﻿366 
  MISS 
  C. 
  A. 
  EAISIN 
  ON 
  THE 
  PETROLOGY 
  [Aug. 
  1 
  897, 
  

  

  (c) 
  Other 
  Hornblende-diabases.— 
  (4, 
  20, 
  9, 
  13, 
  2, 
  14, 
  17, 
  5, 
  25.) 
  In 
  

   one 
  specimen 
  from 
  the 
  west 
  (4 
  : 
  Jebel 
  Raft, 
  west 
  side) 
  and 
  in 
  five 
  

   specimens 
  from 
  the 
  south 
  (Murrat 
  : 
  20, 
  9, 
  13, 
  2, 
  14), 
  where 
  minerals 
  

   are 
  recognizable, 
  felspar 
  and 
  hornblende 
  in 
  various 
  stages 
  of 
  alteration 
  

   are 
  the 
  chief 
  components, 
  and 
  on 
  the 
  weathered 
  surfaces 
  the 
  latter 
  

   may 
  be 
  etched 
  out 
  (4, 
  2). 
  — 
  Microsc. 
  The 
  felspar 
  exhibits 
  plagioclase 
  

   structure 
  either 
  in 
  the 
  interior 
  or 
  as 
  a 
  rim 
  ; 
  and 
  in 
  most 
  of 
  the 
  

   rocks 
  interstitial 
  clear 
  quartz 
  or 
  secondary 
  felspar 
  is 
  present, 
  some 
  of 
  

   which 
  may 
  be 
  traced 
  in 
  No. 
  4 
  originating 
  at 
  the 
  exterior 
  of 
  certain 
  

   crystals, 
  and 
  even 
  connected 
  with 
  a 
  kind 
  of 
  micropegmatitic 
  

   structure. 
  1 
  The 
  hornblende 
  in 
  some 
  cases 
  is 
  markedly 
  idiomorphic, 
  

   and 
  exhibits 
  a 
  change 
  from 
  the 
  brown 
  to 
  a 
  green 
  variety, 
  2 
  as 
  

   in 
  the 
  mottled 
  crystals 
  in 
  20 
  and 
  13. 
  In 
  9 
  the 
  large 
  idio- 
  

   morphic 
  hornblende 
  is 
  entirely 
  replaced 
  by 
  streaky 
  serpentinous 
  or 
  

   chloritic 
  mineral 
  with 
  small 
  included 
  crystals 
  ; 
  in 
  4 
  it 
  is 
  partially 
  

   replaced 
  by 
  filmy 
  viridite. 
  In 
  the 
  former 
  rock 
  some 
  of 
  the 
  enclosures 
  

   are 
  iron- 
  oxide 
  ; 
  but 
  the 
  most 
  interesting 
  are 
  very 
  numerous 
  prisms 
  

   of 
  rutile 
  (and 
  perhaps 
  pseudobrookite), 
  which 
  sometimes 
  are 
  related 
  

   in 
  their 
  position 
  to 
  the 
  principal 
  cleavages 
  of 
  the 
  hornblende. 
  3 
  In 
  

   20 
  a 
  light- 
  coloured 
  part 
  beyond 
  a 
  sharp 
  straight 
  boundary 
  of 
  the 
  

   hornblende- 
  crystal 
  is 
  probably 
  an 
  added 
  growth. 
  4 
  Specimen 
  2, 
  

   somewhat 
  bounded 
  by 
  joint-planes, 
  greyish-green, 
  and 
  externally 
  

   brownish, 
  shows 
  under 
  the 
  microscope 
  aggregates 
  of 
  chlorite 
  

   apparently 
  replacing 
  pyroxenic 
  crystals, 
  and 
  yellowish, 
  slightly 
  

   dichroic 
  flakes, 
  probably 
  altered 
  biotite, 
  and 
  some 
  approaching 
  viridite 
  

   in 
  character. 
  In 
  the 
  microlithic 
  groundmass 
  are 
  granular 
  epidote, 
  

   calcite, 
  and 
  iron-oxide. 
  Clustered 
  rutile 
  seems 
  to 
  have 
  formed 
  within 
  

   ilmenite 
  which 
  is 
  now 
  replaced 
  by 
  leucoxene, 
  or 
  witbin 
  sphene, 
  

   and 
  in 
  13 
  sphene 
  is 
  apparently 
  derived 
  from 
  ilmenite. 
  JSTo. 
  ] 
  4 
  is 
  an 
  

   iron-stained, 
  compact 
  rock 
  which 
  exhibits 
  (microscopically) 
  a 
  very 
  

   irregular 
  mosaic 
  of 
  plagioclase, 
  quartz, 
  colourless 
  chlorite, 
  and 
  a 
  

   high-refracting 
  mineral 
  giving 
  a 
  peculiar 
  blue 
  with 
  crossed 
  nicols. 
  

   It 
  includes 
  grains 
  small 
  and 
  large 
  (some 
  ^ 
  4 
  inch 
  in 
  diameter) 
  of 
  a 
  

   carbonate 
  with 
  a 
  green 
  rim, 
  and 
  of 
  haematite 
  or 
  limonite. 
  Corroded 
  

   remains 
  of 
  larger 
  plagioclase-felspars 
  occur. 
  These 
  two 
  rocks 
  have 
  

   undergone 
  much 
  change, 
  and 
  14 
  is 
  somewhat 
  crushed, 
  but 
  they 
  were 
  

   probably 
  felspathic 
  dolerites 
  or 
  diorites. 
  

  

  Two 
  rocks 
  also 
  from 
  the 
  Southern 
  region 
  (5, 
  17: 
  Murrat) 
  are 
  

   fine-grained 
  and 
  are 
  difficult 
  to 
  identify. 
  No. 
  5 
  is 
  a 
  dark-grey 
  rock, 
  

   with 
  an 
  interrupted 
  silky 
  look 
  due 
  to 
  minute 
  crystals, 
  and 
  with 
  small 
  

   brownish 
  projections 
  on 
  the 
  surfaces 
  smoothed 
  and 
  pitted 
  by 
  sand- 
  

   action. 
  — 
  Microsc. 
  The 
  knobs 
  consist 
  of 
  a 
  quartz-mosaic 
  coloured 
  by 
  

   a 
  brownish 
  carbonate. 
  The 
  cryptocrystalline 
  sericitic 
  groundmass 
  

  

  1 
  The 
  rock 
  reminds 
  me 
  somewhat 
  of 
  specimens 
  examined 
  from 
  the 
  Nile 
  

   Valley 
  and 
  from 
  West 
  Africa. 
  See 
  also 
  Geol. 
  Mag. 
  1893, 
  p. 
  436 
  ; 
  compare 
  

   Nos. 
  27 
  & 
  30, 
  infra, 
  p. 
  367. 
  

  

  2 
  Like 
  the 
  change 
  in 
  certain 
  picrites. 
  See 
  T. 
  G. 
  Bonney, 
  Quart. 
  Journ. 
  Geol. 
  

   Soc 
  vol. 
  xli. 
  (1885) 
  p. 
  520. 
  

  

  3 
  Compare 
  the 
  rutile 
  within 
  serpen 
  tinized 
  olivine, 
  'On 
  the 
  Eauenthal 
  

  

  Serpentine,' 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  liii. 
  (1897) 
  p 
  253. 
  

  

  4 
  See 
  Whitman 
  Cross, 
  Am. 
  Journ. 
  Sci. 
  ser. 
  3, 
  vol. 
  xxxix. 
  (1890) 
  p. 
  360. 
  

  

  