﻿208 
  H. 
  S. 
  Williams 
  — 
  Silurian-Devonian 
  boundary 
  

  

  were 
  marine 
  and 
  not 
  freshwater 
  types, 
  and 
  that 
  some 
  of 
  them 
  

   were 
  identical 
  with 
  species 
  in 
  the 
  formation 
  below 
  (which 
  he 
  

   called 
  Silurian), 
  led 
  Murchison 
  to 
  call 
  this 
  Tilestone 
  the 
  top 
  

   member 
  of 
  the 
  Silurian 
  instead 
  of 
  the 
  bottom 
  of 
  the 
  Old 
  Red. 
  

   In 
  " 
  Silnria 
  " 
  appears 
  a 
  concise 
  description 
  of 
  the 
  transition 
  

   from 
  the 
  Ludlow 
  rocks 
  into 
  the 
  Old 
  Red 
  sandstone, 
  as 
  seen 
  in 
  

   Carmarthenshire 
  and 
  Pembrokeshire 
  : 
  " 
  In 
  all 
  these 
  places, 
  

   strata 
  of 
  dull 
  greenish-gray 
  argillaceous 
  sandstone, 
  minutely 
  

   micaceous, 
  differing 
  chiefly 
  from 
  the 
  type 
  of 
  the 
  Ludlow 
  of 
  

   Shropshire 
  in 
  being 
  harder 
  and 
  thicker 
  bedded, 
  and 
  which 
  

   repose 
  on 
  rocks 
  with 
  Upper 
  Silurian 
  fossils, 
  plunge 
  under 
  red 
  

   and 
  green 
  strata 
  (the 
  red 
  rab 
  of 
  Pembroke), 
  or 
  bottom 
  beds 
  of 
  

   the 
  Old 
  Red 
  sandstone" 
  (p. 
  141). 
  

  

  The 
  typical 
  localities, 
  from 
  which 
  the 
  Tilestone 
  fossils 
  came, 
  

   are 
  Felindre 
  on 
  the 
  Teme 
  and 
  Horeb 
  Chapel, 
  in 
  the 
  valley 
  of 
  

   Cwm 
  Dwr, 
  between 
  Trecastle 
  and 
  Llandovery 
  near 
  Cwn 
  Dwr. 
  

   In 
  the 
  '' 
  Silurian 
  System," 
  the 
  passage 
  beds 
  near 
  Felindre 
  are 
  

   described 
  as 
  " 
  hard, 
  greenish, 
  and 
  reddish, 
  highly 
  micaceous 
  

   sandstone, 
  which 
  contain 
  the 
  Leptaena 
  lata 
  and 
  the 
  Terebratula 
  

   nucula 
  of 
  the 
  Ludlow 
  rock, 
  together 
  with 
  casts 
  of 
  several 
  

   shells, 
  identical 
  with 
  those 
  found 
  in 
  the 
  tilestones 
  of 
  the 
  Cwm 
  

   Dwr, 
  Caermarthenshire, 
  and 
  which 
  have 
  never 
  been 
  found 
  in 
  

   the 
  Silurian 
  System 
  below 
  its 
  junction 
  with 
  the 
  Old 
  Red 
  

   Sandstone" 
  (p.'l91). 
  

  

  The 
  reason 
  for 
  placing 
  the 
  base 
  of 
  the 
  Old 
  Red 
  sandstone 
  

   above 
  these 
  tilestones 
  is 
  stated 
  in 
  the 
  following 
  passage 
  in 
  the 
  

   " 
  Siluria" 
  : 
  "Even 
  then, 
  however, 
  the 
  fossils 
  which 
  were 
  fig- 
  

   ured 
  as 
  characteristic 
  of 
  such 
  tilestones 
  exhibited 
  little 
  else, 
  as 
  

   I 
  showed, 
  than 
  species 
  common 
  to 
  the 
  Ludlow 
  rock 
  itself. 
  

   This 
  zoological 
  fact, 
  and 
  subsequent 
  researches 
  in 
  other 
  parts 
  

   of 
  England, 
  above 
  all 
  those 
  of 
  Professor 
  Sedgwick 
  in 
  West- 
  

   moreland, 
  where 
  the 
  Upper 
  Ludlow 
  strata 
  are 
  much 
  developed, 
  

   have, 
  for 
  eleven 
  years, 
  led 
  me 
  to 
  classify 
  these 
  tilestones 
  with 
  

   the 
  Silurian 
  rocks, 
  of 
  which 
  they 
  form 
  the 
  natural 
  summit. 
  

   For, 
  even 
  in 
  their 
  range 
  from 
  Shropshire 
  through 
  Hereford 
  

   and 
  Radnorshire, 
  into 
  Brecon 
  and 
  Carmarthenshire, 
  whether 
  

   they 
  are 
  of 
  red 
  or 
  yellow 
  colors, 
  they 
  are 
  charged 
  with 
  Ortho- 
  

   ceras 
  bullatum, 
  Chonetes 
  (Leptwna) 
  lata, 
  Spirifer 
  elevatus, 
  

   Orthis 
  lunata, 
  Rhynconella 
  nucula, 
  Cucullella 
  ovata, 
  Bellero- 
  

   johon 
  trilobatus, 
  B. 
  expansus, 
  Trochus 
  helicites, 
  JTolopella 
  

   (Turritella) 
  obsoleta, 
  and 
  the 
  minute 
  bivalved 
  crustacean, 
  Bey- 
  

   richia 
  tuberculata. 
  All 
  of 
  these 
  are 
  the 
  most 
  common 
  fossils 
  

   of 
  the 
  Upper 
  Ludlow 
  rock 
  ; 
  although 
  a 
  few 
  of 
  them 
  descend 
  

   as 
  low 
  as 
  the 
  Caradoc 
  sandstone" 
  (p. 
  139). 
  

  

  From 
  these 
  quotations 
  it 
  is 
  evident 
  that 
  the 
  Tilestone 
  was 
  

   regarded 
  by 
  Murchison 
  as 
  the 
  upper 
  member 
  of 
  the 
  Silurian 
  

  

  