﻿Vol. 
  53.] 
  OF 
  THE 
  NUBIAN 
  DESEKT. 
  369 
  

  

  (vii) 
  Rocks 
  of 
  Igneous 
  Character, 
  but 
  Uncertain 
  Origin. 
  

   (< 
  Schistose 
  ' 
  area 
  : 
  21, 
  24, 
  23.) 
  

  

  (21) 
  Wadi 
  Om 
  Nabadi 
  ; 
  (24) 
  Jebel 
  Banat 
  Eaft. 
  Two 
  rocks 
  from 
  

   different 
  parts 
  of 
  the 
  southern 
  schistose 
  area 
  bear 
  some 
  resemblance 
  

   to 
  each 
  other. 
  Both 
  are 
  pale 
  greenish, 
  with 
  fine-grained 
  structure 
  

   developed, 
  probably 
  by 
  wind-erosion, 
  on 
  the 
  weathered 
  surface. 
  

   They 
  have 
  a 
  slight 
  appearance 
  of 
  schistosity 
  ; 
  and 
  one 
  rock 
  (24) 
  

   exhibits 
  apparent 
  fragments 
  separated 
  by 
  vein-like 
  parts 
  rich 
  in 
  

   epidote. 
  — 
  Microsc. 
  Both 
  rocks 
  include 
  worn 
  and 
  sometimes 
  rounded 
  

   plagioclase-crystals 
  ; 
  the 
  intermediate 
  part 
  being 
  much 
  masked 
  by 
  

   epidote 
  and 
  other 
  secondary 
  minerals. 
  In 
  all 
  the 
  constituents 
  

   microlithic 
  enclosures 
  have 
  formed, 
  often 
  of 
  small 
  flakes 
  of 
  chlorite, 
  

   of 
  which 
  mineral 
  larger 
  crystals 
  also 
  occur 
  in 
  the 
  slide 
  (dichroic, 
  

   changing 
  from 
  green 
  to 
  yellowish). 
  Much 
  scattered 
  iron-oxide 
  is 
  

   found. 
  While 
  some 
  of 
  the 
  microscopic 
  fragmental 
  appearance 
  is 
  

   probably 
  due 
  to 
  subsequent 
  disturbance 
  or 
  crushing, 
  some 
  of 
  it 
  is 
  

   probably 
  original. 
  The 
  rock 
  (21) 
  seems 
  certainly 
  a 
  diabase-grit 
  or 
  

   ash, 
  with 
  inclusion 
  possibly 
  of 
  some 
  andesitic 
  or 
  felsitic 
  fragments 
  ; 
  

   while 
  24, 
  although 
  it 
  may 
  have 
  been 
  also 
  originally 
  clastic, 
  has 
  more 
  

   resemblance 
  to 
  a 
  true 
  igneous 
  rock. 
  The 
  sp. 
  gr. 
  of 
  24 
  is 
  2*77. 
  

  

  (23 
  : 
  Om 
  Nabadi) 
  is 
  a 
  dull 
  greenish-drab, 
  finely-speckled 
  schistose 
  

   rock, 
  in 
  which 
  the 
  plagioclase-crystals 
  are 
  broken 
  and 
  corroded. 
  

   The 
  felsitic 
  groundmass 
  contains 
  greenish 
  microliths 
  (more 
  abundant 
  

   along 
  iron-stained 
  streaks), 
  a 
  carbonate, 
  limonite, 
  ? 
  chalybite, 
  ? 
  epi- 
  

   dote, 
  and 
  black 
  iron-oxide. 
  The 
  brown 
  mica 
  includes 
  small 
  scales 
  

   changing 
  to 
  green, 
  and 
  larger, 
  clearer 
  flakes, 
  transverse 
  to 
  the 
  

   foliation, 
  which 
  thus 
  appear 
  to 
  be 
  secondary, 
  like 
  those 
  described 
  

   by 
  Prof. 
  Bonney 
  in 
  an 
  Alpine 
  schist. 
  1 
  Green 
  dichroic 
  flakes 
  

   occur, 
  probably 
  derived 
  from 
  certain 
  of 
  these 
  larger 
  mica-crystals. 
  

   The 
  clearer 
  areas, 
  enclosed 
  within 
  greenish 
  stringy 
  lines, 
  appear, 
  on 
  

   microscopic 
  examination, 
  to 
  be 
  like 
  normal 
  felsite, 
  and 
  the 
  rock, 
  

   which 
  looks 
  finely 
  fragmental, 
  may 
  owe 
  this 
  appearance 
  to 
  crushing, 
  

   and 
  may 
  be 
  a 
  squeezed 
  porphyrite. 
  The 
  sp. 
  gr. 
  of 
  one 
  fragment 
  

   was 
  found 
  to 
  be 
  2-71, 
  which 
  would 
  agree 
  very 
  well 
  with 
  this 
  

   identification. 
  

  

  (viii) 
  Sedimentary 
  Fragmental 
  Rocks. 
  

  

  One 
  rock 
  from 
  the 
  Southern 
  area 
  is 
  an 
  impure 
  limestone 
  (6 
  : 
  

   Murrat, 
  S.). 
  A 
  strong, 
  compact, 
  dark 
  grey 
  rock, 
  with 
  a 
  somewhat 
  

   foliated 
  look, 
  containing 
  small, 
  elongated, 
  slightly 
  irregular, 
  silver}' 
  

   flakes, 
  very 
  definitely 
  orientated. 
  The 
  rock 
  effervesces 
  briskly 
  with 
  

   hydrochloric 
  acid, 
  is 
  a 
  quartzose 
  or 
  gritty 
  limestone, 
  indurated, 
  and 
  

   from 
  the 
  flaky 
  laminae 
  has 
  the 
  appearance 
  of 
  being 
  pressure-modified, 
  

   but 
  this 
  character 
  cannot 
  be 
  well 
  recognized 
  in 
  the 
  microscope-slide. 
  

   One 
  surface 
  is 
  sand-worn. 
  — 
  Microsc. 
  Calcite, 
  the 
  chief 
  constituent, 
  

   is 
  dusty 
  with 
  opacite 
  (? 
  carbonaceous), 
  and 
  is 
  interrupted 
  by 
  nests 
  

   of 
  impure 
  limonite. 
  The 
  included 
  fragments 
  consist 
  of 
  quartz, 
  of 
  

  

  1 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xlix. 
  (1893) 
  p. 
  104. 
  

   Q.J. 
  G.S. 
  No. 
  211. 
  2 
  c 
  

  

  