﻿CRYSTALLINE 
  ROCKS 
  OF 
  THE 
  MALVERN 
  HILLS. 
  481 
  

  

  of 
  it 
  is 
  in 
  irregular 
  colourless 
  granules, 
  showing 
  high 
  refraction 
  and 
  

   brilliant 
  polarization. 
  Some 
  of 
  this 
  mineral 
  is 
  in 
  larger 
  masses, 
  in 
  

   which 
  can 
  be 
  detected 
  prismatic 
  forms 
  with 
  longitudinal 
  cleavage 
  

   and 
  transverse 
  jointing. 
  Sphene 
  is 
  also 
  present 
  in 
  characteristic 
  

   wedge-like 
  crystals, 
  with 
  very 
  high 
  refraction 
  and 
  a 
  dingy 
  rough- 
  

   ened 
  surface. 
  Much 
  of 
  the 
  epidote 
  seems 
  to 
  have 
  come 
  from 
  the 
  

   hornblende. 
  Chlorite 
  appears 
  in 
  the 
  earlier 
  slides, 
  replacing 
  horn- 
  

   blende. 
  Sometimes 
  it 
  encloses 
  epidote. 
  In 
  the 
  later 
  slides 
  there 
  is 
  

   a 
  little 
  chlorite 
  banded 
  with 
  black 
  mica, 
  and 
  a 
  fair 
  proportion 
  of 
  

   black 
  mica 
  occurs 
  in 
  irregular 
  strings 
  running 
  with 
  the 
  foliation. 
  

  

  Quartz 
  is 
  not 
  in 
  quantity 
  in 
  the 
  first 
  four 
  slides. 
  It 
  appears 
  in 
  

   granules 
  invading 
  the 
  felspar. 
  In 
  the 
  last 
  slide 
  it 
  is 
  very 
  abundant, 
  

   lying 
  in 
  well-marked 
  folia 
  alternating 
  with 
  the 
  other 
  minerals. 
  

  

  A 
  careful 
  study 
  of 
  this 
  series 
  shows 
  that, 
  especially 
  in 
  the 
  later 
  

   slides, 
  most 
  of 
  the 
  minerals 
  are 
  of 
  secondary 
  origin. 
  We 
  may 
  

   assume 
  that 
  the 
  breaking-up 
  of 
  the 
  hornblendes 
  into 
  cleavage-prisms 
  

   lying 
  scattered 
  in 
  elongated 
  patches 
  is 
  a 
  secondary 
  mechanical 
  

   effect. 
  Also 
  it 
  will 
  hardly 
  be 
  denied 
  that 
  the 
  chlorite 
  and 
  the 
  

   epidote 
  are 
  the 
  result 
  of 
  decomposition 
  ; 
  they 
  appear 
  replacing 
  

   hornblende 
  in 
  the 
  clearest 
  manner. 
  Thus 
  it 
  is 
  common 
  for 
  a 
  part 
  

   of 
  an 
  idiomorphic 
  hornblende-crystal 
  to 
  remain 
  intact 
  while 
  the 
  

   place 
  of 
  the 
  other 
  portion 
  is 
  taken 
  by 
  chlorite 
  enclosing 
  epidote. 
  

  

  The 
  black 
  mica 
  has 
  apparently 
  been 
  formed 
  from 
  the 
  chlorite. 
  

   It 
  appears, 
  in 
  the 
  more 
  schistose 
  slides, 
  banding 
  chlorite 
  and 
  some- 
  

   times 
  taking 
  its 
  place. 
  Patches 
  of 
  the 
  mica-crystals 
  surround 
  little 
  

   nests 
  of 
  epidote, 
  and 
  the 
  mica 
  is 
  moulded 
  to 
  the 
  angles 
  of 
  the 
  epi- 
  

   dote, 
  just 
  as 
  in 
  the 
  case 
  of 
  the 
  chlorite. 
  This 
  is 
  well 
  seen 
  in 
  one 
  of 
  

   the 
  later 
  slides 
  of 
  the 
  series 
  (PL 
  XVI. 
  fig. 
  6), 
  in 
  which 
  the 
  hornblende 
  

   has 
  greatly 
  decreased 
  and 
  the 
  field 
  is 
  largely 
  occupied 
  by 
  epidote. 
  

   If 
  the 
  epidote 
  is 
  secondary, 
  so 
  is 
  the 
  biotite 
  *. 
  This 
  mica 
  is 
  also 
  

   moulded 
  on 
  fragmental 
  hornblende 
  in 
  the 
  same 
  slide. 
  Further 
  

   evidence 
  of 
  the 
  conversion 
  of 
  chlorite 
  into 
  black 
  mica 
  will 
  be 
  given 
  

   later 
  on. 
  

  

  The 
  secondary 
  origin 
  of 
  the 
  white 
  mica 
  is 
  very 
  clear. 
  It 
  is 
  

   moulded 
  upon 
  the 
  jagged 
  edges 
  of 
  bent 
  and 
  crushed 
  hornblende- 
  

   crystals, 
  and 
  where 
  the 
  crystals 
  have 
  opened 
  along 
  cleavage- 
  cracks 
  

   mica 
  fills 
  in 
  the 
  opening 
  in 
  the 
  form 
  of 
  wedges. 
  It 
  frequently 
  

   encloses 
  epidote, 
  and 
  in 
  one 
  place 
  it 
  is 
  interlaminated 
  with 
  chlorite. 
  

  

  Most 
  of 
  the 
  felspar 
  also 
  appears 
  to 
  be 
  secondary. 
  Plagioclase, 
  

   clearly 
  poly 
  synthetic, 
  and 
  unstriated 
  felspar 
  are 
  seen 
  moulded 
  upon 
  

   the 
  projecting 
  angles 
  of 
  broken 
  hornblende-crystals, 
  and 
  enclosing 
  

   the 
  minute 
  prismatic 
  fragments 
  into 
  which 
  the 
  mineral 
  breaks 
  up. 
  

   Felspar 
  is 
  also 
  moulded 
  upon 
  the 
  angles 
  of 
  white 
  mica. 
  This 
  felspar 
  

   is 
  clearer 
  and 
  sometimes 
  in 
  much 
  smaller 
  crystals 
  than 
  that 
  of 
  the 
  

   original 
  diorite, 
  in 
  which 
  the 
  felspar 
  of 
  the 
  soundest 
  rock 
  is 
  usually 
  

   somewhat 
  cloudy. 
  

  

  The 
  secondary 
  origin 
  of 
  the 
  quartz 
  I 
  need 
  not 
  dwell 
  upon. 
  

  

  * 
  Prof. 
  Lossen 
  recognizes 
  that 
  "in 
  granite 
  and 
  gabbro 
  contact-zones 
  " 
  biotite 
  

   is 
  developed 
  from 
  chlorite 
  derived 
  from 
  augite 
  or 
  hornblende 
  (' 
  Nature,' 
  Sept. 
  

   27th, 
  1888, 
  p. 
  528). 
  The 
  production 
  of 
  biotite 
  as 
  a 
  contact-effect 
  in 
  sedimentary 
  

   rocks 
  and 
  in 
  some 
  greenstones 
  is, 
  of 
  course, 
  well 
  known. 
  

  

  Q. 
  J. 
  G. 
  S. 
  No. 
  179. 
  2 
  k 
  

  

  