﻿354 
  

  

  DE. 
  C. 
  CALLAWAY 
  ON 
  THE 
  OKIGLN 
  OF 
  

  

  [Aug. 
  1897, 
  

  

  parallel 
  with 
  the 
  foliation 
  are 
  several 
  veins 
  of 
  grey 
  fine-grained 
  

   rock, 
  some 
  of 
  which 
  are 
  a 
  kind 
  of 
  halleflinta, 
  while 
  others 
  are 
  

   foliated. 
  They 
  range 
  in 
  breadth 
  from 
  several 
  inches 
  to 
  less 
  than 
  a 
  

   line. 
  Some 
  of 
  the 
  thicker 
  veins 
  show 
  a 
  gradation 
  within 
  an 
  inch 
  

   or 
  so 
  between 
  halleflinta 
  and 
  gneiss. 
  Similar 
  veins 
  appear 
  in 
  the 
  

   floor 
  of 
  the 
  quarry, 
  and 
  strike 
  across 
  the 
  road, 
  sometimes 
  branching. 
  

   They 
  thin 
  out 
  to 
  a 
  point. 
  It 
  is 
  incredible 
  that 
  this 
  grey 
  rock 
  should 
  

   be 
  sedimentary, 
  and 
  it 
  is 
  almost 
  equally 
  difficult 
  to 
  believe 
  that 
  it 
  

   can 
  be 
  older 
  than 
  the 
  diorite 
  in 
  which 
  it 
  is 
  enclosed. 
  

  

  Fig. 
  1. 
  — 
  Gneiss 
  of 
  primary 
  injection, 
  Llanyaffo. 
  

   w 
  - 
  _______ 
  E- 
  

  

  In 
  thin 
  sections 
  some 
  of 
  the 
  felsite 
  (or 
  halleflinta) 
  exhibits 
  the 
  

   minutely-granulated 
  structure 
  of 
  the 
  grey 
  veins 
  at 
  Llangaffo 
  ; 
  but 
  

   it 
  contains 
  numerous 
  granules 
  of 
  epidote, 
  the 
  effect, 
  perhaps, 
  of 
  

   the 
  chemical 
  action 
  of 
  the 
  enclosing 
  diorite. 
  Junction-specimens 
  

   show 
  this 
  felsite 
  to 
  be 
  interlaminated 
  with 
  bright, 
  clear 
  horn- 
  

   blende 
  (whether 
  by 
  veining 
  or 
  infiltration 
  I 
  cannot 
  say), 
  and 
  

   sometimes 
  we 
  observe 
  the 
  hornblende 
  to 
  alternate 
  with 
  fresh, 
  clear 
  

   plagioclase 
  in 
  long 
  prisms 
  lying 
  in 
  different 
  directions. 
  It 
  would 
  

   thus 
  appear 
  that 
  in 
  this 
  locality 
  there 
  has 
  been 
  an 
  actual 
  fusion 
  of 
  

   some 
  of 
  the 
  constituents, 
  resulting 
  not 
  only 
  in 
  the 
  generation 
  of 
  

   new 
  minerals, 
  but 
  in 
  the 
  regeneration 
  of 
  the 
  hornblende. 
  

  

  Turning 
  into 
  the 
  field 
  close 
  at 
  hand, 
  we 
  come 
  to 
  the 
  typical 
  

   section 
  (p. 
  356) 
  of 
  felsite 
  graduating 
  into 
  grey 
  gneiss, 
  and 
  we 
  are 
  

   thus 
  led 
  to 
  infer 
  that 
  the 
  veins 
  in 
  the 
  road 
  proceed 
  from 
  the 
  same 
  

  

  Section 
  at 
  Forth 
  Gwyfen. 
  

  

  On 
  the 
  shore 
  at 
  this 
  locality 
  there 
  is 
  a 
  very 
  clear 
  exposure 
  of 
  a 
  

   striped 
  gneiss 
  similar 
  to 
  the 
  section 
  shown 
  in 
  fig. 
  1. 
  Grey 
  gneiss 
  

   and 
  dark-green 
  schist 
  run 
  into 
  each 
  other 
  in 
  numerous 
  thin 
  bands. 
  

   The 
  gneiss 
  passes 
  in 
  places 
  into 
  halleflinta, 
  and 
  veins 
  of 
  the 
  grey 
  

   rock, 
  in 
  the 
  form 
  of 
  either 
  halleflinta 
  or 
  gneiss, 
  are 
  also 
  seen 
  in 
  the 
  

   green 
  schist 
  at 
  a 
  distance 
  from 
  the 
  junction 
  of 
  the 
  two 
  rocks. 
  In 
  an 
  

   easterly 
  direction 
  the 
  grey 
  veins 
  come 
  in 
  more 
  and 
  more 
  abundantly, 
  

   till 
  we 
  reach 
  a 
  large 
  mass 
  of 
  grey 
  gneiss. 
  

  

  