﻿^°1' 
  53*1 
  0F 
  THE 
  ^UB^AN 
  DESERT. 
  367 
  

  

  might 
  be 
  derived 
  from 
  a 
  felspathic 
  substance, 
  and 
  numerous 
  brown- 
  

   stained, 
  greenish, 
  dichroic 
  flakes 
  or 
  prisms 
  occur. 
  The 
  rock 
  has 
  

   rather 
  the 
  aspect 
  of 
  a 
  mica-trap, 
  but 
  some 
  of 
  the 
  last-named 
  

   constituent 
  seems 
  to 
  have 
  an 
  oblique 
  extinction, 
  as 
  if 
  it 
  were 
  

   hornblende 
  rather 
  than 
  biotite. 
  No. 
  17 
  has 
  a 
  modified 
  felspathic 
  

   ground 
  somewhat 
  similar 
  to 
  5 
  (with 
  calcite, 
  quartz, 
  limonite, 
  and 
  

   minute 
  ? 
  kyanite) 
  ; 
  but 
  it 
  is 
  penetrated 
  by 
  strings 
  of 
  small 
  horn- 
  

   blende, 
  and 
  encloses 
  large 
  porphyritic 
  felspars, 
  and 
  probably 
  altered 
  

   representatives 
  of 
  others. 
  These 
  form 
  whitish 
  oblongs 
  in 
  the 
  pale 
  

   greenish 
  hand-specimen, 
  which 
  exhibits 
  a 
  platy 
  structure, 
  and 
  the 
  

   rock 
  might 
  be 
  called 
  a 
  hornblende-schist, 
  but 
  is 
  probably 
  a 
  modified 
  

   diorite. 
  

  

  In 
  25 
  (Tilat 
  Abda) 
  from 
  the 
  Eastern 
  area, 
  we 
  return 
  rather 
  nearer 
  

   to 
  the 
  type 
  of 
  2 
  or 
  14. 
  It 
  is 
  a 
  greyish 
  rock 
  crowded 
  with 
  small, 
  

   white, 
  irregular 
  felspars. 
  — 
  Microsc. 
  These 
  are 
  dusty-looking, 
  and 
  

   other 
  crystals, 
  doubtless 
  pyroxenic, 
  consist 
  partly 
  of 
  yellowish, 
  

   generally 
  isotropic 
  substance. 
  As 
  in 
  2, 
  an 
  interstitial 
  clear 
  mineral, 
  

   granular 
  epidote, 
  calcite, 
  and 
  iron-oxide 
  are 
  present. 
  

  

  From 
  the 
  Eastern 
  area 
  also 
  come 
  four 
  other 
  specimens 
  of 
  rather 
  

   different 
  types 
  (29, 
  26, 
  30, 
  27). 
  

  

  (d) 
  Porphyritic 
  Diabase 
  (29). 
  — 
  A 
  dull 
  greenish 
  rock, 
  sand-polished 
  

   and 
  somewhat 
  crushed. 
  — 
  Microsc. 
  The 
  porphyritic 
  crystals 
  are 
  large, 
  

   lath-shaped, 
  rich 
  green, 
  and 
  irregularly 
  terminated. 
  They 
  are 
  

   replaced 
  by 
  chlorite, 
  epidote, 
  granular 
  secondary 
  felspar, 
  sometimes 
  

   deposited 
  as 
  if 
  along 
  cleavage-planes 
  ; 
  if 
  these 
  were 
  felspar 
  originally, 
  

   they 
  have 
  developed 
  an 
  exceptional 
  amount 
  of 
  chlorite. 
  The 
  

   grouudmass 
  is 
  almost 
  completely 
  masked 
  by 
  granular 
  epidote. 
  

  

  (e) 
  Diabase 
  with 
  remains 
  of 
  Augite 
  (26). 
  — 
  Only 
  one 
  rock 
  in 
  the 
  

   collection 
  exhibits 
  augite 
  partially 
  unaltered 
  occurring 
  as 
  porphy- 
  

   ritic 
  crystals, 
  and 
  even 
  here 
  hornblende 
  forms 
  the 
  ferromagnesian 
  

   constituent 
  of 
  the 
  groundraass. 
  Doubtless 
  in 
  many 
  of 
  the 
  rocks 
  

   augite 
  was 
  originally 
  present, 
  the 
  hornblende 
  being 
  secondary. 
  

   This 
  specimen 
  is 
  dark 
  greenish-grey, 
  slightly 
  speckled, 
  containing 
  

   narrow, 
  white, 
  porphyritic 
  felspars, 
  sometimes 
  in 
  radiating 
  clusters, 
  

   and 
  some 
  iron 
  pyrites. 
  — 
  Microsc. 
  The 
  felspar 
  is 
  kaolinized 
  ; 
  the 
  

   augite 
  sometimes 
  well-preserved, 
  occasionally 
  twinned 
  along 
  100 
  ; 
  

   but 
  generally 
  the 
  crystals 
  are 
  changed 
  — 
  along 
  the 
  borders 
  or 
  

   completely 
  — 
  to 
  fibrous 
  hornblende. 
  Clusters 
  of 
  similar 
  actinolite 
  

   with 
  a 
  carbonate 
  and 
  iron-oxide 
  are 
  found. 
  The 
  groundmass 
  

   includes 
  labradorite, 
  and 
  gummy 
  brown 
  mica-like 
  hornblende 
  

   passing 
  into 
  green 
  actinolite. 
  The 
  rock, 
  originally 
  a 
  minutely 
  

   crystalline 
  dolerite, 
  now 
  is 
  rather 
  a 
  hornblendic 
  diabase. 
  

  

  (iv) 
  Quartz-diorite. 
  

  

  In 
  30 
  the 
  quartz 
  is 
  not, 
  perhaps, 
  in 
  sufficient 
  amount 
  to 
  justify 
  

   this 
  term, 
  but 
  it 
  leads 
  up 
  to 
  the 
  better-marked 
  type 
  (27). 
  The 
  

   former 
  consists 
  of 
  flesh-coloured 
  or 
  greenish 
  felspar 
  and 
  of 
  blackish 
  

   hornblende 
  with 
  a 
  little 
  quartz. 
  — 
  Microsc. 
  The 
  plagioclase-felspar 
  

   is 
  intercrystallized 
  with 
  a 
  mosaic, 
  the 
  grains 
  exhibiting 
  a 
  kind 
  of 
  

  

  