﻿Yol. 
  53.] 
  ORIGTX 
  OF 
  THE 
  RATTENTHAL 
  SERPENTINE. 
  259 
  

  

  Although, 
  it 
  may 
  not 
  be 
  possible 
  to 
  show 
  the 
  fate 
  of 
  the 
  surplus 
  

   silica, 
  1 
  this 
  does 
  not 
  offer 
  much 
  difficulty 
  if 
  only 
  a 
  small 
  amount 
  of 
  

   hornblende 
  has 
  been 
  really 
  transformed. 
  In 
  all 
  cases 
  of 
  serpentini- 
  

   zation 
  such 
  a 
  change 
  takes 
  place, 
  but 
  is 
  not 
  evident 
  microscopically 
  ; 
  

   and 
  in 
  the 
  case 
  of 
  sundry 
  hemithrenes, 
  where 
  there 
  is 
  distinct 
  

   evidence 
  of 
  serpentine 
  having 
  formed 
  from 
  malacolite, 
  no 
  visible 
  

   silica 
  can 
  be 
  detected. 
  

  

  We 
  conclude 
  then, 
  in 
  regard 
  to 
  the 
  Rauenthal 
  rock, 
  that 
  serpentini- 
  

   zation 
  occurs 
  of 
  occasional 
  hornblende-crystals, 
  and 
  of 
  some 
  enstatite, 
  

   but 
  that 
  the 
  greater 
  part 
  is 
  to 
  be 
  attributed 
  to 
  the 
  serpen 
  tinization 
  

   of 
  an 
  oli 
  vine-rock, 
  and 
  that 
  the 
  probable 
  origin 
  of 
  the 
  chlorite 
  is 
  a 
  

   modification 
  of 
  a 
  mica. 
  

  

  (7) 
  Structural 
  Characters. 
  

  

  The 
  structural 
  characteristics 
  of 
  the 
  mass 
  suggest 
  some 
  further 
  

   explanations. 
  With 
  regard 
  to 
  the 
  amphibolite, 
  Herr 
  Weigand 
  has 
  

   described 
  its 
  peculiar 
  foliated 
  appearance. 
  If 
  the 
  hypothesis 
  put 
  

   forward 
  by 
  him 
  could 
  not 
  be 
  accepted, 
  there 
  yet 
  seemed 
  a 
  possibility 
  

   that 
  the 
  rock 
  might 
  represent 
  parts 
  of 
  the 
  adjacent 
  gneiss 
  in 
  contact 
  

   with 
  an 
  igneous 
  mass 
  and 
  modified 
  by 
  it. 
  Indeed, 
  my 
  first 
  exami- 
  

   nation 
  of 
  the 
  locality 
  suggested 
  the 
  possibility 
  that 
  one 
  rock 
  might 
  

   have 
  been 
  included 
  and 
  partially 
  melted 
  down 
  by 
  an 
  intruding 
  

   magma, 
  as 
  has 
  been 
  proved 
  to 
  be 
  the 
  case 
  in 
  parts 
  of 
  Cornwall 
  2 
  ; 
  

   but 
  the 
  idea 
  was 
  clearly 
  negatived 
  by 
  further 
  evidence 
  in 
  the 
  field 
  

   and 
  afterwards 
  by 
  that 
  obtained 
  by 
  the 
  microscope. 
  

  

  The 
  amphibolite 
  of 
  the 
  Rauenthal 
  (with 
  a 
  foliated 
  appearance) 
  

   forms 
  actual 
  streaks 
  or 
  layers 
  (sometimes 
  a 
  few 
  inches 
  thick) 
  within 
  

   the 
  serpentine. 
  Under 
  the 
  microscope 
  these 
  are 
  seen 
  to 
  consist 
  

   chiefly 
  of 
  characteristic 
  and 
  unaltered 
  hornblende. 
  But 
  at 
  parts 
  of 
  

   the 
  serpentine 
  itself 
  the 
  thin 
  dull-greenish 
  streaks' 
  (g 
  inch 
  thick) 
  

   previously 
  mentioned 
  are 
  composed 
  of 
  similar 
  hornblende, 
  which, 
  

   however, 
  is 
  partially 
  serpentinized. 
  The 
  chlorite-flakes 
  usually 
  form 
  

   thin 
  layers, 
  which 
  help 
  to 
  give 
  a 
  platy 
  habit 
  to 
  the 
  rock 
  — 
  one 
  of 
  

   its 
  distinctive 
  characteristics 
  (PI. 
  XYII. 
  fig. 
  6). 
  All 
  these 
  structures, 
  

   so 
  far 
  as 
  my 
  observations 
  went, 
  have 
  a 
  general 
  parallelism. 
  3 
  

  

  In 
  the 
  different 
  layers, 
  however, 
  streaks 
  of 
  other 
  composition 
  

   occur 
  ; 
  thus, 
  in 
  an 
  amphibolite-band, 
  we 
  find 
  strings 
  of 
  serpentine 
  

   or 
  occasional 
  chlorite, 
  sometimes 
  in 
  fair 
  abundance 
  (see 
  fig. 
  on 
  p. 
  24.9 
  

   & 
  PL 
  XYII. 
  fig. 
  4). 
  And 
  the 
  various 
  minerals 
  are 
  not 
  confined 
  to 
  

   special 
  la} 
  T 
  ers. 
  Chlorite 
  occurs, 
  as 
  Weigand 
  states, 
  scattered 
  without 
  

   orientation 
  ; 
  the 
  serpentine 
  sometimes 
  has 
  a 
  patchy 
  look, 
  and 
  is 
  

   dark 
  green 
  mottled 
  with 
  pale 
  green 
  — 
  owing 
  to 
  an 
  irregular 
  mixing 
  

   of 
  serpentinized 
  hornblende. 
  As 
  already 
  stated, 
  enstatite 
  can 
  be 
  

  

  1 
  Is 
  it 
  possible 
  that 
  the 
  brownish, 
  somewhat 
  amorphous-looking 
  substance, 
  

   found 
  at 
  places, 
  could 
  be 
  opaline 
  in 
  its 
  nature? 
  

  

  2 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xlvii. 
  (1891) 
  p. 
  471. 
  

  

  3 
  This 
  seems 
  to 
  be 
  in 
  a 
  direction 
  roughly 
  parallel 
  with 
  the 
  boundary 
  of 
  the 
  

   outcropping 
  serpentine 
  ; 
  but 
  the 
  observations 
  on 
  this 
  point 
  were 
  taken 
  in 
  bad 
  

   weather, 
  and 
  must 
  be 
  further 
  tested 
  for 
  confirmation. 
  

  

  