﻿ON 
  THE 
  NORTHERN 
  SLOPES 
  OF 
  CABER 
  JDRIS. 
  435 
  

  

  this 
  side 
  of 
  the 
  mountain 
  the 
  eurite 
  is 
  near 
  some 
  line 
  of 
  emergence 
  

   on 
  the 
  old 
  sea-floor, 
  a 
  consideration 
  that 
  connects 
  it 
  in 
  our 
  minds 
  

   with 
  the 
  Llandeilo 
  lavas 
  of 
  Craig- 
  y-Llam. 
  

  

  But 
  on 
  the 
  northern 
  slopes 
  of 
  Cader 
  Idris 
  the 
  masses 
  are, 
  as 
  

   already 
  stated, 
  very 
  uniform 
  in 
  texture. 
  On 
  Mynydd 
  Moel 
  por- 
  

   phyritic 
  felspars 
  are 
  more 
  easily 
  visible 
  than 
  at 
  other 
  places, 
  but 
  

   even 
  here 
  do 
  not 
  readily 
  catch 
  the 
  eye 
  *. 
  The 
  specific 
  gravity 
  of 
  a 
  

   blue-grey 
  fresh-looking 
  specimen 
  from 
  this 
  point 
  is 
  2*64. 
  In 
  

   section, 
  the 
  same 
  specimen 
  shows 
  clear 
  felspars, 
  mainly 
  plagioclastic, 
  

   with 
  wisp-like 
  yellow 
  and 
  altered 
  biotites 
  and 
  numerous 
  grains 
  of 
  

   quartz 
  ; 
  round 
  these 
  the 
  matrix 
  is 
  exquisitely 
  micropegmatitic. 
  

   Small 
  chloritic 
  patches 
  are 
  frequent. 
  

  

  The 
  typical 
  rock 
  at 
  Llyn-y-Gader, 
  where 
  we 
  have 
  easy 
  access 
  to 
  

   the 
  centre 
  of 
  this 
  huge 
  intrusive 
  sheet, 
  shows 
  the 
  delicate 
  micropeg- 
  

   matite 
  collected 
  into 
  spherulitic 
  forms 
  around 
  the 
  quartz- 
  and 
  felspar- 
  

   crystals. 
  The 
  little 
  bunches 
  and 
  wisps 
  of 
  biotite 
  occur 
  indepen- 
  

   dently 
  of 
  these 
  aggregations. 
  Though 
  simply 
  twinned 
  felspars 
  are 
  

   here 
  more 
  common 
  than 
  at 
  Mynydd 
  Moel, 
  the 
  rock 
  is 
  still 
  allied 
  to 
  

   the 
  granitites 
  or 
  the 
  soda-granites 
  rather 
  than 
  to 
  the 
  normal 
  

   granites. 
  The 
  interstices 
  left 
  between 
  the 
  well-developed 
  spherulites 
  

   are 
  now 
  mainly 
  occupied 
  by 
  chlorite 
  and 
  a 
  few 
  highly 
  refracting 
  

   granules. 
  It 
  is 
  unusual 
  to 
  find 
  in 
  a 
  rock 
  of 
  this 
  description 
  so 
  

   complete 
  a 
  separation 
  between 
  the 
  micropegmatitic 
  spherulites 
  and 
  

   the 
  residual 
  matrix 
  round 
  them. 
  

  

  A 
  specimen 
  collected 
  from 
  the 
  mass 
  to 
  the 
  south-west 
  of 
  Gelli- 
  

   lwyd-fawr 
  and 
  north 
  of 
  the 
  old 
  Towyn 
  road 
  is 
  of 
  slightly 
  coarser 
  

   grain. 
  The 
  micropegmatite 
  is 
  far 
  more 
  distinct, 
  and 
  spherulitic 
  

   grouping 
  does 
  not 
  occur. 
  Here 
  and 
  there 
  the 
  quartz 
  has 
  developed 
  

   ophitically 
  round 
  felspars 
  that 
  lie 
  at 
  random 
  with 
  regard 
  to 
  one 
  

   another. 
  The 
  rock 
  is, 
  however, 
  essentially 
  the 
  same 
  as 
  the 
  main 
  

   eurite 
  of 
  the 
  wall, 
  and 
  has 
  a 
  corresponding 
  specific 
  gravity 
  of 
  2' 
  63. 
  

   Through 
  the 
  kind 
  permission 
  of 
  Prof. 
  Judd 
  we 
  are 
  able 
  to 
  give 
  

   an 
  analysis 
  of 
  the 
  eurite 
  of 
  Cader 
  Idris, 
  which 
  has 
  been 
  made 
  by 
  

   Mr. 
  T. 
  H. 
  Holland, 
  A.N.S.S., 
  in 
  the 
  Geological 
  Laboratory 
  of 
  the 
  

   Normal 
  School 
  of 
  Science 
  and 
  Royal 
  School 
  of 
  Mines. 
  The 
  speci- 
  

   men 
  used 
  was 
  taken 
  from 
  the 
  columns 
  on 
  the 
  south 
  shore 
  of 
  Llyn-y- 
  

   Gader. 
  

  

  Silica 
  72-79 
  

  

  Alumina 
  13*77 
  

  

  Ferric 
  Oxide 
  3-32 
  

  

  Manganous 
  Oxide 
  trace. 
  

  

  Lime 
  1-94 
  

  

  Magnesia 
  '62 
  

  

  Potash 
  2-99 
  

  

  Soda 
  4-12 
  

  

  Loss 
  on 
  Ignition 
  1*08 
  

  

  100-63 
  

  

  * 
  On 
  the 
  copies 
  of 
  the 
  map 
  that 
  we 
  have 
  examined 
  this 
  promontory 
  is 
  

   coloured 
  as 
  interbedded 
  trap, 
  probably 
  by 
  an 
  oversight, 
  since 
  it 
  is 
  clearly 
  

   connected 
  with 
  the 
  mass 
  around 
  Llyn-y-Gader. 
  

  

  2l2 
  

  

  