﻿CRYSTALLINE 
  ROCKS 
  OE 
  THE 
  MALVERN 
  HILLS. 
  477 
  

  

  The 
  differential 
  nature 
  of 
  the 
  movement 
  is 
  seen 
  by 
  taking 
  a 
  

   series 
  of 
  specimens 
  along 
  a 
  straight 
  line 
  at 
  distances 
  of 
  two 
  or 
  three 
  

   incties. 
  "When 
  we 
  examine 
  the 
  slides 
  in 
  succession 
  we 
  can 
  trace 
  

   the 
  stages 
  through 
  which 
  a 
  clear 
  idiomorphic 
  crystal 
  of 
  hornblende 
  

   passes 
  into 
  an 
  irregular 
  opaque 
  string 
  of 
  three 
  or 
  four 
  times 
  the 
  

   original 
  length. 
  A 
  similar 
  change 
  may 
  often 
  be 
  seen 
  even 
  in 
  the 
  

   same 
  slide. 
  One 
  side 
  may 
  be 
  diorite, 
  scarcely 
  modified 
  ; 
  the 
  other, 
  

   a 
  schist. 
  

  

  The 
  planes 
  of 
  movement 
  (shear-planes) 
  can 
  frequently 
  be 
  observed. 
  

   They 
  sometimes 
  appear 
  in 
  the 
  slides 
  as 
  lines 
  of 
  actual 
  separation 
  

   sharply 
  indicated 
  by 
  decomposition-products, 
  such 
  as 
  chlorite 
  or 
  

   ferrite. 
  At 
  these 
  lines 
  the 
  minerals 
  have 
  suffered 
  their 
  maximum 
  

   of 
  distortion 
  and 
  fracture. 
  In 
  a 
  further 
  stage 
  of 
  metamorphism, 
  

   when 
  decomposition 
  has 
  begun 
  to 
  give 
  place 
  to 
  reconstruction, 
  the 
  

   slide 
  is 
  clearer, 
  and 
  frequently, 
  in 
  the 
  case 
  of 
  diorite, 
  black 
  mica 
  

   is 
  formed 
  at 
  the 
  shear-planes, 
  so 
  that 
  the 
  rock 
  splits 
  into 
  thin 
  

   leaves 
  whose 
  surfaces 
  glisten 
  with 
  mica, 
  while 
  the 
  interior 
  of 
  the 
  

   laminae 
  may 
  be 
  dioritic. 
  In 
  a 
  thoroughly 
  reconstructed 
  rock 
  it 
  is 
  

   not 
  always 
  possible 
  to 
  detect 
  the 
  shear-planes. 
  In 
  some 
  schists 
  

   they 
  are 
  marked 
  by 
  lines 
  of 
  mica, 
  and 
  this 
  is 
  perhaps 
  the 
  most 
  

   general 
  indication. 
  

  

  E. 
  Schistosity 
  in 
  Diorite. 
  

  

  Except 
  in 
  the 
  contiguity 
  of 
  granite-veins, 
  I 
  have 
  not 
  observed 
  

   that 
  very 
  material 
  changes 
  occur 
  in 
  diorite 
  under 
  pressure. 
  Paral- 
  

   lelism 
  of 
  structure, 
  the 
  production 
  of 
  shear-planes, 
  the 
  formation 
  of 
  

   decomposition-products, 
  the 
  distortion 
  and 
  fracture 
  of 
  minerals 
  are 
  

   the 
  ordinarv 
  effects 
  noticed 
  in 
  the 
  Malvern 
  district. 
  

  

  C. 
  Schistosity 
  in 
  Granite. 
  

  

  On 
  the 
  margin 
  of 
  a 
  zone 
  the 
  granite 
  is 
  crushed 
  into 
  a 
  mass 
  of 
  

   subparallel 
  wedges, 
  which 
  become 
  progressively 
  thinner 
  and 
  longer. 
  

   In 
  the 
  bands 
  of 
  maximum 
  shearing 
  the 
  cracks 
  are 
  healed 
  up, 
  and 
  

   the 
  ordinary 
  gneissic 
  structure 
  appears. 
  Much 
  of 
  the 
  felspar 
  is 
  

   reconstructed 
  in 
  smaller 
  crystals 
  or 
  in 
  granules, 
  and 
  secondary 
  

   quartz 
  is 
  produced. 
  Yery 
  frequently 
  white 
  mica 
  is 
  also 
  formed. 
  

   This 
  change 
  is, 
  of 
  course, 
  well 
  known, 
  and 
  I 
  have 
  described 
  it 
  from 
  

   a 
  Malvern 
  locality 
  in 
  my 
  former 
  paper 
  (p. 
  528). 
  

  

  D. 
  Schistosity 
  in 
  Felsite. 
  

  

  The 
  felsite, 
  already 
  devitrified, 
  progressively 
  acquires 
  a 
  parallel 
  

   structure, 
  which 
  becomes 
  more 
  and 
  more 
  marked, 
  the 
  individual 
  

   granules 
  breaking 
  up 
  into 
  quartz 
  and 
  mica 
  till 
  a 
  mica-schist 
  is 
  

   formed 
  (ibid. 
  p. 
  530). 
  Yariations 
  occur 
  in 
  the 
  schist, 
  due 
  appa- 
  

   rently 
  to 
  the 
  segregation 
  of 
  the 
  acidic 
  and 
  basic 
  elements 
  respec- 
  

   tively, 
  so 
  that 
  while 
  some 
  seams 
  are 
  a 
  quartz-schist, 
  in 
  others 
  the 
  

   mica 
  predominates 
  over 
  the 
  quartz. 
  

  

  