﻿CRYSTALLINE 
  ROCKS 
  OF 
  THE 
  MALVERN 
  HILLS. 
  485 
  

  

  dichroism. 
  The 
  place 
  of 
  the 
  felspar 
  is 
  mainly 
  taken 
  by 
  qnartz 
  and 
  

   white 
  mica, 
  the 
  latter 
  of 
  which 
  is 
  much 
  more 
  abundant 
  than 
  the 
  

   biotite. 
  Owing 
  to 
  the 
  progressive 
  development 
  of 
  these 
  micas 
  towards 
  

   one 
  side 
  of 
  the 
  slide, 
  the 
  modified 
  diorite 
  passes 
  into 
  a 
  mica-gneiss. 
  

  

  No. 
  313 
  (PL 
  XVI. 
  fig. 
  1) 
  was 
  taken 
  from 
  the 
  edge 
  of 
  a 
  rather 
  large 
  

   granite-vein, 
  12 
  inches 
  from 
  the 
  plexus, 
  to 
  show 
  the 
  effect 
  of 
  the 
  

   granite 
  upon 
  the 
  diorite, 
  even 
  where 
  the 
  compression 
  is 
  not 
  very 
  

   great. 
  The 
  slide 
  includes 
  part 
  of 
  the 
  vein, 
  with 
  a 
  sheath 
  of 
  what 
  

   was 
  once 
  diorite, 
  but 
  is 
  now 
  a 
  sort 
  of 
  kersantite, 
  with 
  a 
  semi- 
  

   gneissic 
  structure 
  due 
  to 
  the 
  laminae 
  of 
  the 
  biotite 
  and 
  the 
  longer 
  

   axes 
  of 
  the 
  felspars 
  being 
  orientated 
  parallel 
  to 
  the 
  margin 
  of 
  the 
  

   vein. 
  The 
  felspar 
  crystals 
  are 
  like 
  those 
  of 
  the 
  adjoining 
  diorite 
  

   (Nos. 
  276, 
  312). 
  Most 
  of 
  them 
  are 
  very 
  cloudy, 
  but 
  some 
  are 
  

   clearly 
  twinned 
  like 
  a 
  plagioelase. 
  The 
  biotite 
  forms 
  a 
  sheath 
  round 
  

   the 
  felspars, 
  and 
  sometimes 
  thickens 
  out 
  into 
  crystals 
  as 
  broad 
  as 
  

   long. 
  Dichroism 
  is 
  strong, 
  the 
  mica 
  being 
  nearly 
  black 
  when 
  the 
  

   striae 
  are 
  parallel 
  to 
  the 
  principal 
  section 
  of 
  the 
  nicol, 
  and 
  clear 
  

   brown 
  when 
  at 
  right 
  angles 
  to 
  it. 
  A 
  little 
  chlorite 
  is 
  banded 
  with 
  it. 
  

   This 
  mica 
  is 
  massed 
  together 
  in 
  linear 
  bundles 
  of 
  larger 
  crystals 
  at 
  

   the 
  contact 
  with 
  the 
  vein. 
  The 
  felspar 
  of 
  the 
  vein 
  is 
  in 
  the 
  usual 
  

   large 
  crystals, 
  which 
  are 
  cloudy 
  and 
  with 
  ill-defined 
  outlines. 
  Some 
  

   plagioelase 
  is 
  present, 
  penetrating 
  the 
  orthoclase 
  from 
  the 
  outside 
  

   irregularly. 
  The 
  quartz 
  sometimes 
  appears 
  running 
  into 
  the 
  felspar 
  

   in 
  little 
  streams 
  which 
  narrow 
  inwards 
  like 
  estuaries. 
  

  

  No. 
  319 
  is 
  from 
  the 
  edge 
  of 
  the 
  plexus, 
  and 
  includes 
  part 
  of 
  a 
  

   granite-vein. 
  The 
  characters 
  are 
  similar 
  to 
  those 
  of 
  the 
  last, 
  with 
  the 
  

   effects 
  of 
  pressure 
  added. 
  The 
  kersantite 
  contains 
  a 
  little 
  white 
  mica 
  

   and, 
  as 
  often 
  happens 
  when 
  this 
  is 
  the 
  case, 
  some 
  chlorite. 
  The 
  

   same 
  kind 
  of 
  kersantite 
  as 
  in 
  No. 
  313 
  appears 
  on 
  one 
  side 
  of 
  the 
  

   vein 
  ; 
  but 
  on 
  the 
  other 
  the 
  biotite 
  runs 
  together 
  into 
  irregular 
  folia. 
  

   Some 
  of 
  the 
  felspar 
  of 
  the 
  kersantite 
  is 
  replaced 
  by 
  a 
  water-clear 
  

   mineral 
  which 
  polarizes 
  like 
  quartz, 
  and, 
  in 
  convergent 
  light, 
  shows 
  

   a 
  dark 
  bar 
  which 
  remains 
  straight 
  during 
  rotation 
  of 
  the 
  stage, 
  and 
  

   the 
  sheath 
  of 
  mica 
  often 
  retains 
  its 
  place 
  round 
  these 
  replacements 
  

   of 
  quartz. 
  The 
  felspar-crystals 
  which 
  are 
  only 
  partially 
  replaced 
  

   have 
  curvilinear 
  margins. 
  In 
  other 
  parts 
  of 
  the 
  slide 
  there 
  is 
  a 
  

   tendency 
  to 
  aggregation 
  ; 
  biotite, 
  quartz, 
  or 
  felspar 
  occurring 
  in 
  

   patches 
  of 
  a 
  roughly 
  oblong 
  shape. 
  Where 
  the 
  quartz 
  predominates, 
  

   the 
  structure 
  is 
  that 
  of 
  quartzite, 
  a 
  few 
  mica-flakes, 
  with 
  orientation 
  

   parallel 
  to 
  the 
  rest 
  of 
  the 
  mica 
  in 
  the 
  slide, 
  or 
  eroded 
  felspar-crystals, 
  

   being 
  scattered 
  through 
  the 
  ground-mass 
  of 
  irregular 
  quartz- 
  

   granules. 
  But 
  the 
  gradation 
  between 
  the 
  kersantite 
  and 
  the 
  gneis- 
  

   soid 
  quartzite 
  is 
  complete. 
  The 
  changes 
  of 
  the 
  kersantite 
  in 
  this 
  

   slide, 
  as 
  in 
  many 
  others, 
  are 
  towards 
  either 
  biotite-gneiss 
  or 
  gneie- 
  

   soid 
  quartzite. 
  In 
  the 
  former 
  case 
  the 
  mica 
  is 
  aggregated 
  into 
  

   more 
  or 
  less 
  regular 
  folia, 
  and 
  the 
  felspar 
  is 
  partly 
  replaced 
  by 
  

   granular 
  quartz 
  ; 
  in 
  the 
  latter 
  the 
  change 
  into 
  quartz 
  is 
  carried 
  

   further, 
  and 
  the 
  mica 
  is 
  almost 
  wanting. 
  There 
  is 
  nothing 
  special 
  

   about 
  the 
  granite-vein, 
  which 
  is 
  irregularly 
  outlined 
  by 
  black 
  mica. 
  

   The 
  orthoclase 
  is 
  penetrated 
  on 
  one 
  side 
  by 
  plagioelase. 
  

  

  