﻿300 
  1/T.-&EN. 
  C. 
  A. 
  MCfaAHON 
  & 
  CAfT. 
  A. 
  H. 
  MtfMAHON 
  [Aug. 
  I§97> 
  

  

  Felspathic 
  Lavas. 
  

  

  No. 
  1434. 
  Sp. 
  gr.= 
  2-751. 
  

  

  From 
  the 
  Shibian 
  Kotal. 
  

  

  „ 
  1429. 
  „ 
  2-847. 
  

  

  „ 
  Amir 
  Chah. 
  

  

  „ 
  1416. 
  „ 
  2-851. 
  

  

  „ 
  Amir 
  Chah. 
  

  

  These 
  rocks 
  seem 
  to 
  lie 
  intermediate 
  between 
  the 
  andesites 
  proper 
  

   and 
  the 
  basaltic 
  lavas. 
  

  

  No. 
  1434 
  is 
  a 
  light 
  greyish, 
  yellow-ochre-coloured, 
  slaty-looking 
  

   felsite. 
  It 
  is 
  composed 
  of 
  crystals 
  of 
  oligoclase 
  set 
  in 
  a 
  micro- 
  

   felsitic 
  base. 
  The 
  felspars 
  look 
  like 
  fragments, 
  and 
  suggest 
  the 
  

   possibility 
  of 
  the 
  rock 
  being 
  an 
  altered 
  ash 
  ; 
  but 
  I 
  think 
  this 
  is 
  prob- 
  

   ably 
  owing 
  to 
  the 
  partial 
  remelting 
  and 
  deep 
  corrosion 
  of 
  the 
  felspars 
  

   by 
  the 
  base 
  when 
  liquid. 
  The 
  rock 
  is 
  considerably 
  altered, 
  being 
  

   dotted 
  over 
  with 
  epidote, 
  which 
  has 
  also 
  formed 
  in 
  some 
  of 
  the 
  felspars. 
  

  

  No. 
  1429 
  is 
  a 
  dark, 
  blackish-grey, 
  compact 
  lava. 
  The 
  slice 
  is 
  

   composed 
  of 
  a 
  matted 
  mass 
  of 
  microliths 
  and 
  idiomorphic 
  pheno- 
  

   crysts 
  of 
  felspar, 
  set 
  in 
  what 
  seems 
  to 
  have 
  been 
  originally 
  a 
  

   glassy 
  base. 
  The 
  base, 
  together 
  with 
  the 
  major 
  part 
  of 
  the 
  large 
  

   felspars, 
  has 
  been 
  altered 
  into 
  a 
  structureless 
  chlorite. 
  Opalescent 
  

   quartz 
  and 
  epidote 
  have 
  been 
  introduced 
  along 
  and 
  adjoining 
  cracks, 
  

   and 
  the 
  slice 
  is 
  dotted 
  over 
  with 
  colourless 
  epidote, 
  and 
  minute 
  spots 
  

   of 
  a 
  mineral, 
  opaque 
  in 
  transmitted 
  but 
  white 
  in 
  reflected 
  light, 
  

   which 
  is 
  probably 
  a 
  leucoxenic 
  variety 
  of 
  sphene. 
  The 
  microliths 
  

   and 
  small 
  felspars 
  have 
  straight 
  extinction, 
  and 
  their 
  refraction 
  is 
  

   precisely 
  that 
  of 
  the 
  Canada 
  balsam 
  in 
  which 
  the 
  slice 
  is 
  set. 
  They 
  

   are 
  probably 
  oligoclase. 
  

  

  No. 
  1416 
  is 
  a 
  light 
  grey 
  lava, 
  mottled 
  with 
  crystals 
  of 
  felspar 
  

   visible 
  to 
  the 
  unaided 
  eye. 
  Under 
  the 
  microscope 
  it 
  is 
  seen 
  to 
  

   be 
  made 
  up 
  of 
  felspar-phenocrysts 
  and 
  a 
  matted 
  mass 
  of 
  micro- 
  

   liths, 
  embedded 
  in 
  a 
  microgranular 
  devitrified 
  base. 
  Some 
  of 
  the 
  

   phenocrysts 
  are 
  quite 
  rounded, 
  others 
  present 
  more 
  or 
  less 
  perfect 
  

   crystalline 
  forms. 
  (See 
  PI. 
  XX, 
  fig. 
  2.) 
  

  

  The 
  porphyritic 
  felspars 
  are 
  so 
  highly 
  altered 
  that 
  their 
  species 
  

   cannot 
  be 
  satisfactorily 
  made 
  out. 
  For 
  the 
  most 
  part 
  they 
  appear 
  

   to 
  possess 
  straight 
  extinction, 
  but 
  it 
  is 
  impossible 
  to 
  say 
  whether 
  this 
  

   is 
  a 
  property 
  of 
  the 
  original 
  felspar 
  or 
  of 
  the 
  pseudomorphous 
  and 
  

   extremely 
  feeble 
  birefringent 
  mineral 
  that 
  has 
  more 
  or 
  less 
  replaced 
  

   it. 
  Two 
  fragments 
  of 
  these 
  felspars 
  yielded 
  respectively 
  specific 
  

   gravities 
  of 
  2-840 
  and 
  2*735. 
  The 
  refractive 
  index 
  of 
  the 
  latter 
  

   proved 
  to 
  be 
  1*557. 
  Taken 
  together, 
  these 
  data 
  point 
  to 
  the 
  felspar 
  

   being 
  labradorite 
  ; 
  but 
  the 
  alteration 
  that 
  the 
  mineral 
  has 
  undergone 
  

   prevents 
  me 
  from 
  pronouncing 
  a 
  definite 
  opinion 
  as 
  to 
  its 
  identity. 
  

  

  The 
  microliths 
  have 
  straight 
  extinction, 
  but 
  the 
  medium-sized 
  

   felspar-prisms 
  extinguish 
  obliquely. 
  There 
  is 
  no 
  uniformity 
  in 
  

   their 
  size 
  or 
  shape. 
  Some 
  are 
  rectangular, 
  some 
  lath-shaped. 
  Some 
  

   are 
  short 
  and 
  stumpy 
  ; 
  others 
  are 
  long 
  and 
  slender. 
  Many 
  of 
  

   them 
  are 
  ragged 
  and 
  ' 
  unfinished 
  ' 
  at 
  their 
  ends, 
  and 
  some 
  have 
  

   rod-like 
  microprisms 
  projecting 
  from 
  their 
  terminal 
  faces. 
  Some 
  

   are 
  quite 
  skeletal, 
  and 
  contain 
  inclusions 
  of 
  the 
  base. 
  A 
  few 
  

   exhibit 
  binary 
  twinning 
  obscurely 
  — 
  none 
  multiple 
  twinning. 
  The 
  

   microliths 
  are 
  presumably 
  oligoclase. 
  

  

  