﻿Vol. 
  53.] 
  Off 
  KOCKS 
  FKOM 
  THE 
  BALtTCSlSTAtf 
  BOEDER. 
  299 
  

  

  rock 
  of 
  the 
  intermediate 
  class 
  which 
  inclines 
  towards 
  the 
  acid 
  type. 
  

   In 
  transmitted 
  light 
  it 
  is 
  of 
  so 
  pale 
  a 
  greenish- 
  white 
  as 
  to 
  be 
  almost 
  

   colourless. 
  The 
  refraction 
  is 
  high, 
  and 
  the 
  surface 
  is 
  rough 
  and 
  

   shagreened. 
  Traces 
  of 
  a 
  fine 
  interrupted 
  cleavage 
  are 
  to 
  be 
  seen, 
  

   and 
  the 
  mineral 
  has 
  straight 
  extinction 
  with 
  reference 
  to 
  this 
  

   cleavage. 
  It 
  has 
  also 
  the 
  deeper 
  and 
  more 
  irregular 
  cracks 
  so 
  

   commonly 
  seen 
  in 
  olivine. 
  The 
  form 
  is 
  sometimes 
  roughly 
  idio- 
  

   morphic, 
  and 
  photographs 
  of 
  two 
  (one 
  from 
  No. 
  1424 
  and 
  the 
  other 
  

   from 
  No. 
  1438) 
  are 
  reproduced 
  in 
  PI. 
  XIX, 
  figs. 
  5 
  & 
  6, 
  which 
  show 
  

   the 
  pointed 
  terminations 
  characteristic 
  of 
  olivine. 
  

  

  Several 
  of 
  these 
  olivines, 
  as 
  in 
  PI. 
  XIX, 
  fig. 
  5, 
  have 
  a 
  deposit 
  of 
  

   the 
  brown-red 
  anthophyllite 
  round 
  a 
  portion 
  of 
  the 
  outer 
  edge 
  of 
  the 
  

   olivine. 
  Eosenbusch 
  1 
  notes 
  the 
  alteration 
  of 
  olivine, 
  beginning 
  with 
  

   the 
  periphery 
  of 
  the 
  latter 
  mineral, 
  into 
  needles 
  of 
  tremolite, 
  actino- 
  

   lite, 
  and 
  anthophyllite, 
  owing 
  to 
  the 
  mutual 
  influence 
  of 
  the 
  olivine 
  

   and 
  the 
  adjacent 
  rock-constituents, 
  and 
  states 
  that 
  this 
  is 
  known 
  to 
  

   occur 
  only 
  in 
  the 
  Archaean 
  rocks. 
  At 
  first 
  sight, 
  the 
  anthophyllite- 
  

   fringe 
  above 
  described 
  would 
  seem 
  to 
  be 
  a 
  case 
  of 
  alteration 
  similar 
  to 
  

   that 
  noted 
  by 
  Eosenbusch 
  ; 
  but 
  in 
  view 
  of 
  the 
  fact 
  already 
  shown, 
  

   that 
  the 
  anthophyllite 
  in 
  these 
  rocks 
  is 
  an 
  original 
  congenital 
  mineral, 
  

   I 
  think 
  it 
  is 
  more 
  probable 
  that 
  the 
  anthophyllite 
  round 
  the 
  margin 
  of 
  

   the 
  olivines 
  is 
  of 
  the 
  nature 
  of 
  an 
  intergrowth. 
  It 
  does 
  not 
  occur 
  in 
  

   needles, 
  and 
  so 
  far 
  from 
  these 
  andesites 
  being 
  of 
  Archaean 
  age, 
  there 
  

   is 
  reason 
  to 
  believe 
  that 
  they 
  are, 
  geologically 
  speaking, 
  compara- 
  

   tively 
  modern. 
  Numerous 
  crystals 
  of 
  anthophyllite 
  occur 
  in 
  these 
  

   slides, 
  in 
  forms 
  that 
  could 
  not 
  possibly 
  be 
  referred 
  to 
  olivine. 
  

  

  In 
  the 
  next 
  two 
  slides, 
  Nos. 
  1425 
  & 
  1426, 
  ordinary 
  monoclinic 
  

   hornblende 
  takes 
  the 
  place 
  of 
  anthophyllite. 
  It 
  is 
  of 
  a 
  brownish- 
  

   green 
  colour 
  — 
  the 
  green 
  element 
  being 
  very 
  distinct 
  ; 
  it 
  is 
  strongly 
  

   dichroic, 
  and 
  the 
  angle 
  of 
  extinction 
  is 
  small. 
  It 
  was 
  evidently 
  one 
  of 
  

   the 
  first 
  minerals 
  to 
  crystallize 
  out, 
  and 
  it 
  has 
  suffered 
  much 
  from 
  the 
  

   corrosive 
  action 
  of 
  the 
  more 
  acid 
  magma. 
  Many 
  of 
  the 
  crystals 
  are 
  

   rounded 
  and 
  corroded, 
  and 
  all 
  have 
  a 
  broad 
  black 
  ' 
  resorption-' 
  

   margin 
  of 
  magnetite. 
  

  

  The 
  condition 
  of 
  the 
  hornblende 
  in 
  these 
  sections 
  confirms 
  the 
  

   conclusion 
  arrived 
  at 
  regarding 
  the 
  anthophyllite 
  : 
  namely, 
  that 
  the 
  

   amphibole 
  in 
  these 
  rocks 
  is 
  an 
  original 
  mineral, 
  and 
  crystallized 
  out 
  

   before 
  the 
  magma 
  ceased 
  to 
  be 
  fluid. 
  

  

  In 
  Nos. 
  1430 
  & 
  1443 
  amphibole 
  gives 
  place 
  to 
  mica. 
  In 
  trans- 
  

   mitted 
  light 
  the 
  mica 
  varies 
  from 
  a 
  yellow-brown 
  to 
  a 
  greenish 
  

   brown. 
  It 
  has 
  suffered 
  much, 
  and 
  has 
  been 
  corroded 
  by 
  the 
  solvent 
  

   action 
  of 
  the 
  liquid 
  magma. 
  It 
  is 
  present 
  in 
  good-sized 
  leaves 
  and 
  

   packets, 
  and 
  also 
  (No. 
  1443) 
  in 
  the 
  form 
  of 
  fibrous 
  microliths. 
  

  

  No. 
  1436 
  closely 
  resembles 
  the 
  rocks 
  above 
  described 
  in 
  its 
  general 
  

   characteristics, 
  but 
  it 
  does 
  not 
  contain 
  any 
  amphibole. 
  It 
  is 
  much 
  

   altered 
  and 
  contains 
  some 
  secondary 
  minerals, 
  such 
  as 
  zoisite 
  and 
  

   ferric 
  oxide, 
  the 
  latter 
  partly 
  infilling 
  some 
  of 
  the 
  felspars. 
  There 
  

   is 
  also 
  apparently 
  the 
  remnant 
  of 
  an 
  augite. 
  

  

  1 
  'Microscopical 
  Physiography 
  of 
  Rock-making 
  Minerals,' 
  transl. 
  & 
  

   abridged 
  by 
  J. 
  P. 
  Iddings, 
  1888, 
  p. 
  217. 
  

  

  