﻿ON 
  THE 
  NORTHERN 
  SLOPES 
  OF 
  CADER 
  IDRIS. 
  437 
  

  

  as 
  Tremadoc. 
  Some 
  of 
  the 
  characteristic 
  ashes 
  of 
  Mynydd-y-Gader, 
  

   occurring 
  on 
  its 
  north-west 
  flanks 
  seven 
  hundred 
  feet 
  below 
  the 
  

   summit, 
  may 
  even 
  fall 
  within 
  the 
  " 
  Dolgelly 
  Group 
  " 
  of 
  Belt. 
  

  

  If, 
  indeed, 
  we 
  are 
  to 
  include 
  all 
  the 
  eruptive 
  deposits 
  in 
  the 
  

   Arenig, 
  the 
  black 
  slates 
  of 
  the 
  Penrhyn-gwyn 
  quarry, 
  lying 
  as 
  they 
  

   do 
  beneath 
  the 
  first 
  slate-tuff, 
  must 
  represent 
  the 
  uppermost 
  Tre- 
  

   madoc; 
  but 
  these 
  occur 
  600 
  feet 
  nearer 
  the 
  sea-level 
  than 
  the 
  summit- 
  

   beds 
  of 
  Mynydd-y-Gader, 
  and 
  we 
  have 
  to 
  face 
  the 
  fact 
  that, 
  taking 
  

   the 
  average 
  dip 
  as 
  only 
  20°, 
  a 
  thickness 
  of 
  some 
  2200 
  feet 
  of 
  ash 
  

   and 
  slate 
  and 
  intercalated 
  sheets 
  intervenes 
  between 
  Penrhyn-gwyn 
  

   and 
  the 
  horizon 
  of 
  the 
  Arenig 
  ironstone 
  near 
  Llyn-y-Gader. 
  Even 
  

   if 
  this 
  thickness 
  is 
  deceptive, 
  even 
  if 
  it 
  is 
  the 
  result 
  of 
  a 
  series 
  of 
  

   repetitions 
  by 
  localized 
  strike-faults, 
  the 
  evidence 
  of 
  the 
  fossils 
  on 
  

   Mynydd-y-Gader 
  is 
  in 
  no 
  way 
  vitiated. 
  

  

  It 
  is 
  of 
  course 
  difficult 
  to 
  compare 
  the 
  thickness 
  of 
  normal 
  sedi- 
  

   ments 
  with 
  those 
  formed 
  contemporaneously 
  near 
  an 
  active 
  volcano 
  ; 
  

   but 
  we 
  feel 
  that, 
  until 
  those 
  best 
  qualified 
  to 
  judge 
  extend 
  the 
  local 
  

   base 
  of 
  the 
  Arenig 
  to 
  the 
  northern 
  front 
  of 
  Mynydd-y-Gader, 
  we 
  

   must 
  regard 
  a 
  large 
  portion 
  of 
  the 
  eruptive 
  series 
  as 
  of 
  Upper 
  or 
  

   even 
  Lower 
  Tremadoc 
  age. 
  

  

  Thus 
  the 
  great 
  eruptions 
  commonly 
  referred 
  to 
  the 
  Arenig 
  and 
  

   Llanvirn 
  periods, 
  or 
  the 
  Llandeilo 
  of 
  older 
  writers, 
  would 
  seem 
  to 
  

   have 
  actively 
  broken 
  out 
  in 
  pre-Ordovician 
  times. 
  The 
  products 
  of 
  

   the 
  volcanic 
  vents 
  in 
  the 
  neighbourhood 
  of 
  Cader 
  Idris 
  became 
  more 
  

   and 
  more 
  highly 
  silicated 
  during 
  the 
  Arenig 
  period, 
  and 
  thus 
  afford 
  

   an 
  early 
  indication 
  of 
  the 
  conditions 
  that 
  afterwards 
  prevailed 
  

   throughout 
  North 
  Wales, 
  when 
  the 
  exceptionally 
  glassy 
  lavas 
  of 
  

   Bala 
  age 
  must 
  have 
  rivalled, 
  over 
  miles 
  of 
  country, 
  the 
  obsidians 
  of 
  

   the 
  Yellowstone 
  Park. 
  While 
  the 
  intrusive 
  sheets, 
  with 
  the 
  

   exception 
  of 
  the 
  eurite, 
  are 
  of 
  more 
  basic 
  character 
  than 
  the 
  ashes 
  

   which 
  are 
  penetrated 
  and 
  altered 
  by 
  them, 
  they 
  none 
  the 
  less 
  appear 
  

   to 
  belong 
  to 
  much 
  the 
  same 
  period 
  of 
  activity. 
  Like 
  the 
  well-known 
  

   instances 
  in 
  the 
  Jurassic 
  strata 
  of 
  the 
  east 
  of 
  Skye, 
  they 
  have 
  fol- 
  

   lowed 
  the 
  planes 
  of 
  bedding 
  with 
  remarkable 
  uniformity. 
  It 
  would 
  

   seem, 
  indeed, 
  that 
  these 
  sheets 
  are 
  lateral 
  offshoots 
  from 
  some 
  

   fairly 
  distant 
  centre, 
  and 
  that 
  they 
  were 
  injected 
  between 
  beds 
  

   already 
  weighted 
  with 
  superincumbent 
  sediment. 
  The 
  contrast 
  

   between 
  the 
  structure 
  thus 
  produced 
  and 
  that 
  formed 
  by 
  dykes 
  on 
  

   the 
  flanks 
  of 
  a 
  volcano 
  is 
  very 
  marked 
  upon 
  the 
  northern 
  slopes 
  of 
  

   Cader 
  Idris. 
  

  

  But 
  these 
  intrusive 
  sheets 
  are 
  not 
  found 
  in 
  the 
  undoubted 
  

   Llandeilo 
  strata 
  towards 
  Machynlleth, 
  and 
  the 
  facts 
  of 
  their 
  distri- 
  

   bution 
  connect 
  them 
  distinctly 
  with 
  the 
  activity 
  that 
  culminated 
  in 
  

   Arenig 
  times. 
  It 
  is 
  quite 
  possible 
  that 
  they 
  represent 
  a 
  reversion 
  

   towards 
  basic 
  conditions 
  at 
  a 
  time 
  when 
  the 
  mass 
  of 
  material 
  already 
  

   erupted 
  prevented 
  further 
  action 
  at 
  the 
  surface, 
  and 
  spread 
  out 
  these 
  

   dolerites 
  and 
  aphanites 
  along 
  easier 
  planes 
  of 
  passage 
  underground. 
  

  

  So 
  far 
  as 
  we 
  have 
  at 
  present 
  examined 
  the 
  evidence 
  afforded 
  by 
  

   Pthobell-fawr, 
  the 
  rocks 
  of 
  that 
  most 
  interesting 
  cone, 
  ranging 
  from 
  

   hornblende-dolerite 
  to 
  basic 
  scoriae, 
  do 
  not 
  seem 
  to 
  point 
  to 
  its 
  

  

  