﻿ON 
  THK 
  NORTHERN 
  SLOPES 
  OF 
  CADER 
  1DRIS. 
  431 
  

  

  liquid-inclusions 
  have 
  developed 
  in 
  the 
  secondary 
  quartz, 
  passing 
  

   from 
  grain 
  to 
  grain 
  as 
  in 
  the 
  spherulitic 
  rock 
  of 
  Digoed 
  *. 
  The 
  

   lavas 
  of 
  Craig-y-Llam 
  thus 
  parallel, 
  even 
  in 
  minor 
  details, 
  the 
  well- 
  

   known 
  " 
  felsites 
  " 
  of 
  the 
  Snowdon 
  area, 
  which 
  are 
  indisputably 
  of 
  

   Eala 
  age. 
  

  

  Similar 
  rocks 
  form 
  a 
  great 
  part 
  of 
  the 
  opposing 
  slopes 
  rising 
  above 
  

   MinfFordd 
  to 
  Llyn-y-Cau. 
  

  

  III. 
  The 
  Dolerites 
  and 
  allied 
  Intrusive 
  Sheets. 
  

  

  While 
  much 
  of 
  the 
  massive 
  " 
  greenstone 
  " 
  apparent 
  on 
  the 
  pub- 
  

   lished 
  maps 
  can 
  be 
  resolved 
  into 
  tuffs 
  and 
  sedimentary 
  layers, 
  there 
  

   remain 
  the 
  more 
  basic 
  intrusive 
  sheets, 
  which 
  have 
  been 
  already 
  

   referred 
  to 
  and 
  which 
  are 
  too 
  numerous 
  to 
  be 
  separately 
  set 
  down 
  

   on 
  the 
  scale 
  of 
  one 
  inch 
  to 
  the 
  mile. 
  

  

  The 
  frequent 
  alternation 
  of 
  the 
  ash-beds 
  with 
  normal 
  marine 
  

   deposits 
  shows 
  that 
  the 
  centres 
  of 
  eruption 
  were 
  at 
  some 
  distance 
  

   from 
  the 
  present 
  mountain 
  ; 
  and 
  the 
  absence 
  of 
  lava-flows 
  and 
  coarse 
  

   agglomerates 
  points 
  distinctly 
  to 
  the 
  same 
  conclusion. 
  Hence 
  true 
  

   volcanic 
  necks 
  are 
  unlikely 
  to 
  occur 
  in 
  the 
  district 
  dealt 
  with 
  in 
  

   this 
  paper, 
  and 
  an 
  examination 
  of 
  the 
  more 
  suggestive 
  and 
  exten- 
  

   sive 
  areas 
  marked 
  as 
  "greenstone" 
  on 
  the 
  map 
  reveals 
  again 
  and 
  

   again 
  the 
  composite 
  character 
  of 
  these 
  masses. 
  Thus 
  the 
  coarse 
  

   dolerite 
  of 
  the 
  quarry 
  between 
  Rhyd-wen 
  and 
  Bron-y-Gader, 
  one 
  

   mile 
  south-west 
  of 
  Dolgelley, 
  proves 
  to 
  be 
  of 
  small 
  extent 
  at 
  the 
  

   surface, 
  and 
  to 
  be 
  itself 
  associated 
  with 
  bedded 
  ash 
  and 
  porphyrites 
  

   of 
  finer 
  grain. 
  

  

  The 
  mass 
  forming 
  the 
  high 
  ridge 
  of 
  Mynydd-y-Gader 
  is, 
  however, 
  

   more 
  continuous, 
  and 
  is 
  one 
  of 
  the 
  most 
  striking 
  features 
  of 
  the 
  

   mountain. 
  The 
  picturesque 
  hollows 
  of 
  its 
  summit, 
  crossed 
  by 
  roaring- 
  

   streamlets, 
  and 
  its 
  northern 
  escarpment, 
  steep 
  and 
  glaciated, 
  exposed 
  

   for 
  some 
  four 
  hundred 
  feet, 
  afford 
  one 
  ample 
  opportunities 
  for 
  obser- 
  

   vation, 
  but 
  force 
  one 
  to 
  regard 
  even 
  this 
  mass 
  as 
  a 
  highly 
  developed 
  

   intrusive 
  sheet. 
  There 
  is 
  no 
  indication 
  here 
  of 
  the 
  great 
  central 
  

   intrusions, 
  the 
  irregular 
  ramifications, 
  the 
  plexus 
  of 
  dykes 
  and 
  veins, 
  

   that 
  characterize 
  the 
  cores 
  of 
  old 
  volcanoes 
  when 
  at 
  length 
  exposed 
  

   by 
  denudation. 
  

  

  The 
  broad 
  " 
  greenstone 
  " 
  band, 
  three 
  miles 
  long, 
  which 
  appears 
  

   upon 
  all 
  recent 
  copies 
  of 
  the 
  map 
  at 
  the 
  base 
  of 
  the 
  cliff 
  of 
  Cader 
  

   Idris, 
  has 
  been 
  already 
  shown 
  to 
  consist 
  of 
  a 
  considerable 
  variety 
  

   of 
  rocks, 
  ranging 
  from 
  slate 
  to 
  dolerite, 
  the 
  whole 
  series 
  having 
  

   the 
  east-and-west 
  strike 
  so 
  persistent 
  on 
  these 
  northern 
  slopes. 
  

   Prof. 
  Ramsay 
  himself, 
  in 
  the 
  Survey 
  memoir, 
  calls 
  attention 
  to 
  its 
  

   complex 
  character. 
  The 
  " 
  tesselated 
  " 
  appearance 
  of 
  the 
  sheets, 
  

   where 
  jointed 
  into 
  columns 
  f 
  , 
  is 
  frequently 
  seen 
  upon 
  surfaces 
  dipping 
  

   at 
  35, 
  40, 
  or 
  even 
  50 
  degrees. 
  

  

  * 
  Quart. 
  Jpurn. 
  Geol. 
  Soc. 
  vol. 
  xlii. 
  pi. 
  ix. 
  fig. 
  5. 
  

  

  t 
  Mem. 
  Geol. 
  Survey 
  of 
  Great 
  Britain, 
  vol. 
  iii. 
  2nd 
  edit. 
  p. 
  31. 
  

  

  