﻿530 
  MAJOR-GENERAL 
  C. 
  A. 
  M'MAHON 
  ON 
  THE 
  HOES 
  BLENDE-SCHISTS 
  

  

  The 
  passage 
  of 
  water 
  from 
  trie 
  upper 
  felspathic 
  band 
  (a-b-c) 
  to 
  the 
  

   lower 
  felspathic 
  band 
  (f-g-h) 
  by 
  the 
  channel 
  (b-g) 
  through 
  the 
  dark 
  

   Jiornblendic 
  band 
  (d-e) 
  is 
  clear] 
  y 
  demonstrated. 
  The 
  fact 
  that 
  the 
  

   liquid 
  that 
  effected 
  the 
  decomposition 
  of 
  the 
  felspar 
  also 
  flowed 
  

  

  Fig. 
  6. 
  — 
  Passage 
  of 
  Water 
  through 
  Banded 
  Hornblende-schists. 
  

  

  along 
  the 
  bands 
  [a-b-c] 
  (f-g-h) 
  is 
  shown 
  by 
  the 
  hard 
  marginal 
  Hue 
  

   left 
  by 
  this 
  liquid 
  along 
  the 
  border 
  of 
  the 
  hornblendic 
  band 
  (d-e) 
  by 
  

   the 
  formation 
  of 
  a 
  secondary 
  product 
  of 
  decomposition, 
  opaque 
  in 
  

   transmitted, 
  and 
  dead 
  white 
  in 
  reflected 
  light. 
  

  

  I 
  suppose, 
  then, 
  to 
  conclude 
  my 
  remarks 
  on 
  this 
  branch 
  of 
  the 
  

   subject, 
  that 
  the 
  banding 
  of 
  the 
  hornblende-schists 
  was 
  produced 
  

   by 
  the 
  capillary 
  flow, 
  or 
  percolation, 
  of 
  heated 
  water 
  through 
  the 
  

   rocks 
  in 
  two 
  ways, 
  namely, 
  by 
  the 
  leeching 
  out 
  of 
  unstable 
  minerals 
  

   (such 
  as 
  pyroxene) 
  from 
  the 
  spaces 
  between 
  the 
  planes 
  of 
  lamination, 
  

   and 
  by 
  the 
  formation 
  of 
  comparatively 
  stable 
  minerals 
  (such 
  as 
  

   hornblende) 
  aloug 
  these 
  planes. 
  

  

  Numerous 
  illustrations 
  of 
  the 
  growth 
  of 
  hornblende 
  in 
  the 
  " 
  wet 
  

   way 
  " 
  are 
  contained 
  in 
  the 
  thin 
  slices 
  under 
  description. 
  In 
  three 
  

   of 
  my 
  specimens 
  of 
  diorite 
  cracks 
  were 
  formed 
  which 
  in 
  some 
  cases 
  

   divided 
  crystals 
  of 
  hornblende 
  in 
  half, 
  and 
  in 
  others 
  separated 
  ad- 
  

   joining 
  crystals, 
  the 
  severed 
  hornblendes 
  being 
  left 
  on 
  opposite 
  sides 
  

   but 
  at 
  the 
  extreme 
  margin 
  of 
  the 
  cracks. 
  In 
  both 
  these 
  cases 
  the 
  

   cracks 
  have 
  been 
  bridged 
  over, 
  not 
  by 
  the 
  mineral 
  that 
  stops 
  them 
  

   in 
  other 
  places, 
  but 
  by 
  hornblende. 
  Sometimes 
  this 
  is 
  in 
  the 
  form 
  

   of 
  actinolite 
  needles, 
  and 
  in 
  others 
  in 
  that 
  of 
  substantial 
  crystals. 
  

   In 
  some 
  cases 
  the 
  crystals, 
  split 
  in 
  half 
  by 
  a 
  crack, 
  have 
  been 
  re- 
  

   paired 
  in 
  the 
  most 
  perfect 
  manner 
  by 
  the 
  insertion 
  of 
  hornblende 
  

   in 
  optical 
  and 
  isomorphic 
  continuity 
  with 
  the 
  severed 
  ends. 
  The 
  

   secondary 
  portion 
  of 
  the 
  composite 
  crystal, 
  however, 
  does 
  not 
  ex- 
  

   hibit 
  the 
  dark 
  colour 
  and 
  strong 
  dichroism 
  of 
  the 
  severed 
  portions 
  

   of 
  the 
  original 
  crystal 
  at 
  the 
  two 
  ends 
  of 
  the 
  new 
  crystal. 
  The 
  

   stopping 
  of 
  these 
  cracks 
  is 
  clearly 
  not 
  due 
  to 
  the 
  exudation 
  of 
  

   uncrysfallized 
  magma 
  from 
  the 
  body 
  of 
  the 
  rock 
  before 
  its 
  complete 
  

   solidification. 
  The 
  extreme 
  sharpness 
  of 
  outline 
  presented 
  by 
  these 
  

   cracks 
  shows 
  that 
  the 
  crystals 
  in 
  the 
  rock 
  were 
  rigid 
  when 
  the 
  

  

  