﻿Vol. 
  53.] 
  THE 
  PORTRAOE 
  LNLIER 
  (CO. 
  DUBLIN). 
  527 
  

  

  felspars, 
  now 
  so 
  much 
  replaced 
  as 
  to 
  show 
  no 
  trace 
  of 
  twinning, 
  

   and 
  also 
  many 
  very 
  fresh 
  porphyritic 
  augites 
  of 
  large 
  size. 
  A 
  

   little 
  chloritic 
  material 
  has 
  come 
  in, 
  filling 
  up 
  vesicles. 
  

  

  In 
  the 
  1-inch 
  Geological 
  Survey 
  map 
  the 
  igneous 
  masses 
  at 
  the 
  

   northern 
  and 
  southern 
  ends 
  of 
  the 
  section 
  are 
  connected 
  by 
  a 
  strip 
  

   of 
  igneous 
  rocks 
  running 
  along 
  the 
  north-western 
  boundary 
  of 
  the 
  

   inlier. 
  jNo 
  trace 
  of 
  this 
  is 
  to 
  be 
  seen 
  now, 
  that 
  part 
  of 
  the 
  country 
  

   being 
  drift-covered, 
  though 
  round 
  the 
  Pigeon 
  House 
  are 
  several 
  

   exposures 
  of 
  ashy 
  shales 
  and 
  calcareous 
  ash. 
  1 
  

  

  V. 
  The 
  Limestone 
  Series. 
  

  

  (a) 
  The 
  Coast-section 
  North, 
  of 
  the 
  Grits. 
  

  

  The 
  limestone 
  series 
  is 
  exposed 
  along 
  the 
  coast 
  from 
  the 
  northern 
  

   martello 
  tower 
  on 
  the 
  north 
  to 
  Priest's 
  Chamber 
  on 
  the 
  south, 
  a 
  

   distance 
  of 
  about 
  ^ 
  mile 
  in 
  a 
  straight 
  line, 
  though 
  if 
  the 
  indenta- 
  

   tions 
  of 
  the 
  coast 
  be 
  followed 
  the 
  distance 
  would 
  be 
  far 
  greater. 
  

  

  Owing 
  to 
  the 
  enormous 
  amount 
  of 
  faulting 
  and 
  folding 
  to 
  which 
  

   the 
  beds 
  have 
  been 
  subjected, 
  it 
  is 
  impossible 
  to 
  ascertain 
  their 
  

   thickness 
  with 
  accuracy, 
  but 
  the 
  general 
  succession 
  from 
  above 
  

   downward 
  is 
  : 
  — 
  

  

  3. 
  Thin-bedded 
  limestone 
  with 
  shaly 
  partings. 
  

  

  2. 
  Beds 
  of 
  compact, 
  grey, 
  crystalline 
  limestone 
  with 
  many 
  fossils. 
  

  

  1. 
  Thin-bedded 
  limestones 
  with 
  shaly 
  partings, 
  the 
  upper 
  beds 
  being 
  

  

  in 
  places 
  crowded 
  with 
  corals, 
  while 
  the 
  lower 
  beds 
  become 
  more 
  

  

  shaly. 
  

  

  The 
  beds 
  may 
  best 
  be 
  described 
  by 
  following 
  the 
  coast-exposure 
  in 
  

   some 
  detail, 
  and 
  understood 
  by 
  referring 
  to 
  the 
  section 
  (PI. 
  XLII). 
  

  

  The 
  most 
  northerly 
  exposures 
  are 
  two 
  knolls 
  of 
  black 
  fissile 
  shales 
  

   which 
  project 
  into 
  the 
  sandy 
  bay 
  south 
  of 
  the 
  northern 
  martello 
  

   tower. 
  They 
  arc 
  considerably 
  crushed 
  and 
  contorted, 
  but 
  have 
  

   yielded 
  obscure 
  traces 
  of 
  graptolites, 
  are 
  probably 
  bounded 
  by 
  faults 
  

   on 
  either 
  side, 
  and 
  are 
  succeeded 
  on 
  the 
  southern 
  boundary 
  of 
  the 
  

   bay 
  by 
  brown 
  shales 
  with 
  thin 
  limestone-bands. 
  These 
  beds 
  are 
  very 
  

   much 
  faulted, 
  crushed, 
  and 
  folded 
  (see 
  fig. 
  2, 
  p. 
  528), 
  and 
  every 
  stage 
  

   can 
  be 
  traced 
  between 
  a 
  continuous 
  limestone-band 
  and 
  oue 
  which 
  

   has 
  been 
  broken 
  up 
  into 
  small 
  rounded 
  fragments, 
  so 
  as 
  to 
  present 
  

   precisely 
  the 
  appearance 
  of 
  a 
  conglomerate. 
  To 
  this 
  type 
  of 
  rock 
  

   we 
  propose 
  to 
  restrict 
  the 
  term 
  ' 
  crush-conglomerate,' 
  applying 
  the 
  

   term 
  ' 
  thrust-conglomerate 
  ' 
  to 
  a 
  conglomerate 
  formed 
  along 
  a 
  

   thrust-plane. 
  

  

  In 
  spite 
  of 
  the 
  disturbance 
  to 
  which 
  these 
  beds 
  have 
  been 
  sub- 
  

   jected, 
  remains 
  of 
  fossils 
  (crinoids 
  and 
  corals) 
  can 
  be 
  detected 
  in 
  the 
  

   limestone. 
  

  

  Before 
  long 
  the 
  shales 
  become 
  less 
  plentiful, 
  and 
  the 
  series 
  passes 
  

   up 
  into 
  a 
  fairly 
  compact 
  band 
  of 
  limestone, 
  perhaps 
  30 
  feet 
  thick, 
  

   and 
  having 
  an 
  easterly 
  dip. 
  This 
  bed 
  forms 
  the 
  coast 
  till 
  close 
  to 
  

  

  1 
  In 
  a 
  small 
  excavation 
  in 
  the 
  railway-cutting 
  immediately 
  south 
  of 
  Donabate 
  

   Station, 
  about 
  2 
  miles 
  to 
  the 
  west, 
  we 
  saw 
  an 
  exposure 
  of 
  a 
  very 
  coarsely 
  

   porphyritic 
  rock 
  with 
  a 
  green 
  groundmass 
  and 
  large 
  white 
  porphyritic 
  felspars. 
  

  

  