﻿368 
  MISS 
  C. 
  A. 
  KAISIN 
  ON 
  THE 
  PETROLOGY 
  [Aug. 
  1 
  897, 
  

  

  micropegmatitic 
  structure. 
  This 
  may 
  show 
  finger-like 
  divisions 
  

   radiating 
  in 
  tufts, 
  or 
  two 
  sets 
  of 
  them 
  at 
  right 
  angles 
  forming 
  a 
  

   kind 
  of 
  basket-work, 
  but 
  usually 
  the 
  structure 
  is 
  less 
  regular. 
  1 
  The 
  

   hornblende 
  is 
  dichroic 
  (myrtle-green 
  to 
  pale 
  greenish-yellow), 
  not 
  

   idiomorphic 
  or 
  fresh 
  -looking, 
  associated 
  with 
  some 
  altered 
  biotite. 
  

   Iron-oxide, 
  epidote, 
  ? 
  apatite, 
  besides 
  quartz, 
  are 
  present. 
  The 
  

   rock 
  originally 
  may 
  have 
  been 
  a 
  diorite. 
  

  

  No. 
  27 
  is 
  a 
  pinkish-white 
  fine-grained 
  rock. 
  — 
  Microsc. 
  The 
  fel- 
  

   spar 
  is 
  plagioclase; 
  the 
  quartz 
  forms 
  a 
  mosaic, 
  containing 
  lines 
  of 
  

   cavities 
  and 
  acicular 
  microliths 
  which 
  extend 
  through 
  adjacent 
  

   grains. 
  The 
  rich 
  green 
  dichroic 
  hornblende, 
  2 
  with 
  twinning 
  along 
  

   100, 
  encloses 
  grains 
  of 
  quartz 
  or 
  felspar. 
  Iron-oxide 
  (? 
  ilmenite), 
  

   epidote, 
  and 
  impure-looking 
  sphene 
  occur. 
  The 
  constituents 
  are 
  

   all 
  rather 
  irregular, 
  and 
  the 
  rock 
  appears 
  to 
  have 
  been 
  somewhat 
  

   affected 
  by 
  pressure 
  followed 
  by 
  reconstitution. 
  

  

  (v) 
  Granite. 
  

  

  One 
  specimen 
  from 
  the 
  Western 
  area 
  (3 
  : 
  Jebel 
  Raft, 
  west 
  side, 
  by 
  

   WadiTonaidba) 
  consists 
  of 
  quartz 
  (projecting 
  on 
  weathered 
  surfaces), 
  

   of 
  much 
  pinkish 
  felspar, 
  and 
  of 
  a 
  reddish-black 
  iron-oxide. 
  — 
  Microsc. 
  

   The 
  felspars 
  are 
  idiomorphic, 
  kaolinized, 
  but 
  at 
  places 
  apparently 
  

   plagioclase. 
  In 
  the 
  interstitial 
  quartz 
  are 
  lines 
  of 
  enclosures. 
  Veins 
  

   also 
  occur 
  which 
  are 
  in 
  mineral 
  continuity 
  with 
  the 
  quartz 
  where 
  

   they 
  pass 
  through 
  it. 
  The 
  haematite 
  or 
  limonite 
  replaces 
  fairly 
  large 
  

   crystals 
  which 
  are 
  posterior 
  to 
  the 
  felspar, 
  but 
  resemble 
  pyroxene, 
  

   and 
  are 
  similar 
  to 
  the 
  replaced 
  pyroxene 
  (?) 
  in 
  rocks, 
  probably 
  

   doleritic, 
  from 
  Abu 
  Seir 
  and 
  West 
  Africa. 
  3 
  

  

  (vi) 
  Felstones. 
  

  

  Two 
  rocks 
  from 
  the 
  Eastern 
  area 
  appear 
  to 
  be 
  cryptocrystalline 
  

   forms 
  of 
  an 
  intermediate 
  or 
  acid 
  type. 
  No. 
  31 
  is 
  jointed, 
  with 
  rather 
  

   polished 
  surfaces. 
  The 
  rock 
  is 
  brown 
  and 
  flinty-looking, 
  speckled 
  

   with 
  white 
  porphyritic 
  felspar-crystals, 
  partly 
  corroded. 
  — 
  Microsc. 
  

   A 
  second 
  generation 
  of 
  small 
  felspars 
  helps 
  to 
  form 
  the 
  ground- 
  

   mass, 
  which 
  includes 
  small 
  green, 
  dichroic, 
  micaceous, 
  or 
  chloritic 
  

   flakes 
  and 
  black 
  iron-oxide. 
  The 
  rock 
  probably 
  is 
  a 
  compact 
  

   quartz-felsite. 
  

  

  No. 
  32 
  is 
  pale 
  grey, 
  with 
  numerous 
  whitish 
  porphyritic 
  felspars, 
  

   often 
  idiomorphic, 
  like 
  orthoclase 
  in 
  shape, 
  but 
  altered, 
  with 
  various 
  

   enclosures 
  — 
  micaceous, 
  pyroxenic, 
  and 
  others. 
  A 
  vein 
  filled 
  with 
  a 
  

   carbonate 
  occurs. 
  The 
  devitrified 
  groundmass, 
  abundantly 
  micro- 
  

   lithic, 
  has 
  afluxional 
  look, 
  and 
  the 
  rock 
  is 
  a 
  felsite 
  probably 
  slightly 
  

   affected 
  by 
  pressure. 
  

  

  1 
  Although 
  a 
  similar 
  formation 
  in 
  other 
  rocks 
  may 
  be 
  original, 
  this 
  suggests 
  a 
  

   secondary 
  deposition 
  along 
  cleavage-planes, 
  resembling 
  the 
  ' 
  quartz 
  de 
  corrosion 
  ' 
  

   of 
  Fouque 
  & 
  Levy, 
  ' 
  Min. 
  Micr.' 
  (Mem. 
  Carte 
  geol. 
  France, 
  1879) 
  p. 
  193. 
  

  

  2 
  a, 
  myrtle-green 
  ; 
  b, 
  olive-green 
  ; 
  c, 
  pale 
  brownish-green. 
  

  

  3 
  Geol. 
  Mag. 
  1893, 
  p. 
  441. 
  

  

  