﻿498 
  DE. 
  C. 
  CALLAWAY 
  ON 
  SECOND 
  AEY 
  3HNERALS 
  IX 
  THE 
  

  

  true 
  veins. 
  The 
  latter 
  in 
  the 
  Malvern 
  district 
  are 
  always 
  different 
  

   in 
  mineral 
  composition 
  from 
  the 
  encasing 
  rock, 
  bnt 
  the 
  flakes, 
  

   allowing 
  for 
  their 
  greater 
  thickness, 
  are 
  mineralogically 
  identical 
  

   with 
  the 
  thinly 
  sheared 
  granite 
  surrounding 
  them. 
  This 
  gneiss 
  

   differs, 
  of 
  course, 
  from 
  the 
  variety 
  produced 
  by 
  the 
  simple 
  shearing 
  

   of 
  granite 
  by 
  the 
  presence 
  of 
  chlorite, 
  biotite, 
  and 
  other 
  minerals 
  

   at 
  the 
  shear- 
  planes. 
  

  

  II. 
  Chemical 
  Considerations. 
  

  

  The 
  conversion 
  of 
  diorite 
  into 
  a 
  highly 
  quartzose 
  or 
  micaceous 
  

   rock 
  involves 
  profound 
  chemical 
  changes. 
  It 
  will 
  be 
  well 
  to 
  trace 
  

   the 
  most 
  important 
  of 
  these. 
  

  

  The 
  removal 
  of 
  bases 
  comes 
  first 
  in 
  order. 
  I 
  have 
  already 
  alluded 
  

   to 
  the 
  silicification, 
  partial 
  or 
  complete, 
  of 
  granite-veins. 
  The 
  

   change 
  is 
  most 
  marked 
  in 
  those 
  of 
  small 
  size. 
  Under 
  the 
  microscope 
  

   the 
  process 
  of 
  alteration 
  becomes 
  apparent. 
  The 
  felspar 
  is 
  seen 
  to 
  

   be 
  invaded 
  by 
  quartz, 
  which 
  runs 
  up 
  into 
  the 
  crystals 
  in 
  streams 
  

   broadening 
  outwards 
  like 
  estuaries. 
  This 
  quartz 
  is 
  in 
  granules, 
  

   polarizing 
  in 
  a 
  mosaic 
  of 
  colours. 
  Sometimes 
  the 
  quartz 
  appears 
  

   dotted 
  here 
  and 
  there 
  in 
  the 
  felspar 
  in 
  single 
  granules 
  or 
  small 
  

   clusters 
  of 
  granules, 
  but 
  these 
  are 
  probably 
  the 
  transverse 
  sections 
  

   of 
  the 
  streams. 
  There 
  is 
  also 
  replacement 
  by 
  quartz 
  in 
  some 
  of 
  

   the 
  felspar 
  of 
  the 
  kersantite, 
  the 
  invasion 
  of 
  the 
  quartz 
  at 
  the 
  

   margin 
  of 
  the 
  felspars 
  giving 
  to 
  the 
  latter 
  a 
  sinuate 
  or 
  dentate 
  

   outline, 
  and 
  the 
  corrosion 
  often 
  goes 
  on 
  till 
  the 
  crystal 
  is 
  destroyed. 
  

   Perforation 
  occurs 
  also 
  in 
  the 
  case 
  of 
  hornblende 
  and 
  black 
  mica, 
  so 
  

   that 
  crystals 
  appear 
  riddled 
  with 
  holes, 
  and 
  in 
  some 
  cases 
  they 
  are 
  

   reduced 
  to 
  shreds. 
  These 
  corroded 
  remnants 
  are 
  usually 
  immersed 
  

   in 
  a 
  translucent 
  mineral, 
  which 
  is 
  frequently 
  quartz 
  *. 
  

  

  A 
  partial 
  account 
  may 
  be 
  given 
  of 
  the 
  bases 
  liberated 
  by 
  the 
  

   separation 
  of 
  silica. 
  The 
  lime 
  of 
  both 
  hornblende 
  and 
  soda-lime 
  

   felspar 
  appears 
  in 
  epidote, 
  that 
  of 
  some 
  of 
  the 
  felspar 
  in 
  zoisite, 
  and 
  

   that 
  of 
  some 
  of 
  the 
  hornblende 
  possibly 
  in 
  sphene 
  ; 
  but 
  the 
  abundance 
  

   of 
  free 
  calcite 
  in 
  some 
  of 
  the 
  more 
  altered 
  rocks 
  shows 
  that 
  much 
  

   of 
  the 
  lime 
  must 
  have 
  percolated 
  in 
  solution 
  through 
  the 
  crust 
  as 
  

   carbonate. 
  The 
  iron 
  of 
  the 
  decomposed 
  hornblende 
  is 
  readily 
  

   accounted 
  for 
  by 
  the 
  epidote, 
  chlorite, 
  biotite, 
  and 
  ferrite. 
  The 
  

   magnesia 
  reappears 
  in 
  chlorite 
  and 
  biotite. 
  Alumina, 
  of 
  course, 
  

   occurs 
  in 
  most 
  of 
  the 
  above 
  alteration-minerals, 
  in 
  the 
  white 
  mica, 
  

   and 
  the 
  garnets. 
  

  

  The 
  chemical 
  energy 
  resulting 
  in 
  the 
  removal 
  of 
  the 
  heavy 
  bases 
  

   must 
  have 
  been 
  accompanied 
  by 
  the 
  liberation 
  of 
  compounds 
  of 
  the 
  

   alkalies. 
  These, 
  as 
  carbonates 
  or 
  silicates, 
  could 
  readily 
  have 
  been 
  

   carried 
  in 
  solution 
  from 
  place 
  to 
  place. 
  

  

  There 
  has 
  been 
  interchange 
  of 
  alkaline 
  bases 
  between 
  the 
  acidic 
  

   and 
  the 
  basic 
  rocks. 
  The 
  Rev. 
  I. 
  H. 
  Timins 
  f 
  stated 
  more 
  than 
  20 
  

  

  * 
  The 
  constituents 
  of 
  the 
  granular 
  rocks 
  of 
  Malvern 
  are 
  often 
  too 
  small 
  for 
  

   the 
  determination 
  of 
  the 
  axial 
  characters, 
  

   t 
  Quart. 
  Journ. 
  Geol. 
  Soc.yol. 
  xxi. 
  p. 
  8b. 
  

  

  