﻿356 
  

  

  BR. 
  C. 
  CALLAWAY 
  ON 
  THE 
  ORIGIN 
  OF 
  

  

  [Aug. 
  1897, 
  

  

  with 
  this 
  hade. 
  On 
  the 
  western 
  side 
  the 
  altered 
  diorite 
  is 
  very 
  

   irregular, 
  projecting 
  into 
  several 
  promontories 
  ; 
  but 
  here 
  also 
  the 
  

   grey 
  gneiss 
  is 
  foliated 
  parallel 
  to 
  the 
  margin 
  of 
  the 
  diorite, 
  the 
  

   laminae 
  following 
  exactly 
  the 
  projections 
  and 
  recesses 
  of 
  the 
  latter. 
  

   This 
  foliation 
  is 
  accentuated 
  by 
  very 
  thin 
  dark 
  lines. 
  

  

  Pig. 
  3. 
  — 
  Gneiss 
  of 
  secondary 
  injection, 
  Llangaffo. 
  

  

  Diorite 
  

  

  passing 
  into 
  

  

  Schist 
  

  

  Grey 
  Gneiss 
  

  

  [The 
  grey 
  gneiss 
  or 
  sheared 
  felsite 
  is 
  injected 
  with 
  decomposition-products 
  from 
  

   the 
  adjacent 
  diorite. 
  The 
  black 
  lines 
  in 
  the 
  grey 
  gneiss 
  represent 
  planes 
  

   of 
  injection, 
  but 
  only 
  a 
  few 
  are 
  shown.] 
  

  

  The 
  explanation 
  of 
  this 
  gneiss 
  is 
  similar 
  to 
  that 
  which 
  I 
  have 
  given 
  l 
  

   of 
  an 
  infiltration-gneiss 
  at 
  Malvern, 
  felsite 
  being 
  substituted 
  for 
  

   granite. 
  The 
  infiltration-products 
  are 
  substantially 
  the 
  same 
  as 
  at 
  

   Malvern, 
  consisting 
  mainly 
  of 
  chlorite, 
  iron-oxide, 
  and 
  epidote 
  ; 
  and 
  

   out 
  of 
  the 
  chlorite 
  and 
  iron-oxide, 
  with 
  the 
  addition 
  of 
  materials 
  

   from 
  the 
  felsite, 
  black 
  mica 
  has 
  in 
  like 
  manner 
  been 
  sometimes 
  

   constructed. 
  The 
  following 
  is 
  a 
  description 
  of 
  a 
  slice 
  of 
  this 
  gneiss. 
  

   It 
  shows 
  a 
  breadth 
  of 
  § 
  inch 
  cut 
  across 
  the 
  foliation, 
  and 
  is 
  taken 
  at 
  

   the 
  junction 
  of 
  the 
  grey 
  gneiss 
  with 
  the 
  altered 
  diorite. 
  

  

  1. 
  A 
  seam 
  of 
  minute 
  transparent 
  granules, 
  generally 
  elongated 
  in 
  

  

  the 
  direction 
  of 
  foliation, 
  many 
  of 
  them 
  being 
  lenticular 
  in 
  

   shape. 
  It 
  is 
  rarely 
  possible 
  to 
  determine 
  which 
  of 
  these 
  

   are 
  quartz 
  and 
  which 
  felspar, 
  bat 
  scattered 
  among 
  them 
  are 
  

   several 
  comparatively 
  large 
  lenticles 
  of 
  quartz. 
  Some 
  minute 
  

   crystals 
  of 
  epidote, 
  elongated 
  with 
  the 
  foliation, 
  are 
  present. 
  

  

  2. 
  A 
  dark 
  seam, 
  chiefly 
  composed 
  of 
  interfelted 
  chlorite, 
  pale 
  green 
  

  

  in 
  ordinary 
  light, 
  almost 
  extinguishing 
  under 
  crossed 
  nicols. 
  

   It 
  occasionally 
  passes 
  into 
  greenish, 
  or 
  even 
  brownish, 
  mica. 
  

   There 
  is 
  a 
  little 
  epidote, 
  and 
  lenticles 
  of 
  clear 
  quartz 
  or 
  felspar 
  

   lie 
  amid 
  the 
  meshwork. 
  

  

  3. 
  A 
  minute 
  interfoliation 
  of 
  several 
  of 
  the 
  felsitic 
  seams 
  with 
  the 
  

  

  dark 
  green 
  material. 
  

  

  4. 
  A 
  comparatively 
  thick 
  folium 
  of 
  the 
  chloritic 
  rock. 
  

  

  Yeins 
  of 
  calcite 
  cross 
  the 
  slide 
  transversely 
  to 
  the 
  foliation. 
  

  

  This 
  specimen 
  may 
  be 
  taken 
  as 
  typical 
  of 
  the 
  structure 
  at 
  the 
  

   junction 
  of 
  the 
  grey 
  and 
  green 
  rocks. 
  It 
  does 
  not 
  contain 
  much 
  

   mica, 
  but 
  in 
  some 
  of 
  the 
  junction-slides 
  there 
  is 
  a 
  fair 
  proportion 
  

   of 
  that 
  mineral, 
  the 
  white 
  variety 
  being 
  predominant. 
  

  

  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xlv. 
  (1889) 
  p. 
  496 
  & 
  vol. 
  xlix. 
  (1893) 
  p. 
  412. 
  

  

  