﻿CRYSTALLINE 
  ROCKS 
  OF 
  THE 
  MALVERN 
  HILLS. 
  499 
  

  

  years 
  ago 
  that 
  the 
  potash 
  in 
  the 
  orthoclase 
  of 
  the 
  Malvern 
  granite 
  

   had 
  been 
  partially 
  replaced 
  by 
  soda. 
  The 
  following 
  analyses 
  by 
  

   Mr. 
  J. 
  H. 
  Player 
  *, 
  F.G.S., 
  tend 
  to 
  confirm 
  this 
  statement. 
  The 
  

   alkalies 
  only 
  are 
  here 
  given 
  : 
  — 
  

  

  Soda. 
  Potash. 
  

  

  A. 
  " 
  Quartz-felspar, 
  Malvern, 
  North 
  Hill 
  " 
  37 
  4*9 
  

  

  B. 
  '' 
  Quartz-felspar, 
  Malvern, 
  coarse 
  graphic 
  texture"... 
  1"9 
  7'1 
  

   0. 
  " 
  Quartz-felspar, 
  Malvern, 
  red 
  rock, 
  fine-grained 
  "... 
  2 
  - 
  6 
  4'0 
  

  

  This 
  " 
  quartz-felspar 
  " 
  is 
  undoubtedly 
  the 
  ordinary 
  granite 
  of 
  the 
  

   district. 
  The 
  proportion 
  of 
  soda 
  in 
  (B) 
  is 
  about 
  normal, 
  but 
  in 
  the 
  

   other 
  two 
  examples 
  there 
  has 
  been 
  a 
  gain 
  of 
  soda 
  and 
  a 
  loss 
  of 
  

   potash. 
  

  

  We 
  have 
  seen 
  that 
  in 
  the 
  vicinity 
  of 
  granite-veins, 
  one 
  of 
  two 
  

   changes 
  is 
  ordinarily 
  seen 
  in 
  diorite, 
  either 
  chlorite 
  is 
  converted 
  

   into 
  black 
  mica, 
  or 
  white 
  mica 
  appears 
  in 
  the 
  place 
  of 
  soda-lime 
  

   felspar 
  f. 
  Potash 
  must 
  therefore 
  have 
  been, 
  taken 
  up. 
  Two 
  

   analyses 
  of 
  diorite, 
  kindly 
  furnished 
  me 
  in 
  MS. 
  by 
  Mr. 
  Player, 
  con- 
  

   firm 
  the 
  evidence 
  of 
  the 
  microscope. 
  I 
  am 
  able 
  to 
  identify 
  the 
  

   varieties 
  from 
  slides 
  sent 
  by 
  Mr. 
  Player. 
  D 
  is 
  a 
  typical 
  coarse 
  

   grey 
  diorite 
  with 
  plenty 
  of 
  hornblende. 
  E 
  is 
  the 
  medium 
  black 
  

   variety, 
  and 
  has 
  undergone 
  partial 
  metamorphism, 
  a 
  narrow 
  seam 
  

   of 
  biotite-gneiss 
  running 
  across 
  the 
  middle 
  of 
  the 
  slide. 
  

  

  Soda. 
  Potash. 
  

  

  D. 
  " 
  Diorite, 
  Malvern, 
  North 
  Hill 
  " 
  23 
  43 
  

  

  E. 
  " 
  Basic 
  rock, 
  Malvern 
  " 
  22 
  39 
  

  

  The 
  chemical 
  processes 
  described 
  in 
  this 
  paper 
  are 
  in 
  harmony 
  

   with 
  what 
  is 
  known 
  of 
  mineral 
  changes. 
  On 
  the 
  whole 
  they 
  illus- 
  

   trate 
  a 
  transition 
  from 
  the 
  unstable 
  to 
  the 
  stable. 
  Stability 
  is, 
  of 
  

   course, 
  relative 
  to 
  environment, 
  so 
  that 
  biotite 
  in 
  the 
  presence 
  of 
  

   solutions 
  of 
  potash 
  may 
  be 
  as 
  stable 
  as 
  chlorite 
  in 
  their 
  absence. 
  

   The 
  unstable 
  minerals, 
  hornblende, 
  augite, 
  potash-felspar, 
  and 
  soda- 
  

   lime 
  felspars, 
  forming 
  the 
  bulk 
  of 
  the 
  igneous 
  rocks, 
  are, 
  in 
  the 
  

   schists, 
  replaced 
  by 
  quartz 
  and 
  white 
  mica, 
  black 
  mica, 
  epidote, 
  

   chlorite, 
  garnets, 
  actinolite, 
  iron-oxides, 
  calcite, 
  and 
  natrolite 
  J. 
  

   It 
  seems 
  probable 
  that 
  the 
  only 
  original 
  minerals 
  are 
  apatite, 
  horn- 
  

   blende, 
  and 
  augite, 
  with 
  part 
  of 
  the 
  quartz, 
  felspars, 
  and 
  iron-ores. 
  

   The 
  capital 
  facts 
  to 
  be 
  noted 
  are 
  the 
  secondary 
  origin 
  of 
  all 
  the 
  

   micas, 
  the 
  abundant 
  generation 
  of 
  quartz, 
  and 
  the 
  reconstruction 
  of 
  

   felspar. 
  

  

  III. 
  Temperature 
  of 
  Metamorphism. 
  

  

  The 
  facts 
  adduced 
  seem 
  to 
  prove 
  that 
  the 
  metamorphism 
  has 
  to 
  a 
  

   large 
  degree 
  taken 
  place 
  in 
  rocks 
  already 
  consolidated. 
  Whether 
  

   in 
  granite 
  or 
  diorite 
  the 
  foliation 
  is 
  connected 
  with 
  a 
  shearing- 
  

  

  * 
  British 
  Association 
  Report, 
  1886. 
  

  

  t 
  From 
  the 
  frequent 
  appearance 
  of 
  untwinned 
  felspar 
  in 
  diorite 
  near 
  granite- 
  

   veins 
  it 
  would 
  appear 
  probable 
  that 
  plagioclase 
  passes 
  into 
  muscovite 
  through 
  

   the 
  intermediate 
  form 
  of 
  orthoclase 
  ; 
  but 
  further 
  inquiry 
  is 
  needed. 
  

  

  J 
  Butley, 
  Quart. 
  Jo 
  urn. 
  Geol. 
  Soc. 
  vol. 
  xliii. 
  p. 
  501. 
  

  

  2n2 
  

  

  