﻿352 
  DR. 
  C. 
  CALLA.WAY 
  ON 
  THE 
  ORIGIN 
  OF 
  [Aug. 
  1 
  897, 
  

  

  constituents 
  have 
  also 
  been 
  somewhat 
  crushed. 
  There 
  is 
  also 
  present 
  

   a 
  fair 
  sprinkling 
  of 
  small 
  highly-refracting 
  crystals. 
  Amongst 
  them 
  

   are 
  both 
  epidote 
  and 
  sphene. 
  A 
  few 
  small 
  patches 
  of 
  quartz 
  are 
  

   scattered 
  through 
  the 
  slide. 
  This 
  felsite 
  passes 
  within 
  an 
  inch 
  or 
  

   two 
  into 
  a 
  rock 
  which, 
  megascopically, 
  appears 
  rather 
  less 
  compact, 
  

   and 
  would 
  be 
  called 
  a 
  halleninta. 
  Microscopically 
  (418), 
  it 
  does 
  

   not 
  essentially 
  differ 
  from 
  the 
  last, 
  but 
  there 
  is 
  a 
  slightly 
  parallel 
  

   structure 
  among 
  the 
  granules, 
  and 
  quartz 
  has 
  been 
  developed 
  in 
  

   a 
  more 
  streaky 
  manner, 
  the 
  streaks 
  running 
  with 
  the 
  incipient 
  

   foliation. 
  A 
  very 
  minute 
  quantity 
  of 
  chlorite 
  and 
  white 
  mica 
  

   accentuates 
  the 
  parallel 
  arrangement, 
  and 
  streams 
  of 
  minute 
  sphenes 
  

   run 
  in 
  the 
  same 
  direction. 
  Another 
  slide 
  (380) 
  at 
  about 
  the 
  same 
  

   distance 
  from 
  the 
  first 
  shows 
  a 
  structure 
  similar 
  to 
  that 
  of 
  no. 
  418 
  ; 
  

   but 
  the 
  parallel 
  arrangement 
  is 
  rather 
  more 
  marked, 
  and 
  the 
  

   colourless 
  mica 
  is 
  somewhat 
  more 
  abundant. 
  The 
  hand-specimen, 
  

   however, 
  from 
  which 
  the 
  slide 
  is 
  cut 
  is 
  a 
  fairly 
  typical 
  halleninta, 
  

   the 
  schistosity 
  and 
  the 
  presence 
  of 
  mica 
  being 
  scarcely 
  perceptible. 
  

   The 
  streaks 
  of 
  quartz 
  do 
  not 
  occur 
  in 
  this 
  or 
  any 
  of 
  the 
  following 
  

   slides. 
  

  

  Seven 
  more 
  thin 
  sections, 
  taken 
  at 
  intervals 
  of 
  a. 
  few 
  inches, 
  show 
  

   the 
  progressive 
  change 
  into 
  a 
  well-marked 
  gneiss. 
  In 
  381 
  the 
  

   mica 
  is 
  more 
  abundant 
  than 
  in 
  the 
  last. 
  Pel 
  spar 
  can 
  be 
  identified 
  

   in 
  distinct 
  folia 
  of 
  idiomorphic 
  crystals, 
  both 
  plagioclase 
  and 
  micro- 
  

   cline 
  being 
  present. 
  No. 
  382 
  contains 
  rather 
  less 
  mica. 
  JNos. 
  383 
  

   and 
  384 
  are 
  generally 
  similar 
  to 
  the 
  two 
  preceding 
  slides. 
  A 
  little 
  

   brown 
  mica 
  appears 
  in 
  385, 
  seemingly 
  in 
  distinct 
  shear-planes, 
  as 
  if 
  

   it 
  might 
  have 
  resulted 
  from 
  infiltration, 
  the 
  diorite 
  in 
  mass 
  being 
  at 
  

   no 
  great 
  distance. 
  No. 
  386 
  is 
  very 
  gneissic, 
  white 
  mica 
  being 
  fairly 
  

   abundant, 
  and 
  several 
  distinct 
  folia 
  of 
  fresh-looking 
  felspar 
  being 
  

   present. 
  The 
  last 
  slide 
  (387) 
  of 
  the 
  series 
  is 
  a 
  beautifully 
  foliated 
  

   gneiss. 
  Micas, 
  both 
  white 
  and 
  brown, 
  are 
  plentiful, 
  the 
  latter, 
  as 
  

   before, 
  appearing 
  in 
  well-marked 
  shear-planes. 
  I 
  do 
  not 
  mean 
  to 
  

   suggest 
  that 
  the 
  white 
  mica 
  also 
  is 
  not 
  generated 
  at 
  shear-planes 
  : 
  

   but 
  with 
  the 
  brown 
  mica 
  the 
  planes 
  are 
  very 
  conspicuous, 
  being 
  

   indicated, 
  where 
  the 
  biotite 
  thins 
  out, 
  by 
  brown 
  iron-oxide. 
  The 
  

   folia 
  intervening 
  between 
  the 
  mica-seams 
  display 
  mosaic 
  polarization, 
  

   and 
  very 
  little 
  felspar 
  can 
  be 
  indentified. 
  Strain-shadows 
  are 
  well- 
  

   marked 
  in 
  this 
  slide. 
  A 
  few 
  small 
  garnets 
  are 
  present. 
  

  

  I 
  have 
  traced 
  similar 
  gradations 
  in 
  rocks 
  at 
  Holland 
  Arms, 
  on 
  the 
  

   Menai 
  Straits, 
  and 
  elsewhere. 
  It 
  is 
  easy 
  to 
  obtain 
  hand-specimens 
  

   which 
  show 
  halleninta 
  in 
  one 
  part 
  and 
  mica-gneiss 
  in 
  another, 
  with 
  

   gradations 
  between. 
  I 
  have 
  examined 
  a 
  large 
  number 
  of 
  micro- 
  

   scopic 
  slides 
  of 
  these 
  specimens. 
  The 
  part 
  which 
  would 
  be 
  described 
  

   as 
  halleninta 
  usually 
  shows 
  signs 
  of 
  great 
  crushing, 
  and 
  the 
  specimen 
  

   (379) 
  from 
  Y 
  Graig 
  still 
  remains 
  the 
  only 
  example 
  which 
  can 
  be 
  

   positively 
  identified 
  as 
  felsite. 
  

  

  We 
  are 
  not, 
  however, 
  left 
  to 
  the 
  unsupported 
  testimony 
  of 
  the 
  

   microscope, 
  for, 
  as 
  we 
  shall 
  see 
  in 
  the 
  next 
  division 
  of 
  this 
  paper, 
  

   the 
  halleninta 
  behaves 
  like 
  a 
  true 
  eruptive, 
  penetrating 
  other 
  rocks 
  

   in 
  veins, 
  and 
  enclosing 
  blocks 
  of 
  the 
  adjacent 
  masses. 
  

  

  