﻿258 
  MISS 
  C. 
  A. 
  KAISIN 
  ON 
  THE 
  NATURE 
  AND 
  [May 
  1 
  897, 
  

  

  the 
  magma 
  there 
  was 
  poor 
  in 
  the 
  lime, 
  1 
  alumina, 
  and 
  alkali 
  necessary 
  

   to 
  form 
  mica, 
  the 
  iron 
  may 
  have 
  been 
  used 
  up 
  in 
  the 
  composition 
  of 
  

   the 
  olivine. 
  

  

  If, 
  however, 
  anyone 
  maintains 
  that 
  the 
  evidence 
  just 
  given 
  for 
  

   an 
  original 
  mica 
  is 
  not 
  conclusive, 
  difficulties 
  still 
  remain 
  in 
  

   Weigand's 
  hypothesis. 
  First, 
  patches 
  of 
  the 
  chlorite 
  occur 
  in 
  the 
  

   Bauenthal 
  rock 
  in 
  parts 
  where 
  no 
  hornblende 
  can 
  be 
  traced, 
  or 
  

   flakes 
  are 
  isolated 
  within 
  oli 
  vine-serpentine 
  (PI. 
  XVII. 
  fig. 
  3). 
  

   Secondly, 
  in 
  rock 
  from 
  other 
  localities 
  there 
  is 
  serpentinized 
  

   hornblende, 
  but 
  no 
  indication 
  of 
  chlorite. 
  Thirdly, 
  the 
  chlorite 
  

   occurs 
  in 
  Rauenthal 
  specimens 
  embedded 
  among 
  completely 
  fresh 
  

   hornblende, 
  and 
  thus 
  apparently 
  not 
  associated 
  with 
  its 
  serpen- 
  

   tinization. 
  Fourthly, 
  so 
  far 
  from 
  the 
  presence 
  of 
  the 
  chlorite 
  

   differentiating, 
  as 
  we 
  are 
  told, 
  this 
  serpentine 
  from 
  serpentines 
  

   which 
  have 
  originated 
  from 
  olivine-rocks, 
  the 
  mineral 
  in 
  question 
  

   is 
  found 
  in 
  the 
  latter, 
  as 
  Prof. 
  Bonney 
  has 
  pointed 
  out, 
  2 
  and 
  as 
  

   will 
  be 
  described 
  shortly 
  in 
  several 
  examples. 
  

  

  Finally, 
  specimens 
  were 
  found 
  from 
  Bonhomme, 
  and 
  afterwards 
  

   from 
  the 
  Eauenthal, 
  in 
  which 
  a 
  transition 
  from 
  mica 
  to 
  a 
  pale 
  

   chlorite 
  could 
  be 
  traced 
  ; 
  and, 
  as 
  will 
  be 
  shown 
  later, 
  the 
  structure 
  

   of 
  the 
  platy 
  serpentine 
  would 
  agree 
  with 
  this 
  origin 
  of 
  the 
  chlorite. 
  

  

  (0) 
  Chemical 
  Changes. 
  

  

  If 
  the 
  chlorite 
  be 
  thus 
  a 
  modification 
  of 
  an 
  original 
  mica, 
  the 
  process 
  

   for 
  the 
  formation 
  of 
  the 
  serpentine 
  from 
  a 
  hornblende-rock 
  would 
  

   be 
  quite 
  inexplicable. 
  The 
  alumina 
  (and 
  possibly 
  other 
  constituents 
  

   of 
  the 
  supposed 
  hornblende), 
  if 
  not 
  used 
  in 
  the 
  formation 
  of 
  the 
  

   chlorite, 
  would 
  have, 
  as 
  Weigand 
  suggests, 
  no 
  place. 
  In 
  addition 
  

   to 
  this, 
  however, 
  the 
  difficulty 
  in 
  the 
  supposed 
  chemical 
  changes 
  of 
  

   the 
  disappearance 
  of 
  silica 
  or 
  accession 
  of 
  magnesia 
  is 
  not 
  touched. 
  

   Thus, 
  if 
  we 
  assume 
  the 
  quantity 
  of 
  the 
  most 
  persistent 
  substance, 
  

   alumina, 
  to 
  be 
  represented 
  by 
  unity, 
  and 
  take 
  the 
  amounts 
  of 
  the 
  

   other 
  components 
  as 
  ratios 
  to 
  that 
  in 
  the 
  amphibolite 
  and 
  in 
  the 
  

   bulk 
  serpentine 
  3 
  respectively, 
  we 
  find 
  how 
  Yery 
  different 
  is 
  the 
  

   composition 
  of 
  the 
  supposed 
  original 
  rock 
  and 
  of 
  that 
  into 
  which 
  it 
  is 
  

   stated 
  to 
  change. 
  Thus 
  

  

  the 
  silica 
  would 
  be 
  27*305 
  : 
  1 
  instead 
  of 
  6-898 
  : 
  1 
  ; 
  

   the 
  magnesia 
  „ 
  26-623 
  : 
  1 
  „ 
  3-902 
  : 
  1> 
  

  

  1 
  Since 
  it 
  was 
  shown 
  that 
  the 
  chlorite 
  contained 
  lime, 
  then 
  the 
  original 
  mica 
  

   should 
  have 
  been 
  a 
  variety 
  which 
  included 
  that 
  constituent. 
  The 
  amount 
  of 
  

   lime, 
  however., 
  in 
  the 
  bulk 
  analysis 
  is 
  slightly 
  greater 
  than 
  would 
  be 
  explained 
  

   by 
  the 
  quantity 
  in 
  the 
  chlurite. 
  The 
  presence 
  of 
  perofskite 
  might 
  partly 
  

   account 
  for 
  this. 
  

  

  2 
  Quart. 
  Journ. 
  G-eol. 
  Soc. 
  vol. 
  xlvii. 
  (1891) 
  p. 
  466. 
  

  

  3 
  That 
  this 
  serpentine 
  represents 
  a 
  normal 
  average 
  specimen 
  may 
  be 
  seen 
  by 
  

   combining 
  the 
  analyses 
  of 
  the 
  chlorite 
  and 
  of 
  dissolved 
  serpentine 
  on 
  the 
  

   estimate 
  that 
  the 
  chlorite 
  forms 
  roughly 
  about 
  T 
  \ 
  or 
  J 
  v 
  of 
  the 
  bulk 
  of 
  the 
  rock, 
  

   when 
  a 
  composition 
  is 
  obtained 
  very 
  near 
  that 
  of 
  the 
  bulk 
  analysis. 
  

  

  4 
  Or 
  compare 
  the 
  calculation 
  given 
  by 
  Prof. 
  Bonney, 
  Geol. 
  Mag. 
  1887, 
  p. 
  67, 
  

   and 
  note. 
  

  

  