﻿CRYSTALLINE 
  ROCKS 
  OF 
  THE 
  MALVERN 
  HILLS. 
  479 
  

  

  ness 
  from 
  several 
  feet 
  to 
  two 
  or 
  three 
  lines. 
  Some 
  of 
  them 
  have 
  

   siibparallel 
  margins, 
  others 
  form 
  slender 
  tongues 
  ending 
  upwards 
  

   in 
  a 
  point, 
  while 
  one 
  is 
  narrow 
  below 
  but 
  swells 
  out 
  upwards 
  into 
  

   a 
  club-shaped 
  mass. 
  Where 
  the 
  veins 
  attain 
  a 
  thickness 
  of 
  a 
  few 
  

   inches 
  they 
  are 
  encased 
  in 
  a 
  sheath 
  of 
  kersantite 
  (diorite 
  modified 
  

   by 
  the 
  contact), 
  the 
  mica 
  being 
  most 
  abundant 
  at 
  the 
  contact 
  of 
  the 
  

   kersantite 
  with 
  the 
  granite. 
  

  

  At 
  the 
  northern 
  end 
  of 
  the 
  section 
  the 
  veined 
  rock 
  passes 
  rapidly 
  

   into 
  a 
  banded 
  gneiss, 
  with 
  a 
  dip 
  to 
  the 
  north 
  of 
  70°. 
  This 
  sheared 
  

   portion 
  is 
  about 
  a 
  yard 
  wide, 
  and 
  probably 
  extends 
  further 
  under 
  

   the 
  turf. 
  The 
  ground-mass 
  (modified 
  diorite) 
  is 
  blackish-green 
  in 
  

   colour, 
  and 
  is 
  striped 
  with 
  several 
  greyish 
  bands 
  (modified 
  granite 
  ) 
  

   varying 
  in 
  thickness 
  from 
  four 
  inches 
  to 
  a 
  line 
  or 
  two. 
  The 
  diorite 
  

   and 
  granite 
  become 
  progressively 
  crushed 
  towards 
  the 
  shear-zone 
  ; 
  

   but 
  where 
  the 
  oblique 
  dip 
  sets 
  in, 
  the 
  parallelism 
  of 
  the 
  constituents, 
  

   whether 
  rock 
  or 
  mineral, 
  grows 
  very 
  marked, 
  and 
  there 
  is 
  much 
  

   mineral 
  replacement. 
  The 
  following 
  are 
  the 
  chief 
  changes 
  to 
  be 
  

   noted 
  : 
  — 
  

  

  The 
  granite 
  (453 
  *) 
  in 
  the 
  angle 
  between 
  the 
  last 
  vertical 
  vein 
  

   (v 
  r 
  , 
  fig. 
  I) 
  and 
  the 
  first 
  clear 
  dip 
  of 
  gneiss 
  already 
  begins 
  to 
  display 
  

   a 
  parallel 
  structure. 
  The 
  felspar 
  is 
  largely 
  microcline, 
  showing 
  very 
  

   clearly 
  the 
  cross-hatched 
  structure. 
  The 
  longer 
  axes 
  of 
  the 
  larger 
  

   crystals 
  and 
  one 
  system 
  of 
  lamellae 
  lie 
  in 
  the 
  direction 
  of 
  the 
  rough 
  

   foliation. 
  The 
  felspars 
  are 
  much 
  corroded 
  at 
  the 
  margins 
  and 
  in 
  the 
  

   interior 
  by 
  a 
  water-clear 
  mineral, 
  probably 
  quartz, 
  so 
  that 
  isomor- 
  

   phic 
  forms 
  are 
  rare. 
  There 
  are 
  also 
  in 
  the 
  slide 
  many 
  small 
  crystal- 
  

   line 
  granules 
  of 
  microcline 
  and 
  of 
  unstriated 
  felspar, 
  forming, 
  with 
  

   granular 
  t 
  quartz, 
  thin 
  irregular 
  folia, 
  such 
  as 
  are 
  usual 
  in 
  a 
  gneiss 
  

   formed 
  from 
  granite. 
  Quartz 
  also 
  occurs 
  as 
  long 
  fingers 
  running 
  

   into 
  and 
  between 
  felspar 
  crystals, 
  and, 
  being 
  parallel 
  to 
  the 
  elonga- 
  

   tion 
  of 
  the 
  latter, 
  they 
  accentuate 
  the 
  schistosity. 
  

  

  Another 
  specimen 
  of 
  granite 
  (405), 
  taken 
  just 
  at 
  the 
  margin 
  of 
  

   the 
  thoroughly 
  schistose 
  zone, 
  has 
  a 
  more 
  distinctly 
  foliated 
  aspect. 
  

   The 
  felspar, 
  much 
  of 
  which 
  is 
  microcline, 
  is 
  in 
  small 
  granules. 
  The 
  

   granules 
  of 
  quartz 
  are 
  much 
  larger 
  and 
  are 
  elongated 
  with 
  the 
  

   foliation. 
  White 
  mica, 
  showing 
  characteristic 
  cleavage, 
  parallel 
  

   extinction, 
  and 
  vivid 
  colours 
  at 
  oblique 
  angles, 
  occurs 
  in 
  crystals 
  

   with 
  sections 
  (normal 
  to 
  the 
  basal 
  plane) 
  whose 
  four 
  sides 
  are 
  about 
  

   equal. 
  Some 
  of 
  it 
  is 
  moulded 
  on 
  rounded 
  granules 
  of 
  quartz, 
  but 
  

   sometimes 
  quartz 
  is 
  moulded 
  on 
  angles 
  of 
  the 
  mica. 
  

  

  A 
  few 
  inches 
  within 
  the 
  zone, 
  and 
  near 
  where 
  the 
  oblique 
  dip 
  

   abuts 
  on 
  the 
  vertical 
  veining, 
  a 
  band 
  of 
  the 
  granite 
  is 
  sheared 
  into 
  

   a 
  gneiss 
  with 
  thin 
  regular 
  folia, 
  which 
  are 
  strangely 
  contorted 
  into 
  

   almost 
  circular 
  folds 
  an 
  inch 
  or 
  two 
  in 
  diameter. 
  This 
  contortion 
  

   appears 
  to 
  be 
  due 
  to 
  the 
  friction 
  of 
  the 
  rock, 
  during 
  shearing, 
  against 
  

   the 
  less 
  yielding 
  mass 
  in 
  contact 
  with 
  it. 
  Under 
  the 
  microscope 
  (408) 
  

  

  ~* 
  The 
  numbers 
  in 
  brackets 
  refer 
  to 
  the 
  slides 
  in 
  my 
  cabinet, 
  

   t 
  In 
  view 
  of 
  the 
  various 
  senses 
  in 
  which 
  the 
  term 
  " 
  granulitic" 
  has 
  been 
  used, 
  

   I 
  have 
  throughout 
  this 
  paper 
  refrained 
  from 
  employing 
  it. 
  

  

  