﻿ON 
  TUE 
  NORTHERN 
  SLOPES 
  OF 
  CADER 
  IDRIS. 
  

  

  423 
  

  

  temporaneous 
  porphyries 
  " 
  from 
  the 
  

   Barmouth 
  estuary 
  across 
  Cader 
  Idris 
  

   to 
  Machynlleth, 
  recognizing 
  his 
  Ffesti- 
  

   niog 
  and 
  Tremadoc 
  group 
  in 
  the 
  foot- 
  

   hills 
  near 
  Arthog 
  and 
  Dolgelley. 
  

  

  Above 
  these 
  picturesque 
  and 
  wooded 
  

   promontories, 
  some 
  six 
  or 
  seven 
  hun- 
  

   dred 
  feet 
  in 
  height, 
  the 
  ground 
  rises 
  

   for 
  another 
  eleven 
  hundred 
  feet 
  as 
  a 
  

   broken 
  grassy 
  upland, 
  set 
  with 
  little 
  

   scattered 
  cliffs, 
  the 
  slopes 
  being 
  easy 
  

   of 
  ascent 
  except 
  to 
  the 
  west 
  of 
  Tyrau 
  

   Mawr, 
  where 
  the 
  valley 
  between 
  the 
  

   lower 
  and 
  upper 
  ranges 
  is 
  rapidly 
  nar- 
  

   rowing 
  to 
  its 
  head. 
  On 
  the 
  east, 
  the 
  

   glaciated 
  front 
  of 
  Mynydd-y-Gader, 
  

   the 
  " 
  Stony 
  Mountain 
  " 
  of 
  Mr. 
  Aikin, 
  

   forms 
  a 
  bold 
  contrast 
  to 
  these 
  green 
  

   and 
  softer 
  ledges. 
  

  

  Prom 
  this 
  level 
  the 
  great 
  wall 
  of 
  

   Cader 
  Idris 
  rises, 
  with 
  several 
  pictu- 
  

   resque 
  lakelets 
  at 
  its 
  foot, 
  a 
  precipice 
  

   some 
  seven 
  miles 
  long 
  and 
  at 
  its 
  boldest 
  

   point 
  nine 
  hundred 
  feet 
  in 
  height. 
  

   Across 
  it, 
  however, 
  there 
  are 
  several 
  

   easy 
  foot-tracks, 
  and 
  from 
  its 
  crest 
  the 
  

   dip-slope 
  of 
  the 
  hills 
  can 
  be 
  seen 
  de- 
  

   scending 
  southward, 
  smooth 
  and 
  grass- 
  

   covered, 
  to 
  the 
  Towyn 
  valley 
  and 
  Tal- 
  

   y-llyn. 
  

  

  II. 
  The 
  Tuffs 
  and 
  Sedimentary 
  

   Deposits. 
  

  

  Starting 
  southward 
  from 
  the 
  level 
  

   of 
  Llyn 
  Gwernen, 
  on 
  the 
  old 
  Dolgelley 
  

   and 
  Towyn 
  road, 
  the 
  first 
  rocks 
  en- 
  

   countered 
  on 
  these 
  northern 
  slopes 
  are 
  

   the 
  black 
  slates 
  so 
  well 
  displayed 
  in 
  

   the 
  deep 
  pit-like 
  quarry 
  of 
  Penrhyn- 
  

   gwyn. 
  The 
  cleavage 
  is 
  almost 
  perpen- 
  

   dicular 
  to 
  the 
  bedding, 
  which 
  dips 
  here 
  

   somewhat 
  west 
  of 
  south. 
  On 
  the 
  up- 
  

   permost 
  ledge 
  of 
  the 
  excavation 
  the 
  

   first 
  evidence 
  of 
  contemporaneous 
  vol- 
  

   canic 
  activity 
  occurs. 
  A 
  grey 
  amygda- 
  

   loidal 
  andesite, 
  five 
  to 
  six 
  feet 
  thick, 
  

   rests 
  upon 
  the 
  cleavage-edges 
  of 
  the 
  

   slates, 
  being 
  itself 
  only 
  slightly 
  affected 
  

   by 
  the 
  earth-pressures 
  of 
  the 
  district. 
  

  

  w 
  

  

  ro'ty 
  

  

  *^> 
  

  

  to 
  

  

  