﻿556 
  PROF. 
  C. 
  LAPWORTH 
  ON 
  THE 
  G-IRVAN 
  SUCCESSION. 
  

  

  long-continued 
  action 
  of 
  the 
  weather, 
  contrast 
  trenchantly 
  with 
  the 
  

   dark-green 
  conglomerates 
  of 
  the 
  cliffs 
  above. 
  The 
  limestone 
  is 
  di- 
  

   stinctly 
  bedded 
  in 
  strata 
  of 
  flag-like 
  character, 
  from 
  a 
  few 
  inches 
  to 
  

   two 
  feet 
  in 
  thickness. 
  The 
  planes 
  of 
  bedding 
  are, 
  as 
  a 
  rule, 
  per- 
  

   fectly 
  regular 
  ; 
  but 
  occasionally 
  their 
  upper 
  surface 
  is 
  more 
  or 
  less 
  

   wrinkled 
  or 
  undulated, 
  and 
  the 
  little 
  hollows 
  thus 
  formed 
  are 
  filled 
  

   with 
  flakes 
  of 
  greenish 
  shale, 
  calcareous, 
  and 
  more 
  or 
  less 
  concre- 
  

   tionary 
  in 
  structure. 
  

  

  The 
  thicker 
  limestones, 
  when 
  split 
  open, 
  show, 
  on 
  rare 
  occasions, 
  

   an 
  irregularly 
  laminated 
  interior, 
  and 
  the 
  mass 
  admits 
  of 
  being 
  

   broken 
  down 
  into 
  irregular 
  flakes 
  or 
  sheets 
  of 
  an 
  inch 
  or 
  so 
  in 
  thick- 
  

   ness. 
  But, 
  as 
  a 
  general 
  rule, 
  the 
  interior 
  of 
  the 
  rock 
  is 
  quite 
  homo- 
  

   geneous. 
  It 
  is 
  of 
  a 
  dull 
  grey 
  tint, 
  passing 
  into 
  black 
  in 
  its 
  more 
  

   compact 
  parts, 
  and 
  shading 
  off 
  into 
  a 
  dull 
  greyish 
  green 
  where 
  of 
  

   a 
  more 
  open 
  character. 
  In 
  texture 
  it 
  is 
  usually 
  firm 
  and 
  compact, 
  

   detached 
  plates 
  ringing 
  under 
  the 
  hammer 
  like 
  clinkstone. 
  When 
  

   fractured 
  it 
  flies 
  off 
  in 
  conchoidal 
  fragments 
  having 
  much 
  the 
  

   appearance 
  of 
  hornstone 
  or 
  flint, 
  with 
  clouded 
  surface 
  and 
  trans- 
  

   lucent 
  edges. 
  

  

  In 
  the 
  more 
  massive 
  beds 
  no 
  trace 
  of 
  a 
  fossil 
  is 
  discernible, 
  and 
  

   even 
  in 
  the 
  less 
  hardened 
  parts 
  there 
  is 
  generally 
  little 
  evidence 
  of 
  

   their 
  presence 
  beyond 
  certain 
  dim 
  nebulous 
  outlines, 
  hardly 
  suggestive 
  

   of 
  specific 
  form. 
  When 
  greatly 
  weathered, 
  however, 
  as 
  in 
  the 
  many 
  

   stone 
  walls 
  of 
  the 
  neighbouring 
  fields, 
  the 
  fossiliferous 
  nature 
  of 
  

   the 
  nodular 
  limestones 
  is 
  apparent 
  at 
  a 
  glance 
  ; 
  the 
  faces 
  of 
  the 
  

   bedding-planes 
  are 
  covered 
  with 
  elevated 
  knots 
  and 
  patches, 
  due 
  to 
  

   the 
  former 
  presence 
  of 
  corals 
  and 
  shells, 
  the 
  finer 
  structure 
  of 
  the 
  

   former 
  and 
  the 
  half- 
  obliterated 
  outlines 
  of 
  the 
  latter 
  being 
  easily 
  

   recognizable 
  by 
  the 
  eye. 
  The 
  spiral 
  lines 
  characteristic 
  of 
  the 
  

   natural 
  sections 
  of 
  the 
  beautiful 
  genus 
  Maclurea 
  are 
  by 
  no 
  means 
  rare 
  

   under 
  these 
  circumstances 
  ; 
  and 
  the 
  delicate 
  structure 
  of 
  the 
  peculiar 
  

   coral-like 
  Tetradlum 
  is 
  even 
  more 
  abundant 
  : 
  while 
  on 
  rarer 
  occasions 
  

   we 
  find 
  patches 
  covered 
  with 
  the 
  roe-like 
  Saccammina, 
  occasional 
  

   fragments 
  of 
  Trilobites, 
  fractured 
  Brachiopoda, 
  and 
  the 
  like. 
  

  

  These 
  limestones 
  fall 
  very 
  naturally 
  into 
  two 
  main 
  subdivisions 
  

   — 
  a 
  higher 
  group 
  of 
  hard 
  limestones 
  (Ab 
  3 
  ), 
  with 
  rare 
  dividing 
  seams 
  

   of 
  calcareous 
  shale 
  and 
  but 
  few 
  fossils 
  ; 
  and 
  a 
  lower 
  group 
  of 
  impure 
  

   nodular 
  and 
  flaggy 
  beds 
  (Ab 
  2 
  ), 
  which 
  show, 
  when 
  fully 
  weathered, 
  

   abundant 
  shaly 
  partings 
  and 
  frequent 
  examples 
  of 
  Maclurea 
  Logani. 
  

  

  In 
  these 
  compact 
  and 
  nodular 
  beds 
  the 
  calcareous 
  matter 
  is 
  

   tolerably 
  pure, 
  and 
  the 
  limestones 
  admit 
  of 
  being 
  burnt 
  for 
  agricul- 
  

   tural 
  purposes 
  ; 
  while 
  in 
  the 
  underlying 
  laminated 
  beds 
  next 
  to 
  be 
  

   described 
  the 
  strata 
  are 
  largely 
  impregnated 
  with 
  aluminous 
  matter, 
  

   and 
  are 
  of 
  no 
  special 
  economical 
  value. 
  

  

  It 
  is 
  difficult, 
  if 
  not 
  impossible, 
  to 
  give 
  an 
  exact 
  estimate 
  of 
  the 
  

   united 
  thickness 
  of 
  the 
  compact 
  and 
  nodular 
  limestones. 
  To 
  judge 
  

   from 
  the 
  space 
  they 
  cover 
  upon 
  the 
  ground, 
  and 
  from 
  their 
  vertical 
  

   extent 
  when 
  plotted 
  in 
  section, 
  it 
  cannot 
  be 
  less 
  than 
  60 
  or 
  70 
  feet 
  ; 
  

   but 
  in 
  no 
  locality 
  elsewhere 
  do 
  they 
  actually 
  reach 
  this 
  thickness. 
  

  

  Orthis-conjinis 
  Flags. 
  — 
  In 
  the 
  locality 
  under 
  description 
  the 
  

  

  