﻿436 
  MESSES. 
  G. 
  A. 
  J. 
  COLE 
  AND 
  A. 
  V. 
  JENNINGS 
  

  

  It 
  is 
  interesting 
  to 
  possess 
  this 
  analysis 
  of 
  a 
  rock 
  with 
  which 
  

   almost 
  every 
  British 
  geologist 
  is 
  familiar, 
  and 
  it 
  confirms 
  the 
  re- 
  

   lationship 
  of 
  this 
  eurite 
  to 
  the 
  granites 
  rich 
  in 
  soda. 
  

  

  "We 
  believe 
  that 
  the 
  mass 
  must 
  have 
  been 
  fairly, 
  if 
  minutely, 
  

   crystalline 
  at 
  the 
  time 
  of 
  its 
  original 
  consolidation 
  ; 
  but 
  it 
  is 
  obvious 
  

   that 
  any 
  discussion 
  of 
  the 
  "granophyric" 
  structure 
  of 
  Rosenbusch 
  

   must 
  affect 
  our 
  views 
  of 
  this 
  rock 
  of 
  Cader 
  Idris, 
  in 
  company 
  with 
  

   a 
  host 
  of 
  other 
  British 
  examples, 
  closely 
  agreeing 
  with 
  it 
  in 
  texture 
  

   and 
  in 
  chemical 
  composition. 
  

  

  V. 
  The 
  Age 
  and 
  Duration 
  of 
  the 
  Eruptions. 
  

  

  Such 
  being 
  the 
  relations 
  and 
  general 
  characters 
  of 
  the 
  rocks 
  

   forming 
  the 
  northern 
  slopes 
  of 
  Cader 
  Idris, 
  we 
  have 
  now 
  to 
  consider 
  

   their 
  position 
  in 
  the 
  stratigraphical 
  series. 
  

  

  It 
  appears 
  to 
  be 
  generally 
  admitted 
  that 
  no 
  indications 
  of 
  erup- 
  

   tive 
  action 
  occur 
  in 
  this 
  district 
  throughout 
  the 
  Lingula 
  Flags 
  ; 
  and 
  

   volcanic 
  activity 
  is 
  believed 
  to 
  have 
  commenced 
  in 
  early 
  Arenig 
  

   times*. 
  This 
  view 
  has 
  been 
  supported 
  by 
  the 
  fact 
  that 
  on 
  the 
  

   more 
  recently 
  issued 
  maps 
  of 
  Cader 
  Idris 
  the 
  igneous 
  rocks 
  occurring 
  

   upon 
  the 
  lower 
  horizons 
  of 
  the 
  mountain 
  are 
  coloured 
  as 
  entirely 
  

   intrusive. 
  The 
  section, 
  however, 
  published 
  by 
  the 
  Geological 
  Survey 
  

   in 
  1852, 
  and 
  copies 
  of 
  the 
  map 
  prior 
  to 
  1881, 
  correctly 
  represent 
  

   them 
  as 
  consisting 
  largely 
  of 
  ashes, 
  and 
  in 
  the 
  section 
  they 
  are 
  

   included 
  in 
  the 
  old 
  extensive 
  Lingula 
  Flags. 
  

  

  The 
  presence 
  of 
  the 
  Lower 
  Tremadoc 
  beds 
  upon 
  the 
  flanks 
  of 
  

   Mynydd-y-Gader 
  is 
  well 
  shown 
  by 
  Belt 
  t, 
  who 
  records 
  the 
  discovery 
  

   of 
  Niohe 
  Homfrayi 
  and 
  Asaphus 
  innotatus, 
  accompanied 
  by 
  fossils 
  

   recalling 
  the 
  earlier 
  Cambrian 
  fauna. 
  Beneath 
  these 
  beds 
  he 
  

   recognized 
  the 
  " 
  Dolgelly 
  Group 
  " 
  of 
  the 
  Lingula 
  Flags 
  %. 
  Mr. 
  

   Homfray, 
  of 
  Portmadoc, 
  who 
  went 
  over 
  the 
  ground 
  with 
  Belt, 
  

   informs 
  us 
  that 
  the 
  junction 
  of 
  the 
  Tremadoc 
  and 
  the 
  Arenig 
  is 
  

   obscured 
  by 
  the 
  intrusive 
  masses 
  ; 
  but 
  he 
  states 
  that 
  the 
  Tremadoc 
  

   beds 
  lie 
  upon 
  the 
  top 
  of 
  Mynydd-y-Gader, 
  the 
  ironstone, 
  with 
  

   characteristic 
  Trilobites 
  above 
  it, 
  occurring 
  near 
  the 
  base 
  of 
  the 
  

   Arenig 
  and 
  being 
  included 
  in 
  that 
  group. 
  This 
  correlation 
  has 
  been 
  

   accepted 
  by 
  the 
  author 
  of 
  the 
  Survey 
  memoir 
  and 
  by 
  subsequent 
  

   writers, 
  who 
  regard 
  the 
  ironstone 
  as 
  the 
  representative 
  of 
  the 
  

   similar 
  bed 
  at 
  Pen-y-morfa, 
  and 
  who 
  indicate 
  in 
  their 
  sections 
  a 
  

   considerable 
  thickness 
  of 
  Tremadoc 
  strata 
  between 
  it 
  and 
  the 
  crest 
  of 
  

   Mynydd-y-Gader, 
  or 
  on. 
  corresponding 
  horizons 
  of 
  the 
  mountain 
  §. 
  

   It 
  is 
  clear, 
  then, 
  that 
  the 
  series 
  described 
  by 
  us 
  as 
  slaty 
  tuffs 
  and 
  

   metamorphosed 
  ashes 
  coincides 
  in 
  large 
  part 
  with 
  the 
  beds 
  accepted 
  

  

  * 
  Dr. 
  Hicks, 
  in 
  1875, 
  in 
  discussing 
  the 
  conditions 
  under 
  which 
  the 
  Cambrian 
  

   and 
  Silurian 
  rocks 
  were 
  deposited, 
  places 
  the 
  commencement 
  of 
  eruption 
  at 
  the 
  

   close 
  of 
  the 
  Arenig 
  period. 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxi. 
  p. 
  558. 
  

  

  t 
  Geol. 
  Mag. 
  1868, 
  p. 
  8. 
  

  

  % 
  Ibid. 
  1867, 
  p. 
  542. 
  

  

  § 
  Mem. 
  Geol. 
  Survey 
  of 
  Gt, 
  Britain, 
  vol. 
  iii. 
  2nd 
  ed. 
  pp. 
  35, 
  38, 
  39, 
  &c. 
  

  

  Hicks, 
  Congres 
  Geol 
  Internat. 
  Londres, 
  1888, 
  Explication 
  des 
  Excursions, 
  

   fig. 
  16. 
  

  

  