﻿

  CRYSTALLINE 
  ROCKS 
  OF 
  THE 
  MALVERN 
  HILLS. 
  487 
  

  

  The 
  two 
  shear-zones 
  of 
  which 
  I 
  have 
  here 
  given 
  an 
  account 
  may 
  be 
  

   taken 
  as 
  types. 
  In 
  the 
  former 
  the 
  production 
  of 
  white 
  mica 
  in 
  the 
  

   soda-lime 
  felspar 
  is 
  perhaps 
  the 
  most 
  prominent 
  character 
  ; 
  in 
  the 
  

   latter 
  the 
  conversion 
  of 
  hornblende 
  into 
  biotite 
  is 
  the 
  most 
  striking 
  

   feature. 
  The 
  reconstruction 
  of 
  plagioclase 
  and 
  the 
  generation 
  of 
  

   granular 
  quartz 
  is 
  seen 
  in 
  both. 
  These 
  veined 
  complexes 
  give 
  us 
  

   the 
  " 
  injection-schists 
  " 
  of 
  my 
  former 
  paper 
  (p. 
  532). 
  

  

  I 
  am 
  not 
  prepared 
  to 
  explain 
  why 
  the 
  schistosity 
  of 
  the 
  Malvern 
  

   region 
  is 
  zonal 
  ; 
  but 
  I 
  think 
  the 
  frequency 
  with 
  which 
  a 
  shear-zone 
  

   coincides 
  with 
  a 
  veined 
  complex 
  can 
  hardly 
  be 
  accident. 
  Though 
  

   there 
  is 
  shearing 
  in 
  granite 
  only, 
  and 
  in 
  diorite 
  only, 
  it 
  is 
  more 
  

   common 
  where 
  the 
  diorite 
  is 
  interlaced 
  with 
  veins, 
  especially 
  small 
  

   veins. 
  Shearing 
  would, 
  of 
  course, 
  occur 
  at 
  weak 
  places 
  in 
  the 
  crust, 
  

   and 
  it 
  would 
  seem 
  that 
  a 
  veined 
  complex 
  should 
  more 
  readily 
  yield 
  

   to 
  pressure 
  than 
  a 
  homogeneous 
  mass 
  ; 
  or 
  both 
  veining 
  and 
  shearing 
  

   may 
  be 
  results 
  of 
  an 
  original 
  weakness. 
  

  

  Parallelism 
  in 
  the 
  arrangement 
  of 
  minerals 
  without 
  regional 
  

   pressure 
  has 
  been 
  noticed 
  by 
  previous 
  writers. 
  Thus, 
  in 
  sedimentary 
  

   rocks 
  altered 
  at 
  their 
  contact 
  with 
  granite-veins 
  there 
  is 
  sometimes 
  

   a 
  production 
  of 
  minerals 
  in 
  lines 
  parallel 
  to 
  the 
  margins 
  of 
  the 
  veins. 
  

   We 
  have 
  seen 
  this 
  principle 
  illustrated 
  in 
  Nos. 
  274 
  and 
  276, 
  and 
  

   similar 
  examples 
  frequently 
  occur 
  at 
  Malvern. 
  But 
  this 
  cause 
  is 
  

   only 
  of 
  local 
  effect, 
  and 
  it 
  cannot 
  account 
  for 
  the 
  general 
  schistosity 
  

   of 
  the 
  Malvern 
  rocks. 
  

  

  The 
  phenomena 
  here 
  described 
  from 
  two 
  localities 
  are 
  common 
  

   throughout 
  the 
  Malvern 
  Hills. 
  The 
  most 
  obvious 
  fact 
  observable 
  in 
  

   the 
  field 
  is 
  the 
  production 
  of 
  the 
  biotite. 
  Granite-veins, 
  if 
  of 
  large 
  

   size, 
  that 
  is, 
  6 
  inches 
  or 
  more 
  in 
  diameter, 
  are 
  often 
  surrounded 
  by 
  

   a 
  sheath 
  of 
  kersantite 
  two 
  or 
  more 
  inches 
  in 
  thickness. 
  Indeed, 
  so 
  

   uniform 
  is 
  the 
  effect 
  of 
  the 
  contact 
  that 
  the 
  vicinity 
  of 
  veins 
  may 
  

   usually 
  be 
  safely 
  inferred 
  from 
  the 
  appearance 
  of 
  biotite 
  in 
  the 
  

   diorite. 
  Thin 
  granite-veins, 
  unless 
  very 
  numerous 
  and 
  close 
  to- 
  

   gether, 
  or 
  accompanied 
  by 
  shearing, 
  do 
  not 
  always 
  produce 
  biotite 
  ; 
  

   they 
  may 
  give 
  rise 
  merely 
  to 
  decomposition 
  without 
  subsequent 
  

   synthesis. 
  If 
  the 
  veins 
  are 
  abundant 
  or 
  large, 
  the 
  chlorite 
  nearest 
  

   the 
  granite 
  is 
  banded 
  with 
  black 
  mica, 
  or 
  is 
  entirely 
  replaced 
  by 
  

   it. 
  I 
  am 
  not 
  satisfied 
  that 
  hornblende 
  in 
  this 
  district 
  is 
  ever 
  con- 
  

   verted 
  into 
  biotite 
  directly. 
  

  

  Shearing 
  has 
  greatly 
  intensified 
  the 
  metamorphic 
  effects 
  of 
  the 
  

   granitic 
  intrusions. 
  The 
  production 
  of 
  biotite 
  at 
  shear-planes 
  may 
  

   only 
  in 
  part 
  result 
  directly 
  from 
  mechanical 
  energy. 
  The 
  shear- 
  

   planes 
  would 
  certainly 
  permit 
  the 
  infiltration 
  of 
  mineral 
  solutions, 
  

   and 
  these 
  might 
  of 
  course 
  give 
  rise 
  to 
  chemical 
  reactions. 
  But 
  some 
  

   direct 
  proof 
  of 
  the 
  infiltration 
  of 
  mineral 
  matter 
  will 
  be 
  given 
  further 
  

   on 
  (p. 
  496). 
  

  

  2. 
  Further 
  Mineral 
  Changes 
  : 
  White 
  Mica 
  from 
  Biotite 
  and 
  from 
  

   Chlorite 
  ; 
  Garnets 
  and 
  Zoisite 
  ; 
  S±)hene 
  ; 
  Actinolite 
  ; 
  H&matite 
  ; 
  

   Calcite. 
  

  

  White 
  Mica 
  from 
  Biotite. 
  — 
  This 
  change 
  has 
  been 
  noticed 
  by 
  writers 
  

  

  