﻿^0\. 
  53.] 
  OF 
  THE 
  NUBIAN 
  DESERT. 
  371 
  

  

  elongated 
  and 
  orientated. 
  In 
  these 
  the 
  opaque 
  metallic 
  haematite 
  (?) 
  

   is 
  disseminated 
  in 
  fine 
  particles, 
  but 
  it 
  is 
  aggregated 
  as 
  crystals 
  or 
  

   thicker 
  streaks 
  among 
  the 
  cementing 
  secondary 
  quartz. 
  In 
  12 
  

   the 
  grains 
  of 
  the 
  mosaic 
  are 
  larger, 
  more 
  sharply 
  defined, 
  more 
  

   crystal-shaped, 
  and 
  freer 
  from 
  opacite. 
  

  

  Summary. 
  

  

  1. 
  Classes 
  of 
  Rocks. 
  

  

  With 
  the 
  exception 
  of 
  two 
  or 
  three 
  felstones, 
  the 
  igneous 
  rocks 
  

   are 
  holocrystalline. 
  One 
  is 
  apparently 
  granitic, 
  and 
  one 
  gneiss 
  may 
  

   be 
  a 
  modification 
  of 
  an 
  igneous 
  rock, 
  but 
  the 
  great 
  majority 
  belong 
  

   to 
  a 
  rather 
  basic 
  or 
  to 
  an 
  intermediate 
  type, 
  and 
  are 
  generally 
  much 
  

   altered 
  ; 
  they 
  are 
  diorites, 
  diabases, 
  dolerites, 
  or 
  gabbros. 
  Of 
  none 
  

   of 
  the 
  rocks 
  can 
  we 
  say 
  with 
  certainty 
  tbat 
  it 
  originated 
  as 
  a 
  

   lava-flow, 
  only 
  a 
  slight 
  indication 
  of 
  fluxional 
  structure 
  being 
  found 
  

   in 
  one 
  felstone, 
  and 
  certain 
  specimens 
  (such 
  as 
  the 
  gabbro 
  and 
  the 
  

   granite) 
  doubtless 
  can 
  only 
  be 
  from 
  intrusive 
  masses 
  ; 
  the 
  mode 
  of 
  

   occurrence 
  of 
  others 
  must 
  be 
  decided 
  by 
  field 
  evidence. 
  

  

  The 
  few 
  fragmental 
  rocks 
  come 
  from 
  the 
  ' 
  Schistose 
  ' 
  area, 
  except 
  

   •one 
  gritty 
  limestone 
  apparently 
  caught 
  in 
  among 
  igneous 
  masses. 
  

   Two 
  are 
  much 
  crushed, 
  probably 
  phyllites 
  ; 
  and 
  one 
  at 
  least 
  consists 
  

   chiefly 
  of 
  igneous 
  materials, 
  but 
  probably 
  originated 
  from 
  the 
  denu- 
  

   dation 
  of 
  doleritic 
  or 
  diabasic 
  rocks, 
  and 
  not 
  as 
  tuff 
  or 
  agglomerate. 
  

  

  2. 
  Mineral 
  Changes. 
  

  

  The 
  rock-constituents 
  afford 
  evidence 
  of 
  many 
  and 
  various 
  altera- 
  

   tions. 
  These 
  are 
  especially 
  illustrated 
  in 
  the 
  numerous 
  specimens 
  

   belonging 
  to 
  the 
  doleritic 
  or 
  allied 
  groups. 
  

  

  Felspar. 
  — 
  In 
  nearly 
  all 
  cases 
  the 
  felspars 
  are 
  much 
  changed, 
  in 
  some 
  

   completely 
  so. 
  Examples 
  occur 
  of 
  the 
  common 
  dusty 
  alteration, 
  while 
  

   much 
  of 
  the 
  felspar 
  has 
  developed 
  flakes 
  or 
  films, 
  either 
  scattered 
  or 
  

   massed 
  together, 
  of 
  a 
  minute 
  mica-like 
  mineral 
  which 
  may 
  be 
  some- 
  

   times 
  a 
  hydrous 
  mica, 
  sometimes 
  kaolin. 
  Epidote 
  occurs 
  in 
  crystals, 
  

   or 
  often 
  as 
  a 
  granular 
  aggregate 
  ; 
  but 
  some 
  of 
  the 
  felspars 
  are 
  replaced 
  

   by 
  zoisite 
  or 
  by 
  a 
  mixture 
  of 
  zoisite 
  and 
  epidote, 
  and 
  other 
  groups 
  of 
  

   these 
  minerals 
  probably 
  are 
  similarly 
  derived. 
  Occasionally 
  a 
  small 
  

   amount 
  of 
  a 
  greenish 
  mineral 
  resembling 
  a 
  serpentinous 
  product 
  is 
  

   present, 
  and 
  in 
  one 
  slide 
  a 
  quantity 
  of 
  chlorite 
  is 
  found 
  within 
  

   crystals 
  which 
  probably 
  are 
  felspar, 
  a 
  less 
  usual 
  alteration, 
  and 
  one 
  

   which 
  is 
  only 
  possible 
  by 
  the 
  interchange 
  of 
  constituents. 
  1 
  

  

  Pyroxenic 
  Minerals. 
  — 
  In 
  only 
  one 
  slide 
  is 
  there 
  any 
  ordinary 
  

   unchanged 
  augite, 
  and 
  even 
  there 
  many 
  of 
  the 
  crystals 
  are 
  either 
  

   partially 
  or 
  completely 
  altered 
  to 
  an 
  aggregate 
  mostly 
  of 
  an 
  actino- 
  

   litic 
  character. 
  Several 
  of 
  the 
  rocks 
  contain 
  common, 
  brown, 
  

   well-cleaved 
  hornblende, 
  nearly 
  always 
  exhibiting 
  alteration 
  to 
  a 
  

   mottled 
  and 
  pale 
  green 
  or 
  colourless 
  variety 
  or 
  to 
  actinolite. 
  The 
  

   lustre-mottling 
  of 
  the 
  crystals 
  is 
  interesting 
  ; 
  they 
  sometimes 
  

  

  1 
  See 
  T. 
  G. 
  Bonney, 
  ' 
  On 
  some 
  Schistose 
  Greenstones, 
  etc.,' 
  Quart. 
  Journ. 
  Geol. 
  

   Soc. 
  vol. 
  xlix. 
  (1893) 
  p. 
  103. 
  

  

  2c2 
  

  

  