﻿T. 
  1PKENNY 
  HUGHES 
  ON 
  THE 
  GEOLOGY 
  OF 
  ANGLESEY. 
  

  

  27 
  

  

  and 
  shales 
  are 
  so 
  rotten 
  that 
  it 
  is 
  almost 
  impossible 
  to 
  examine 
  them 
  

   except 
  where 
  a 
  little 
  baked 
  and 
  caked 
  by 
  the 
  metamorphic 
  action. 
  

  

  I 
  take 
  it 
  that 
  the 
  Graptolite-bearing 
  beds 
  of 
  Caemawr 
  and 
  Llys- 
  

   dnlas, 
  and 
  those 
  of 
  Llangwyllog, 
  described 
  in 
  my 
  former 
  paper, 
  all 
  

   belong 
  to 
  the 
  same 
  series 
  and 
  are 
  of 
  about 
  the 
  same 
  age 
  as 
  the 
  beds 
  

   of 
  Pontseiont, 
  south 
  of 
  Caernarvon, 
  described 
  by 
  Mr. 
  Marr 
  (Q. 
  J. 
  G. 
  S. 
  

   vol. 
  xxxii. 
  p. 
  134), 
  and 
  as 
  the 
  beds 
  in 
  the 
  south 
  arm 
  of 
  Abereiddy 
  

   Bay, 
  and 
  that 
  we 
  had 
  better 
  call 
  them 
  all 
  Arenig 
  for 
  the 
  present. 
  

  

  The 
  highest 
  horizon 
  in 
  the 
  black 
  slates 
  from 
  which 
  I 
  have 
  pro- 
  

   cured 
  any 
  fossils 
  is 
  in 
  the 
  black 
  slates 
  between 
  the 
  two 
  great 
  felsite 
  

   ribs 
  in 
  the 
  Mona 
  mines 
  of 
  Paris 
  Mountain, 
  where, 
  after 
  we 
  had 
  

   searched 
  for 
  some 
  time 
  and 
  found 
  only 
  a 
  few 
  obscure 
  traces, 
  Mr. 
  

   Panning 
  Evans, 
  whose 
  kind 
  assistance 
  I 
  take 
  this 
  opportunity 
  of 
  

   acknowledging, 
  had 
  the 
  good 
  luck 
  to 
  pick 
  up 
  a 
  slab 
  which 
  has 
  yielded 
  

   no 
  less 
  than 
  eight 
  species, 
  viz. 
  : 
  — 
  

  

  Rastrites 
  (Monograptus) 
  triangulatua, 
  Harkn. 
  

   Monograptus 
  gregarius, 
  Lapw. 
  

  

  attenuates, 
  Hopk. 
  

  

  Diplograptus 
  tamariscus, 
  Nich. 
  

  

  acuminatus, 
  Nich. 
  

  

  rnodestus, 
  Lapw. 
  

  

  Climacograptus 
  rectangularis, 
  M'Coy. 
  

   normalis, 
  Lapw. 
  

  

  Professor 
  Lapworth, 
  who 
  determined 
  these 
  for 
  me, 
  considers 
  that 
  

   they 
  indicate 
  an 
  horizon 
  equivalent 
  to 
  the 
  M. 
  gregarius 
  zone 
  of 
  the 
  

   Birkhill 
  shales 
  or 
  the 
  lowest 
  zone 
  of 
  the 
  Graptolitic 
  mudstone 
  of 
  

   the 
  Lake 
  District, 
  i. 
  e. 
  distinctly 
  Silurian. 
  

  

  It 
  would 
  appear 
  therefore 
  that 
  we 
  have 
  in 
  Anglesey 
  : 
  — 
  

  

  1. 
  An 
  upper 
  slate 
  group, 
  in 
  which 
  I 
  have 
  fixed 
  two 
  life-zones, 
  

   which 
  must 
  be 
  referred 
  to 
  the 
  Silurian, 
  of 
  the 
  boundaries 
  and 
  sub- 
  

   divisions 
  of 
  which 
  I 
  hope 
  hereafter 
  to 
  offer 
  the 
  Society 
  further 
  de- 
  

   tails. 
  

  

  2. 
  A 
  lower 
  group 
  of 
  slates 
  and 
  shales 
  in 
  which 
  Arenig 
  fossils 
  have 
  

   been 
  found 
  in 
  several 
  localities, 
  and 
  in 
  which, 
  after 
  correcting 
  the 
  

   determination 
  of 
  a 
  species 
  of 
  Orthis, 
  we 
  have 
  reason 
  to 
  suspect 
  that 
  

   Tremadoc 
  and 
  even 
  Menevian 
  beds 
  may 
  be 
  recognized 
  ; 
  and 
  below 
  

   these 
  are 
  

  

  3. 
  The 
  basement 
  beds 
  of 
  the 
  Cambrian, 
  consisting 
  of 
  conglome- 
  

   rates, 
  grits, 
  and 
  sandstones, 
  of 
  which 
  this 
  paper 
  chiefly 
  treats. 
  

  

  Speaking 
  generally 
  on 
  the 
  correlation 
  of 
  the 
  beds, 
  I 
  think 
  we 
  

   have 
  in 
  this 
  Anglesey 
  section 
  an 
  approach 
  to 
  the 
  Scandinavian 
  type 
  ; 
  

   in 
  both 
  there 
  is 
  a 
  thinning 
  out 
  of 
  the 
  lower 
  groups 
  (the 
  Harlech, 
  

   Menevian, 
  and 
  Lingula 
  Plags) 
  against 
  an 
  old 
  shore-line 
  ; 
  but 
  certain 
  

   forms 
  of 
  life 
  which, 
  when 
  the 
  older 
  rocks 
  are 
  well 
  developed, 
  have 
  

   not 
  been 
  found 
  so 
  low 
  down, 
  here 
  occur 
  near 
  the 
  bottom 
  of 
  the 
  local 
  

   series. 
  A 
  conglomerate 
  passing 
  up 
  into 
  fucoidal 
  sandstone 
  forms 
  

   our 
  base 
  ; 
  but 
  above 
  that 
  the 
  first 
  life-zone 
  is 
  represented 
  as 
  yet 
  by 
  

   only 
  two 
  species, 
  of 
  one 
  of 
  which, 
  the 
  Trilobite, 
  only 
  a 
  fragment 
  of 
  

   one 
  individual 
  has 
  yet 
  been 
  found, 
  while 
  the 
  other 
  I 
  have 
  shown 
  

   above 
  is 
  a 
  doubtful 
  species. 
  The 
  next 
  two 
  life-zones 
  are 
  both 
  

   Arenig. 
  Above 
  this 
  nothing 
  is 
  clear 
  till 
  we 
  reach 
  the 
  Silurian. 
  

  

  