﻿400 
  ME. 
  E. 
  X. 
  WOETH 
  ON 
  THE 
  ELYANS 
  AXD 
  

  

  Some 
  of 
  the 
  characteristics 
  of 
  the 
  Shillamill 
  elvan 
  are 
  set 
  forth 
  by 
  

   Mr. 
  Raitley, 
  in 
  his 
  ' 
  Eruptive 
  Rocks 
  of 
  Brent 
  Tor 
  ' 
  * 
  ; 
  but 
  a 
  far 
  finer 
  

   section 
  than 
  that 
  exposed 
  at 
  the 
  time 
  of 
  his 
  visit 
  has 
  recently 
  

   been 
  opened 
  across 
  the 
  dyke 
  in 
  the 
  construction 
  of 
  the 
  new 
  London 
  

   and 
  South- 
  Western 
  extension, 
  and 
  I 
  do 
  not 
  know 
  of 
  any 
  spot 
  where 
  

   extreme 
  features 
  of 
  elvanic 
  variation 
  can 
  now 
  be 
  studied 
  to 
  better 
  

   advantage. 
  The 
  centre 
  of 
  the 
  dyke 
  is, 
  to 
  quote 
  Mr. 
  Butley's 
  words, 
  

   " 
  a 
  quartzose 
  felspar-porphyry 
  of 
  the 
  usual 
  elvanitic 
  type;" 
  but 
  on 
  

   each 
  margin 
  the 
  rock 
  is 
  dark 
  grey, 
  in 
  part 
  granular 
  in 
  texture, 
  in 
  

   part 
  massive, 
  and 
  with 
  unevenly 
  distributed 
  dots 
  and 
  patches 
  of 
  dark 
  

   greenish 
  hue. 
  Some 
  parts, 
  again, 
  would 
  fall 
  under 
  the 
  old 
  name 
  of 
  

   " 
  claystone-porphyry." 
  The 
  different 
  phases 
  graduate 
  into 
  each 
  

   other, 
  but 
  it 
  would 
  be 
  easy 
  to 
  select 
  what, 
  on 
  a 
  mere 
  casual 
  survey, 
  

   might 
  be 
  taken 
  to 
  represent 
  half 
  a 
  dozen 
  different 
  rocks 
  from 
  half 
  

   a 
  dozen 
  distinct 
  localities. 
  Mr. 
  Rutley 
  notes 
  that 
  while 
  the 
  three 
  

   sections 
  which 
  he 
  describes 
  "are 
  identical 
  in 
  their 
  original 
  com- 
  

   position, 
  yet 
  they 
  differ 
  greatly 
  in 
  their 
  general 
  aspect 
  ;" 
  and 
  this 
  is 
  

   emphasized 
  by 
  the 
  results 
  of 
  my 
  examination 
  of 
  sections 
  from 
  this 
  

   new 
  exposure. 
  

  

  Microscopic 
  investigation 
  shows 
  that 
  while 
  the 
  leading 
  constituents 
  

   are, 
  as 
  Mr. 
  Rutley 
  says, 
  orthoclase, 
  quartz, 
  and 
  magnesian 
  mica, 
  

   with 
  a 
  varying 
  amount 
  of 
  felsitic 
  matter, 
  and 
  that 
  the 
  porphyritic 
  

   felspars 
  have 
  been 
  much 
  decomposed, 
  there 
  is 
  also 
  present 
  in 
  portions 
  

   of 
  the 
  dyke 
  a 
  considerable 
  proportion 
  of 
  dark-green 
  hornblende, 
  

   chiefly 
  associated 
  with 
  the 
  more 
  definite 
  quartz 
  and 
  felspar. 
  The 
  

   occurrence 
  of 
  hornblende 
  is 
  so 
  unusual 
  in 
  the 
  Dartmoor 
  granites 
  and 
  

   granitoid 
  rocks 
  that 
  this 
  is 
  especially 
  noteworthy. 
  The 
  proportion 
  

   of 
  felsitic 
  matter 
  varies 
  greatly 
  in 
  different 
  portions 
  of 
  the 
  dyke, 
  

   and 
  some 
  parts 
  are 
  distinctly 
  granular. 
  As 
  accessories 
  there 
  occur 
  

   casually 
  iron 
  and 
  copper 
  pyrites 
  and 
  some 
  chlorite. 
  This 
  elvan 
  has 
  

   been 
  the 
  subject 
  of 
  great 
  alteration, 
  and 
  there 
  is 
  hardly 
  any 
  that 
  

   will 
  better 
  repay 
  careful 
  study. 
  

  

  The 
  other 
  elvan 
  that 
  claims 
  special 
  mention 
  is 
  the 
  Grenofen, 
  which 
  

   is 
  half 
  a 
  mile 
  south 
  of 
  the 
  Shillamill, 
  and 
  stretches 
  from 
  east 
  to 
  

   west 
  four 
  miles. 
  Mr. 
  Rutley 
  observed 
  this 
  at 
  the 
  Lower 
  Grenofen 
  

   quarry, 
  where 
  it 
  has 
  a 
  well-marked 
  porphyritic 
  character, 
  and 
  con- 
  

   tains 
  so 
  little 
  felsitic 
  matter 
  that 
  it 
  is 
  essentially 
  a 
  fine-grained 
  

   porphyritic 
  granite, 
  partially 
  syenitic, 
  seeing 
  that 
  hornblende 
  is 
  

   irregularly 
  developed. 
  

  

  This 
  elvan, 
  unlike 
  the 
  Shillamill, 
  retains 
  its 
  general 
  characteristics 
  

   in 
  breadth, 
  but 
  varies 
  greatly 
  in 
  its 
  length. 
  It 
  has 
  recently 
  been 
  

   intersected 
  in 
  a 
  tunnel 
  on 
  the 
  new 
  railway 
  not 
  far 
  from 
  Shillamill, 
  

   and 
  is 
  there 
  essentially 
  an 
  even-grained, 
  loose-textured 
  granite, 
  the 
  

   kaolinization 
  of 
  the 
  felspar 
  having 
  proceeded 
  so 
  far 
  as 
  to 
  make 
  the 
  

   rock 
  quite 
  rotten. 
  Still 
  further 
  west, 
  about 
  a 
  mile 
  distant, 
  on 
  

   Morwell 
  Down, 
  the 
  dyke 
  yields 
  a 
  variety 
  with 
  a 
  compact 
  semi- 
  

   vitreous 
  ground-mass, 
  in 
  which 
  felspar, 
  quartz, 
  and 
  mica 
  are 
  por- 
  

   phyritically 
  developed. 
  Under 
  the 
  microscope 
  the 
  felsitic 
  base 
  is 
  

   seen 
  to 
  be 
  remarkably 
  even-textured, 
  resembling 
  ground 
  glass, 
  and 
  

   * 
  Pp. 
  24, 
  41, 
  42. 
  

  

  