Chloritic 
  Sands, 
  species 
  of 
  Rhynchonellce 
  are 
  alone 
  met 
  with, 
  the 
  

   Terebratulce 
  being 
  rare 
  or 
  absent, 
  and 
  only 
  as 
  depression 
  advances 
  

   becoming 
  predominant. 
  The 
  ffliynclionellce 
  appear 
  also 
  to 
  have 
  

   been 
  capable 
  of 
  existing 
  in 
  the 
  conglomeratic 
  beds 
  of 
  the 
  Peninsular 
  

   Division, 
  where, 
  so 
  far 
  as 
  I 
  am 
  aware, 
  Terebratulce, 
  have 
  never 
  yet 
  

  

  1 
  ' 
  Building 
  of 
  the 
  British 
  Isles,' 
  1888, 
  p. 
  177. 
  

  

  2 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  Hi. 
  (1896) 
  p. 
  69. 
  

  

  