﻿CRYSTALLINE 
  ROCKS 
  OF 
  THE 
  MALVERN 
  HILLS. 
  493 
  

  

  " 
  On 
  one 
  side 
  of 
  the 
  slide 
  are 
  many 
  little 
  scales 
  of 
  a 
  colourless 
  

   mineral 
  showing 
  same 
  characteristics 
  as 
  are 
  seen 
  and 
  described 
  

   in 
  327. 
  

  

  " 
  When 
  the 
  schistose 
  structure 
  is 
  plainest, 
  the 
  chlorite 
  appears 
  

   banded 
  with 
  lamellae 
  of 
  biotite, 
  which 
  are 
  recognized 
  both 
  by 
  strong 
  

   pleochroism 
  and 
  by 
  the 
  very 
  high 
  interference-colours. 
  The 
  horn- 
  

   blende 
  has 
  almost 
  entirely 
  disappeared. 
  

  

  " 
  Quartz 
  is 
  apparently 
  wanting. 
  Calcite 
  is 
  present 
  in 
  scattered 
  

   grains. 
  

  

  " 
  No. 
  327. 
  Hornblende 
  has 
  almost 
  totally 
  disappeared, 
  only 
  small 
  

   fragments 
  being 
  left 
  surrounded 
  by 
  chlorite. 
  The 
  latter 
  is 
  very 
  

   abundant, 
  and 
  by 
  the 
  parallel 
  arrangement 
  of 
  the 
  plates 
  gives 
  an 
  

   eminently 
  schistose 
  structure 
  to 
  the 
  rock. 
  

  

  " 
  Biotite 
  is 
  scarcely 
  noticeable, 
  while 
  the 
  muscovite, 
  noted 
  sparsely 
  

   in 
  325, 
  and 
  frequently 
  in 
  326, 
  is 
  here 
  very 
  abundant. 
  It 
  is 
  negative, 
  

   the 
  axis 
  of 
  greatest 
  elasticity 
  being 
  at 
  right 
  angles 
  to 
  the 
  cleavage. 
  

   It 
  is 
  not 
  attacked 
  by 
  hydrochloric 
  acid. 
  The 
  very 
  minute 
  scales, 
  

   isolated 
  from 
  rock-powder 
  by 
  means 
  of 
  the 
  heavy 
  solution 
  of 
  iodide 
  

   of 
  mercury 
  and 
  iodide 
  of 
  potassium, 
  give 
  upon 
  the 
  cleavage-faces 
  in 
  

   convergent 
  polarized 
  light 
  a 
  vertical 
  negative 
  bisectrix 
  with 
  mode- 
  

   rate 
  axial 
  angle, 
  such 
  as 
  muscovite 
  generally 
  shows. 
  The 
  size 
  

   of 
  the 
  axial 
  angle 
  excludes 
  talc. 
  Feldspar 
  is 
  apparently 
  entirely 
  

   changed 
  into 
  a 
  very 
  fine 
  mass 
  of 
  epidote 
  grains. 
  Calcite, 
  as 
  in 
  the 
  

   last, 
  occurs 
  in 
  scattered 
  grains. 
  The 
  rock 
  effervesces 
  with 
  hydro- 
  

   chloric 
  acid. 
  Reddish 
  scales 
  of 
  iron-oxide 
  and 
  grains 
  of 
  magnetite 
  

   or 
  ilmenite 
  are 
  abundant. 
  The 
  last 
  are 
  generally 
  surrounded 
  by 
  a 
  

   zone 
  of 
  titanite. 
  

  

  <; 
  The 
  above-mentioned 
  muscovite 
  never 
  shows 
  a 
  tendency 
  to 
  be 
  

   intergrown 
  with 
  chlorite 
  as 
  is 
  the 
  case 
  with 
  biotite." 
  

  

  The 
  series 
  just 
  described 
  was 
  selected 
  to 
  show 
  in 
  a 
  compact 
  form 
  

   some 
  of 
  the 
  changes 
  produced 
  at 
  an 
  igneous 
  complex, 
  but 
  there 
  is 
  

   no 
  good 
  sh^ar-zone 
  here, 
  and 
  the 
  metamorphism 
  has 
  not 
  been 
  

   carried 
  to 
  a 
  very 
  advanced 
  stage. 
  

  

  No. 
  328, 
  from 
  the 
  ridge 
  north 
  of 
  the 
  Wych, 
  shows 
  the 
  same 
  

   variety 
  of 
  diorite 
  penetrated 
  by 
  a 
  few 
  minute 
  granite-veins, 
  three 
  of 
  

   which 
  appear 
  in 
  the 
  slide. 
  It 
  was 
  selected 
  to 
  illustrate 
  the 
  fact 
  

   that 
  the 
  degree 
  of 
  metamorphism 
  bears 
  some 
  proportion 
  to 
  the 
  

   number 
  and 
  size 
  of 
  the 
  granite-veins 
  plus 
  the 
  amount 
  of 
  shearing. 
  

   Here 
  the 
  forces 
  are 
  acting 
  with 
  comparative 
  feebleness, 
  and 
  the 
  

   result 
  is 
  some 
  decomposition, 
  but 
  little 
  recomposition. 
  

  

  One 
  side 
  of 
  the 
  slide 
  is 
  diorite, 
  which 
  " 
  shows 
  a 
  less 
  degree 
  of 
  

   change 
  in 
  structure 
  and 
  composition 
  than 
  any 
  of 
  the 
  foregoing 
  

   (325-327). 
  The 
  twinning 
  of 
  the 
  plagioclase 
  is 
  easily 
  recognizable, 
  

   and 
  the 
  hornblende 
  is 
  tolerably 
  fresh 
  and 
  unaltered." 
  " 
  Quartz 
  

   has 
  been 
  developed 
  in 
  considerable 
  quantities. 
  It 
  is 
  apparently 
  

   secondary 
  in 
  origin. 
  Feldspar 
  is 
  partly 
  changed 
  to 
  epidote." 
  " 
  At 
  

   the 
  junction 
  of 
  this 
  band 
  with 
  the 
  granite 
  -veins, 
  the 
  diorite 
  is 
  

   sheared, 
  and 
  at 
  the 
  same 
  time 
  much 
  decomposed. 
  Hornblende 
  is 
  

   replaced 
  by 
  chlorite. 
  Larger 
  crystals 
  of 
  epidote 
  are 
  developed 
  at 
  

   the 
  junction 
  with 
  columnar 
  form. 
  In 
  addition 
  to 
  the 
  above-men- 
  

  

  