﻿PROF. 
  C. 
  LAPWORTH 
  ON 
  THE 
  GIRVAN 
  SUCCESSION. 
  557 
  

  

  impure 
  Maclur 
  ea-limestones, 
  already 
  noticed, 
  pass 
  downwards 
  into 
  

   a 
  similar 
  thickness 
  of 
  calcareous 
  flagstones, 
  with 
  way-boards 
  and 
  

   intercalated 
  seams 
  and 
  flakes 
  of 
  greenish 
  or 
  brownish 
  shales 
  and 
  

   mudstones 
  (Ab 
  1 
  ). 
  These 
  are 
  well 
  exposed 
  in 
  the 
  banks 
  and 
  bed 
  

   of 
  the 
  little 
  stream 
  of 
  Ben 
  an 
  Burn 
  ; 
  and 
  their 
  inferiority 
  to 
  the 
  

   limestones 
  is 
  demonstrated 
  by 
  the 
  fact 
  that 
  they 
  can 
  be 
  followed 
  by 
  

   the 
  eye 
  coming 
  out 
  from 
  below 
  the 
  latter 
  in 
  the 
  steep 
  slopes 
  in 
  

   regular 
  order, 
  and 
  insensibly 
  graduating 
  from 
  them 
  in 
  their 
  petro- 
  

   graphical 
  characters. 
  

  

  In 
  the 
  upper 
  limestones, 
  as 
  we 
  have 
  shown, 
  the 
  texture 
  of 
  the 
  

   rock 
  is 
  usually 
  firm 
  and 
  compact 
  ; 
  and 
  the 
  calcareous 
  matter 
  

   being 
  evenly 
  distributed, 
  the 
  rock 
  itself 
  is 
  solid 
  and 
  homogeneous 
  

   throughout. 
  In 
  these 
  underlying 
  flagstones, 
  on 
  the 
  other 
  hand, 
  the 
  

   lime 
  is 
  not 
  only 
  much 
  smaller 
  in 
  amount, 
  but 
  it 
  is 
  much 
  more 
  

   capriciously 
  distributed, 
  and 
  the 
  rock 
  weathers 
  more 
  irregularly 
  

   under 
  the 
  influence 
  of 
  the 
  atmosphere. 
  Instead 
  of 
  the 
  hard, 
  white, 
  

   compact, 
  and 
  flinty-looking 
  sheets 
  of 
  the 
  upper 
  beds, 
  we 
  now 
  

   find 
  the 
  strata 
  degenerating 
  into 
  a 
  kind 
  of 
  impure 
  flagstone 
  or 
  

   calcareous 
  sandstone, 
  containing 
  nodules 
  of 
  lime, 
  and 
  assuming 
  

   in 
  places 
  a 
  honeycombed 
  structure, 
  whose 
  hollows 
  are 
  filled 
  

   exteriorly 
  with 
  soft 
  rottenstone, 
  which 
  gives 
  to 
  the 
  group 
  a 
  peculiarly 
  

   dull 
  clive-brown 
  colour. 
  In 
  the 
  steeper 
  cliffs 
  the 
  soft 
  rottenstone 
  

   becomes 
  wholly 
  washed 
  away, 
  and 
  the 
  deeply 
  honeycombed 
  flaggy 
  

   rock 
  puts 
  on 
  a 
  most 
  peculiar 
  and 
  characteristic 
  appearance. 
  

  

  In 
  the 
  compact 
  limestones 
  above 
  fossils 
  are, 
  as 
  we 
  have 
  shown, 
  rare 
  

   and 
  excessively 
  difficult 
  of 
  extraction 
  ; 
  in 
  these 
  impure 
  flagstones 
  

   below 
  they 
  are, 
  on 
  the 
  contrary, 
  generally 
  abundant 
  and 
  easily 
  procured. 
  

   They 
  are, 
  however, 
  almost 
  exclusively 
  Brachiopoda, 
  viz. 
  Orthis 
  con- 
  

   finis, 
  Salter, 
  0. 
  altemata, 
  Salter, 
  Strophomena 
  grandis, 
  &c. 
  Of 
  these 
  

   the 
  species 
  Orthis 
  con 
  finis, 
  Salt., 
  is 
  emphatically 
  the 
  most 
  abundant 
  

   and 
  characteristic, 
  occurring 
  everywhere 
  throughout 
  the 
  region 
  

   where 
  these 
  strata 
  are 
  exhibited, 
  and 
  occasionally 
  abounding 
  to 
  the 
  

   exclusion 
  of 
  all 
  others. 
  

  

  Transitional 
  Sandstone. 
  — 
  The 
  bottom 
  beds 
  of 
  these 
  Orthis-confinis 
  

   flagstones 
  take 
  on 
  a 
  dull 
  purple 
  colour, 
  and, 
  as 
  we 
  descend 
  the 
  suc- 
  

   cession, 
  they 
  gradually 
  part 
  Avith 
  their 
  calcareous 
  nodules 
  and 
  

   degenerate 
  into 
  a 
  group 
  of 
  purple 
  grits 
  and 
  flagstones 
  (Aa 
  2 
  ), 
  which, 
  

   in 
  many 
  of 
  their 
  essential 
  features, 
  remind 
  us 
  of 
  some 
  of 
  the 
  typical 
  

   beds 
  of 
  the 
  Old 
  Hed 
  Sandstone. 
  

  

  In 
  the 
  present 
  locality 
  these 
  purple 
  sandstones 
  are 
  very 
  fairly 
  

   exposed, 
  and 
  have 
  an 
  estimated 
  thickness 
  of 
  about 
  forty 
  feet. 
  As 
  

   a 
  rule, 
  they 
  are 
  simply 
  coarse-grained, 
  well-bedded 
  grits, 
  including 
  

   many 
  seams 
  of 
  coarse 
  sandy 
  shales. 
  Above, 
  they 
  pass 
  insensibly 
  

   upwards 
  into 
  the 
  Orthis-confinis 
  flagstones 
  last 
  noticed 
  ; 
  below, 
  they 
  

   contain 
  seams 
  of 
  pebbles, 
  first 
  about 
  the 
  size 
  of 
  a 
  marble, 
  next 
  of 
  

   the 
  size 
  of 
  the 
  fist, 
  till 
  finally 
  they 
  graduate 
  downwards 
  into 
  a 
  massive 
  

   purple 
  conglomerate, 
  which 
  will 
  be 
  described 
  in 
  the 
  sequel. 
  

  

  Such 
  palaeontological 
  characters 
  as 
  these 
  purple 
  sandstones 
  possess 
  

   ally 
  them 
  to 
  the 
  overlying 
  Orthis-confinis 
  flagstones. 
  An 
  occasional 
  

   seam 
  is 
  met 
  with 
  amongst 
  them, 
  affording 
  a 
  few 
  casts 
  of 
  Orthis 
  con- 
  

  

  