﻿528 
  MAJOE-GEXEEAL 
  C. 
  A. 
  M'MAHON 
  OX 
  THE 
  HOEISTBLENDE-SCHISTS 
  

  

  felspars 
  set 
  in 
  an 
  interstitial 
  paste. 
  Under 
  the 
  microscope 
  it 
  is 
  

   seen 
  to 
  consist 
  of 
  decomposed 
  felspar 
  through 
  which 
  water 
  has 
  

   percolated 
  in 
  all 
  directions. 
  These 
  streams 
  are 
  always 
  winding 
  and 
  

   tortuous 
  except 
  when, 
  here 
  and 
  there, 
  they 
  have 
  temporarily 
  fol- 
  

   lowed 
  the 
  course 
  of 
  a 
  crack. 
  These 
  streams, 
  which 
  alternately 
  

   widen 
  out 
  into 
  broad 
  lacunae 
  and 
  contract 
  into 
  narrow 
  channels, 
  

   are 
  marked 
  by 
  fringing 
  borders 
  of 
  opacite, 
  limonite, 
  and 
  magnetite, 
  

   and 
  they 
  are 
  stopped 
  in 
  part 
  by 
  an 
  isotropic 
  serpentinous 
  mineral, 
  

   and 
  in 
  part 
  by 
  a 
  rich 
  red, 
  strongly 
  dichroic 
  mica, 
  which, 
  in 
  every 
  

   case, 
  is 
  orientated 
  in 
  the 
  direction 
  of 
  the 
  serpentinous 
  stream 
  that 
  

   contains 
  it, 
  the 
  basal 
  cleavage 
  of 
  the 
  mica 
  being 
  strict]y 
  parallel 
  to 
  

   the 
  sides 
  of 
  the 
  stream. 
  "Whatever 
  view 
  may 
  be 
  adopted 
  regarding 
  

   the 
  origin 
  of 
  this 
  rock, 
  there 
  can, 
  I 
  think, 
  be 
  no 
  doubt 
  that 
  the 
  mica, 
  

   like 
  the 
  serpentinous 
  matter 
  that 
  surrounds 
  it, 
  is 
  a 
  secondary 
  pro- 
  

   duct 
  of 
  aqueous 
  action. 
  

  

  The 
  access 
  of 
  water 
  to 
  solid 
  rocks 
  and 
  the 
  heating 
  of 
  this 
  water 
  

   in 
  rocks 
  deeply 
  buried 
  under 
  superincumbent 
  strata 
  are 
  ideas 
  which 
  

   are 
  so 
  familiar 
  to 
  geologists 
  that 
  I 
  need 
  not 
  spend 
  any 
  time 
  on 
  this 
  

   branch 
  of 
  the 
  inquiry, 
  but 
  pass 
  on 
  to 
  point 
  out 
  how 
  heated 
  water 
  

   may 
  have 
  produced 
  the 
  banding 
  of 
  the 
  Lizard 
  hornblende-schists. 
  

  

  I 
  need 
  also, 
  I 
  should 
  think, 
  devote 
  little 
  space 
  to 
  showing 
  the 
  

   high 
  probability 
  that 
  fine-grained 
  ash-beds 
  of 
  subaqueous 
  origin*, 
  

   after 
  consolidation 
  by 
  the 
  pressure 
  of 
  superposed 
  deposits, 
  would 
  favour 
  

   the 
  passage 
  of 
  underground 
  water 
  along 
  the 
  planes 
  of 
  lamination 
  

   more 
  readily 
  than 
  in 
  a 
  direction 
  perpendicular 
  to 
  those 
  planes. 
  

  

  When 
  inspecting 
  jails 
  in 
  India, 
  the 
  process 
  of 
  paper-making 
  by 
  

   hand 
  interested 
  me 
  much 
  and 
  furnished 
  an 
  illustration 
  capable 
  of 
  a 
  

   geological 
  application. 
  The 
  material 
  of 
  which 
  the 
  paper 
  is 
  made 
  

   floats 
  in 
  a 
  vat 
  of 
  water. 
  A 
  sieve 
  is 
  plunged 
  into 
  this 
  vat, 
  and 
  the 
  

   pulpy 
  matter 
  suspended 
  in 
  the 
  water 
  is 
  allowed 
  to 
  settle 
  on 
  it 
  for 
  

   a 
  few 
  seconds. 
  As 
  soon 
  as 
  the 
  requisite 
  thickness 
  is 
  attained 
  the 
  

   sieve 
  is 
  withdrawn, 
  and 
  the 
  newly 
  formed 
  film 
  is 
  there 
  and 
  then 
  

   deposited 
  on 
  a 
  heap 
  of 
  similar 
  films 
  until 
  a 
  large 
  block 
  is 
  formed. 
  

   One 
  would 
  naturally 
  expect 
  the 
  whole 
  series 
  of 
  wet 
  films 
  to 
  con- 
  

   solidate 
  into 
  an 
  amorphous 
  lump, 
  but, 
  contrary 
  to 
  one's 
  expectations, 
  

   sheet 
  after 
  sheet 
  is 
  removed 
  without 
  difficulty, 
  and 
  after 
  having 
  been 
  

   dried 
  in 
  the 
  sun, 
  is 
  polished 
  and 
  used 
  as 
  writing-paper. 
  This 
  process 
  

   has 
  often 
  struck 
  me 
  as 
  a 
  pretty 
  illustration 
  of 
  how 
  pauses, 
  of 
  even 
  

   short 
  duration, 
  in 
  the 
  deposition 
  of 
  sedimentar}^ 
  rocks 
  may 
  help 
  to 
  pro- 
  

   duce 
  a 
  fissile 
  structure. 
  This 
  tendency, 
  moreover, 
  would 
  be 
  greatly 
  

   increased 
  by 
  the 
  compression 
  caused 
  by 
  the 
  weight 
  of 
  superposed 
  

   strata. 
  The 
  slates 
  of 
  the 
  Himalayas 
  are 
  nearly 
  all 
  lamination- 
  

   slates, 
  but 
  they 
  are 
  as 
  truly 
  fissile 
  in 
  the 
  direction 
  of 
  the 
  lamina- 
  

   tion 
  as 
  English 
  slates 
  are 
  in 
  the 
  direction 
  of 
  cleavage. 
  

  

  Let 
  us 
  suppose 
  that 
  fine-grained 
  ash-beds 
  divided 
  by 
  planes 
  of 
  

   sedimentation, 
  or 
  of 
  cleavage 
  parallel 
  to 
  the 
  bedding, 
  were 
  subjected 
  

   to 
  the 
  action 
  of 
  heated 
  water 
  in 
  which 
  a 
  periodical 
  movement, 
  however 
  

   languid, 
  had 
  been 
  set 
  up 
  (see 
  fig. 
  5). 
  During 
  the 
  pauses 
  in 
  the 
  flow, 
  

   the 
  heated 
  water 
  in 
  the 
  plane 
  (a-b) 
  would 
  percolate 
  by 
  gravitation 
  and 
  

   * 
  See 
  Bonney, 
  ante. 
  

  

  