﻿438 
  MESSES. 
  G, 
  A. 
  J. 
  COLE 
  AXD 
  A. 
  T. 
  JKNA1NGS 
  

  

  connexion 
  with 
  either 
  the 
  tuffs 
  or 
  the 
  intrusive 
  sheets 
  of 
  Cader. 
  

   The 
  mapping 
  of 
  this 
  rugged 
  area 
  as 
  one 
  continuous 
  mass 
  of 
  intrusive 
  

   ' 
  ; 
  greenstone" 
  has 
  doubtless 
  led 
  many 
  besides 
  ourselves 
  to 
  mentally 
  

   exaggerate 
  its 
  importance 
  before 
  becoming 
  acquainted 
  with 
  it 
  in 
  

   the 
  field. 
  Its 
  composite 
  character 
  is, 
  however, 
  referred 
  to 
  in 
  the 
  

   careful 
  Survey 
  memoir. 
  

  

  It 
  may 
  be 
  that 
  the 
  volcanic 
  action 
  in 
  the 
  Cader 
  Idris 
  district 
  was 
  

   almost 
  entirely 
  explosive, 
  and 
  that 
  the 
  actual 
  vent 
  has 
  been 
  lost 
  

   amid 
  the 
  distribution 
  of 
  its 
  products 
  by 
  the 
  sea. 
  That 
  local 
  uncon- 
  

   formities 
  on 
  an 
  appreciable 
  scale 
  must 
  have 
  been 
  produced, 
  is 
  shown 
  

   by 
  the 
  ejected 
  sedimentary 
  flakes 
  in 
  which 
  the 
  tuffs 
  abound. 
  

  

  The 
  age 
  of 
  the 
  eurite 
  itself 
  can 
  only 
  be 
  inferred 
  from 
  the 
  cha- 
  

   racter 
  of 
  the 
  lava-flows 
  of 
  middle 
  Ordovician 
  times. 
  Just 
  as 
  the 
  

   intrusive 
  masses 
  at 
  Mynydd 
  Mawr 
  and 
  other 
  points 
  round 
  Snowdon 
  

   may 
  represent 
  the 
  centres 
  from 
  which 
  the 
  Bala 
  rhyolites 
  flowed, 
  so 
  

   this 
  enormous 
  sill 
  may 
  have 
  been 
  connected 
  with 
  the 
  acid 
  eruptions 
  

   of 
  which 
  we 
  have 
  evidence 
  among 
  the 
  Llandeilo 
  slates 
  of 
  Craig-y- 
  

   Llam. 
  It 
  may 
  even 
  be 
  possible 
  to 
  trace 
  on 
  the 
  southern 
  slopes 
  of 
  

   Cader, 
  or 
  at 
  some 
  point 
  further 
  to 
  the 
  north-east, 
  the 
  continuity 
  of 
  

   part 
  of 
  the 
  eurite 
  with 
  these 
  products 
  of 
  surface-eruption. 
  

  

  It 
  is 
  interesting 
  to 
  note, 
  in 
  conclusion, 
  that 
  the 
  intrusive 
  granite 
  

   between 
  Ffestiniog 
  and 
  the 
  Moelwyn 
  range, 
  with 
  its 
  abundant 
  

   quartz 
  and 
  its 
  poorness 
  in 
  ferro-magnesian 
  silicates, 
  is 
  just 
  such 
  a 
  

   rock 
  as 
  might 
  have 
  resulted 
  had 
  more 
  complete 
  crystallization, 
  

   under 
  slower 
  conditions 
  of 
  cooling, 
  taken 
  place 
  in 
  the 
  eurite 
  of 
  Cader 
  

   Idris 
  *. 
  

  

  In 
  preparing 
  the 
  notes 
  here 
  brought 
  together, 
  we 
  have 
  several 
  

   times 
  received 
  generous 
  assistance 
  from 
  Mr. 
  G. 
  J. 
  Williams, 
  F.G.S., 
  

   whose 
  intimate 
  knowledge 
  of 
  Ordovician 
  strata 
  in 
  Xorth 
  Wales 
  is 
  

   always 
  at 
  the 
  service 
  of 
  his 
  friends. 
  We 
  are 
  also 
  indebted 
  to 
  Prof. 
  

   J. 
  W. 
  Judd, 
  alt. 
  D. 
  Homfray, 
  and 
  Dr. 
  Hicks, 
  for 
  kind 
  help 
  given 
  

   and 
  the 
  removal 
  of 
  some 
  of 
  the 
  difficulties 
  that 
  we 
  have 
  met 
  with. 
  

   We 
  feel 
  that 
  the 
  stratigraphical 
  questions 
  raised 
  in 
  the 
  concluding 
  

   section 
  of 
  this 
  paper 
  can 
  only 
  be 
  settled 
  by 
  detailed 
  mapping 
  and 
  

   correlation 
  of 
  the 
  several 
  fossiliferous 
  horizons. 
  We 
  have 
  sought 
  

   to 
  show, 
  however, 
  that 
  contemporaneous 
  volcanic 
  rocks 
  form 
  a 
  large 
  

   part 
  of 
  Cader 
  Idris 
  at 
  a 
  level 
  far 
  below 
  that 
  assigned 
  to 
  the 
  base 
  

   of 
  the 
  Arenig. 
  

  

  Discrssiox. 
  

  

  The 
  Peesldext 
  was 
  struck 
  by 
  the 
  fact 
  that 
  the 
  crystalline 
  volcanic 
  

   beds 
  appeared 
  to 
  be 
  intrusive, 
  the 
  tuffs 
  only 
  being 
  contemporaneous. 
  

   Some 
  years 
  ago 
  Mr. 
  David 
  Forbes 
  had 
  argued 
  that 
  a 
  submarine 
  

   lava-flow 
  was 
  impossible. 
  He 
  believed 
  that 
  this 
  view 
  had 
  not 
  

   been 
  universally 
  accepted, 
  but 
  he 
  thought 
  it 
  might 
  account 
  for 
  the 
  

   great 
  thickness 
  of 
  many 
  submarine 
  tuffs. 
  

  

  Dr. 
  Hicks 
  said, 
  with 
  regard 
  to 
  the 
  volcanic 
  series, 
  that 
  it 
  was 
  not 
  

   quite 
  certain 
  what 
  Tremadoc 
  beds 
  were 
  found 
  in 
  the 
  region 
  ; 
  the 
  

  

  * 
  This 
  granite 
  has 
  been 
  described 
  by 
  Mr. 
  Teall 
  in 
  his 
  ' 
  British 
  Petrography,' 
  

   p. 
  319. 
  

  

  