﻿^°1- 
  53-] 
  THE 
  ^ORTRAINE 
  INLIER 
  (CO. 
  DUBLIN). 
  521 
  

  

  a 
  list 
  of 
  some 
  fifty 
  fossils 
  determined 
  by 
  W. 
  H. 
  Baily 
  ! 
  ; 
  later 
  Prof. 
  

   Cole 
  writes 
  of 
  the 
  area 
  in 
  his 
  articles 
  on 
  ' 
  County 
  Dublin, 
  Past 
  and 
  

   Present 
  ' 
  2 
  ; 
  while 
  the 
  most 
  recent 
  account 
  is 
  that 
  by 
  Prof. 
  Sollas, 
  

   written 
  for 
  the 
  visit 
  of 
  the 
  Geologists' 
  Association 
  to 
  the 
  district 
  

   in 
  1893. 
  3 
  

  

  II. 
  Description 
  of 
  the 
  Area 
  and 
  its 
  Geological 
  Structure. 
  

  

  The 
  area 
  with 
  which 
  we 
  are 
  concerned 
  forms 
  a 
  blunt-ended 
  

   promontory, 
  limited 
  on 
  the 
  north 
  by 
  the 
  estuary 
  on 
  which 
  Rush 
  

   is 
  built, 
  and 
  on 
  the 
  south 
  by 
  that 
  on 
  which 
  Malahide 
  is 
  situated. 
  

   The 
  pre-Devonian 
  rocks 
  are 
  bounded 
  on 
  the 
  east 
  by 
  the 
  sea 
  ; 
  on 
  the 
  

   north 
  by 
  the 
  waters 
  of 
  the 
  Rush 
  estuary 
  ; 
  on 
  the 
  west 
  by 
  a 
  coarse 
  

   conglomerate, 
  referred 
  by 
  the 
  officers 
  of 
  the 
  Geological 
  Survey 
  to 
  the 
  

   Old 
  Red 
  Sandstone 
  period 
  ; 
  and 
  on 
  the 
  south 
  by 
  an 
  area 
  of 
  blown 
  

   sand. 
  Our 
  account 
  of 
  them 
  deals 
  mainly 
  with 
  the 
  coast-section, 
  

   for 
  inland 
  exposures 
  are 
  scanty, 
  the 
  country 
  being 
  much 
  covered 
  

   by 
  drift. 
  

  

  Two 
  martello 
  towers 
  have 
  been 
  built 
  on 
  the 
  promontory, 
  the 
  

   northern 
  one 
  being 
  used 
  as 
  a 
  Coastguard 
  station, 
  and 
  they 
  will 
  be 
  

   frequently 
  referred 
  to, 
  as 
  they 
  form 
  convenient 
  landmarks. 
  (See 
  

   Map, 
  PI. 
  XLIII.) 
  

  

  Geologically 
  the 
  rocks 
  forming 
  the 
  inlier 
  are 
  divisible 
  into 
  several 
  

   well-marked 
  groups. 
  At 
  the 
  northern 
  end 
  is 
  an 
  area 
  of 
  volcanic 
  

   rocks, 
  succeeded 
  to 
  the 
  south 
  by 
  an 
  area 
  of 
  limestone 
  and 
  shale, 
  

   which 
  is 
  followed 
  by 
  an 
  exposure 
  of 
  grits. 
  Then 
  comes 
  a 
  smaller 
  

   development 
  of 
  limestone 
  and 
  shale, 
  and, 
  finally, 
  at 
  the 
  southern 
  

   end 
  of 
  the 
  inlier 
  occurs 
  a 
  second 
  area 
  of 
  volcanic 
  rocks. 
  Owing 
  

   to 
  the 
  great 
  amount 
  of 
  faulting 
  and 
  folding 
  which 
  has 
  taken 
  place, 
  

   the 
  area 
  has 
  been 
  made 
  very 
  complicated. 
  Not 
  one 
  of 
  the 
  groups 
  

   of 
  rocks 
  that 
  we 
  have 
  mentioned 
  can 
  be 
  seen 
  to 
  rest 
  conformably 
  

   on 
  another, 
  the 
  junction 
  being 
  apparently 
  in 
  every 
  case 
  a 
  faulted 
  

   one. 
  The 
  divisional 
  plane 
  between 
  the 
  grit 
  series 
  and 
  the 
  limestone- 
  

   and-shale 
  series 
  is, 
  wherever 
  visible, 
  a 
  thrust-plane, 
  marked 
  by 
  the 
  

   development 
  of 
  most 
  interesting 
  conglomerates 
  due 
  to 
  earth-move- 
  

   ments. 
  

  

  Although 
  we 
  have 
  no 
  definite 
  proof 
  of 
  their 
  age, 
  the 
  grit 
  series 
  

   apparently 
  forms 
  the 
  newest 
  group, 
  and 
  the 
  volcanic 
  rocks 
  the 
  

   oldest, 
  of 
  the 
  inlier, 
  the 
  district 
  as 
  a 
  whole 
  forming 
  a 
  much 
  

   denuded 
  half-syncline. 
  

  

  III. 
  The 
  Igneous 
  and 
  Associated 
  Rocks 
  at 
  the 
  Northern 
  End 
  

   of 
  the 
  Inlier. 
  

  

  ' 
  At 
  the 
  north-western 
  corner 
  of 
  the 
  inlier 
  the 
  coarse 
  quartzose 
  

   conglomerate 
  mentioned 
  above 
  is 
  seen 
  dipping 
  N.W. 
  at 
  35°, 
  and 
  from 
  

   beneath 
  it 
  appear 
  with 
  a 
  south-easterly 
  dip 
  a 
  series 
  of 
  purple 
  and 
  

   green 
  andesitic 
  rocks 
  which 
  form 
  the 
  foreshore 
  up 
  to, 
  and 
  just 
  beyond, 
  

  

  1 
  Explan. 
  Mem. 
  Sheets 
  102 
  & 
  1J2 
  Geol. 
  Surv. 
  Irel., 
  2nd 
  ed., 
  Dublin, 
  1875. 
  

  

  2 
  ' 
  Irish 
  Naturalist,' 
  vol. 
  i. 
  (1892) 
  p. 
  31. 
  

  

  3 
  ' 
  The 
  Geology 
  of 
  Dublin 
  and 
  its 
  Neighbourhood,' 
  Proc. 
  Geol. 
  Assoc, 
  

   vol. 
  xiii. 
  p. 
  91. 
  

  

  