﻿298 
  LT.-GEN. 
  C. 
  A. 
  MCMAHON 
  & 
  CAPT. 
  A. 
  H. 
  MCMAHON 
  [Aug. 
  1 
  897 
  

  

  contain, 
  in 
  addition 
  to 
  the 
  minerals 
  above 
  mentioned, 
  a 
  rhombic 
  

   amphibole. 
  In 
  transmitted 
  light 
  it 
  is 
  of 
  a 
  rich 
  brown-red 
  colour 
  

   and 
  is 
  powerfully 
  dichroic, 
  changing 
  from 
  a 
  golden 
  yellow, 
  when 
  

   the 
  direction 
  of 
  elongation 
  of 
  the 
  crystal 
  is 
  at 
  right 
  angles 
  to 
  the 
  

   principal 
  section 
  (longer 
  diagonal) 
  of 
  the 
  polarizer, 
  to 
  a 
  rich 
  brown- 
  

   red, 
  or 
  red-brown, 
  when 
  the 
  direction 
  of 
  elongation 
  is 
  parallel 
  

   to 
  the 
  longer 
  diagonal 
  of 
  the 
  nicol. 
  The 
  mineral 
  possesses 
  straight 
  

   extinction, 
  and 
  it 
  appears 
  to 
  be 
  anthophyllite. 
  

  

  Slice 
  No. 
  1424 
  contains 
  some 
  very 
  characteristic 
  cross-sections 
  of 
  

   prisms 
  of 
  this 
  mineral, 
  which 
  show 
  the 
  intersection 
  of 
  the 
  prism- 
  

   faces 
  and 
  the 
  intersection 
  of 
  the 
  prismatic 
  cross-cleavages. 
  They 
  

   are 
  quite 
  typical 
  cross-sections 
  of 
  amphibole. 
  The 
  major 
  ( 
  + 
  ) 
  axis 
  

   is, 
  as 
  in 
  rhombic 
  amphibole, 
  at 
  right 
  angles 
  to 
  the 
  direction 
  of 
  

   elongation. 
  Moreover, 
  I 
  isolated 
  fragments 
  of 
  this 
  mineral 
  and 
  

   determined 
  its 
  specific 
  gravity 
  with 
  the 
  aid 
  of 
  a 
  heavy 
  liquid 
  and 
  

   Westphal's 
  balance, 
  and 
  found 
  it 
  to 
  be 
  exactly 
  that 
  of 
  anthophyllite 
  : 
  

   namely, 
  between 
  3*1 
  and 
  3*2. 
  

  

  The 
  anthophyllite 
  appears 
  to 
  be 
  an 
  original 
  mineral 
  in 
  these 
  rocks. 
  

   Crystals 
  of 
  felspars 
  are 
  caught 
  up 
  in 
  it, 
  aud 
  in 
  PI. 
  XIX, 
  fig. 
  3, 
  a 
  case 
  

   is 
  depicted 
  where 
  a 
  large 
  augite 
  has 
  caught 
  up 
  and 
  enclosed 
  a 
  frag- 
  

   mentary-looking 
  crystal 
  of 
  anthophyllite. 
  That 
  the 
  latter 
  is 
  an 
  

   original 
  inclusion 
  in 
  the 
  augite 
  appears 
  plain, 
  from 
  the 
  fact 
  that 
  

   contact-action 
  has 
  produced 
  a 
  dark 
  halo 
  round 
  the 
  anthophyllite. 
  

  

  The 
  case 
  represented 
  in 
  PL 
  XIX, 
  fig. 
  4, 
  is 
  an 
  interesting 
  one, 
  and 
  

   bears 
  directly 
  on 
  the 
  question 
  at 
  issue. 
  The 
  anthophyllite, 
  when 
  it 
  

   began 
  to 
  crystallize, 
  formed 
  on 
  the 
  lower 
  half 
  of 
  a 
  felspar-mi 
  crolith. 
  

   The 
  growth 
  of 
  the 
  felspar 
  in 
  that 
  direction 
  was 
  arrested, 
  but 
  it 
  con- 
  

   tinued 
  to 
  grow 
  and 
  widen 
  along 
  its 
  upper 
  half 
  where 
  the 
  anthophyllite 
  

   did 
  not 
  interfere 
  with 
  it. 
  Similar 
  cases 
  of 
  interference 
  between 
  

   growing 
  crystals 
  of 
  augite 
  and 
  felspar 
  are 
  often 
  seen 
  in 
  basalts 
  ; 
  

   but 
  the 
  case 
  has 
  a 
  special 
  importance 
  in 
  the 
  study 
  of 
  these 
  rocks, 
  as 
  

   it 
  shows 
  conclusively 
  that 
  the 
  crystallization 
  of 
  the 
  smaller 
  felspars 
  

   and 
  the 
  anthophyllite 
  was 
  contemporaneous. 
  

  

  Rosenbusch, 
  in 
  his 
  work 
  on 
  rocks, 
  1 
  notes 
  that 
  a 
  brown 
  hornblende 
  

   with 
  an 
  extinction-angle 
  ranging 
  from 
  small 
  to 
  nothing 
  is 
  known 
  

   to 
  occur 
  in 
  some 
  andesites. 
  In 
  the 
  slices 
  above 
  described 
  the 
  angle 
  

   of 
  extinction 
  is 
  uniformly 
  nil. 
  

  

  In 
  Nos. 
  1424, 
  1438, 
  and 
  1464 
  all 
  the 
  amphibole 
  consists 
  of 
  antho- 
  

   phyllite 
  ; 
  but 
  the 
  first 
  two 
  contain 
  in 
  addition 
  a 
  considerable 
  amount 
  

   of 
  augite. 
  In 
  transmitted 
  light 
  it 
  is 
  of 
  a 
  somewhat 
  pale 
  brown- 
  

   green 
  colour, 
  sometimes 
  putting 
  on 
  a 
  purplish 
  tint 
  : 
  it 
  shows 
  no 
  

   dichroism. 
  A 
  single 
  cleavage 
  predominates, 
  but 
  more 
  or 
  less 
  distinct 
  

   traces 
  of 
  a 
  cross-cleavage 
  may 
  sometimes 
  be 
  seen. 
  The 
  augites 
  

   occasionally 
  exhibit 
  crystallographic 
  outlines, 
  but 
  are 
  more 
  frequently 
  

   allotriomorphic. 
  

  

  The 
  augite-hornblende-andesif 
  es, 
  Xos. 
  1424 
  & 
  1438, 
  are 
  remarkable 
  

   for 
  containing 
  crystals 
  of 
  olivine. 
  Hosenbusch, 
  in 
  his 
  work 
  already 
  

   referred 
  to, 
  mentions 
  that 
  olivine 
  sometimes 
  occurs 
  in 
  andesites, 
  

   but 
  one 
  would 
  certainly 
  not 
  expect 
  to 
  meet 
  with 
  this 
  mineral 
  in 
  a 
  

  

  1 
  ' 
  Mikroskopische 
  Physiograpbie 
  der 
  Massigen 
  Gesteine,' 
  2nd 
  ed. 
  (1887) 
  

   p. 
  659. 
  

  

  