﻿PROF. 
  C. 
  LArWOKTH 
  ON 
  THE 
  GTRVAN 
  SUCCESSION. 
  639 
  

  

  section 
  of 
  some 
  30 
  or 
  40 
  feet, 
  and 
  the 
  next 
  strata 
  seen 
  are 
  very 
  dif- 
  

   ferent 
  in 
  their 
  mineralogical 
  characters. 
  They 
  protrude 
  in 
  little 
  

   bosses 
  from 
  the 
  sand 
  of 
  the 
  small 
  beach 
  immediately 
  north 
  of 
  the 
  

   Horse 
  Rock 
  itself, 
  and 
  consist 
  of 
  a 
  few 
  feet 
  of 
  highly 
  calcareous 
  

   flagstone, 
  or 
  " 
  bastard 
  limestone," 
  much 
  altered 
  and 
  disturbed, 
  but 
  

   abounding 
  in 
  casts 
  of 
  Pentamerus 
  ohlongus 
  and 
  other 
  Brachiopoda, 
  

   together 
  with 
  well-preserved 
  examples 
  of 
  Alveolites 
  Lahechei, 
  Favo- 
  

   sites 
  gotJilandica, 
  and 
  other 
  corals. 
  

  

  After 
  a 
  second 
  hiatus 
  of 
  less 
  geographical 
  importance 
  than 
  the 
  

   former, 
  these 
  Coralline 
  Limestones 
  are 
  succeeded 
  by 
  a 
  thin 
  group 
  of 
  

   Graptolitiferous 
  shales, 
  which 
  are 
  traceable 
  from 
  the 
  back 
  of 
  the 
  

   smithy, 
  parallel 
  to 
  the 
  two 
  zones 
  already 
  noticed, 
  for 
  about 
  60 
  yards 
  

   out 
  to 
  sea. 
  They 
  consist 
  of 
  dark 
  greyish-blue 
  and 
  somewhat 
  flaggy 
  

   beds 
  veined 
  by 
  seams 
  of 
  carbonaceous 
  matter. 
  They 
  are 
  thrown 
  

   into 
  innumerable 
  wrinkles 
  and 
  contortions, 
  which 
  are 
  beautifully 
  

   exhibited 
  on 
  the 
  wave-washed 
  shore 
  at 
  the 
  back 
  of 
  the 
  Forge. 
  

  

  These 
  shales 
  contain 
  an 
  abundance 
  of 
  fairly 
  preserved 
  Graptolites 
  

   of 
  the 
  species 
  Diplograptus 
  modestus, 
  Lapw., 
  Clima 
  cog 
  raphes 
  nor- 
  

   malise 
  Lapw., 
  Monograptus 
  tenuis, 
  Portl., 
  and 
  M. 
  cyjphus, 
  Lapw., 
  

   together 
  with 
  forms 
  of 
  JDictyonema 
  and 
  Orthoceras. 
  

  

  The 
  Graptolitic 
  shales 
  are 
  followed 
  abruptly 
  by 
  a 
  conspicuous 
  

   group 
  of 
  conglomeratic 
  sandstones. 
  They 
  form 
  a 
  broad 
  band 
  lying 
  

   to 
  the 
  north 
  of 
  the 
  shales, 
  and 
  ranging 
  outwards 
  from 
  the 
  roadway 
  

   into 
  the 
  deep 
  water 
  beyond 
  the 
  Horse 
  Rock. 
  These 
  superior 
  beds 
  

   are 
  emphatically 
  bedded 
  sandstones, 
  much 
  softer 
  and 
  looser 
  in 
  

   texture 
  than 
  the 
  generality 
  of 
  the 
  Lower 
  Palaeozoic 
  rocks 
  ; 
  they 
  

   contain 
  an 
  abundance 
  of 
  white 
  quartz 
  pebbles, 
  often 
  rounded, 
  but 
  

   occasionally 
  angular, 
  scattered 
  confusedly 
  through 
  the 
  main 
  body 
  of 
  

   the 
  rock. 
  The 
  basal 
  beds 
  are 
  filled 
  with 
  -patches 
  and 
  angular 
  frag- 
  

   ments 
  of 
  the 
  underlying 
  shales, 
  and 
  the 
  two 
  formations 
  are 
  dove- 
  

   tailed 
  into 
  each 
  other 
  in 
  a 
  most 
  intricate 
  manner, 
  rendering 
  the 
  

   detection 
  of 
  their 
  natural 
  relationship 
  more 
  a 
  matter 
  of 
  speculation 
  

   than 
  of 
  absolute 
  certainty. 
  

  

  The 
  base 
  of 
  these 
  conglomeratic 
  sandstones 
  must 
  originally 
  have 
  

   been 
  unconformable 
  with 
  the 
  Graptolitiferous 
  shales 
  with 
  which 
  

   they 
  are 
  in 
  contact. 
  Although 
  the 
  two 
  strikingly 
  distinct 
  rock- 
  

   groups 
  are 
  greatly 
  crumpled 
  and 
  intermixed 
  along 
  the 
  line 
  of 
  junc- 
  

   tion, 
  a 
  cautious 
  study 
  of 
  the 
  phenomena 
  apparent 
  places 
  the 
  fact 
  of 
  

   their 
  original 
  discordance 
  almost 
  beyond 
  question. 
  

  

  Abundant 
  small 
  fragments 
  and 
  many 
  large 
  slabs 
  of 
  the 
  neigh- 
  

   bouring 
  grey 
  shales 
  occur 
  in 
  the 
  heart 
  of 
  the 
  sandstones, 
  sometimes 
  

   lying 
  parallel 
  with 
  the 
  plane 
  of 
  bedding, 
  sometimes 
  inclined 
  at 
  a 
  

   wide 
  angle 
  thereto. 
  Again, 
  the 
  lowest 
  visible 
  zone 
  of 
  the 
  sand- 
  

   stone 
  conglomerate 
  is 
  filled 
  with 
  pieces 
  of 
  the 
  same 
  striped 
  shales, 
  

   which 
  are 
  surrounded 
  and 
  buried 
  up 
  by 
  the 
  coarser 
  pebblj* 
  rock, 
  as 
  

   if 
  they 
  had 
  projected 
  from 
  the 
  sea-floor 
  at 
  the 
  time 
  of 
  the 
  formation 
  

   of 
  the 
  sandstone, 
  and 
  had 
  been 
  enveloped 
  and 
  buried 
  by 
  the 
  latter 
  

   being 
  deposited 
  around 
  and 
  above 
  them. 
  Thus 
  it 
  is 
  highly 
  probable 
  

   that 
  the 
  conglomeratic 
  sandstones 
  which 
  now 
  dip 
  generally 
  with 
  

   the 
  underlying 
  beds 
  were 
  originally 
  somewhat 
  discordant 
  with 
  them, 
  

  

  