﻿362 
  CAPT. 
  H. 
  G. 
  LYONS 
  ON 
  THE 
  [Aug. 
  1 
  897, 
  

  

  the 
  spot, 
  but 
  the 
  small 
  size 
  of 
  the 
  stones 
  used 
  and 
  the 
  piles 
  of 
  coarse 
  

   broken 
  (but 
  not 
  crushed 
  or 
  ground) 
  quartz 
  lying 
  about 
  would 
  lead 
  

   one 
  to 
  think 
  that 
  the 
  schists 
  rather 
  than 
  the 
  quartz 
  had 
  been 
  

   worked 
  as 
  the 
  source 
  of 
  the 
  metals, 
  the 
  quartz 
  having 
  been 
  thrown 
  

   aside 
  as 
  rubbish. 
  I 
  could 
  see 
  no 
  signs 
  of 
  gold 
  in 
  the 
  pieces 
  of 
  quartz 
  

   which 
  I 
  examined. 
  At 
  this 
  part 
  the 
  strike 
  of 
  the 
  schists 
  and 
  of 
  

   the 
  veins 
  of 
  white 
  quartz 
  is 
  W.S.W.-E.N.E. 
  

  

  North 
  of 
  Bir 
  Tilat 
  Abda, 
  and 
  as 
  far 
  as 
  Bir 
  Ongat, 
  the 
  massive 
  

   crystalline 
  rocks 
  alone 
  are 
  seen, 
  while 
  to 
  the 
  north 
  of 
  Bir 
  Ab 
  Araga 
  

   is 
  a 
  very 
  large 
  exposure 
  of 
  greyish-red 
  granite. 
  Not 
  until 
  north- 
  

   west 
  of 
  Bir 
  Ongat 
  do 
  the 
  schistose 
  rocks 
  again 
  appear, 
  and 
  along 
  

   Wadi 
  Nayit 
  their 
  strike 
  coincides 
  generally 
  with 
  the 
  direction 
  of 
  

   the 
  wadi 
  (from 
  N.-S. 
  to 
  N.N.W.-S.S.E.), 
  while 
  granite 
  forms 
  a 
  

   low 
  chain 
  of 
  hills 
  to 
  the 
  west. 
  

  

  Down 
  the 
  Wadi 
  Allaki, 
  which 
  takes 
  all 
  the 
  drainage 
  of 
  this 
  part, 
  

   schistose 
  rocks 
  alone 
  appear, 
  having 
  a 
  strike 
  generally 
  at 
  right 
  angles 
  

   to 
  the 
  valley, 
  that 
  is, 
  N.E.-S.W. 
  

  

  (3) 
  Pre-Cretaceous 
  (?) 
  Conglomerate. 
  

  

  On 
  the 
  crystalline 
  rock-mass 
  of 
  Jebel 
  Raft, 
  and 
  apparently 
  over- 
  

   lain 
  by 
  the 
  Nubian 
  Sandstone, 
  there 
  is 
  a 
  very 
  coarse 
  conglomerate 
  

   formed 
  of 
  fragments 
  of 
  the 
  crystalline 
  rock, 
  both 
  rounded 
  and 
  

   subangular, 
  with 
  intercalated 
  patches 
  of 
  sandstone 
  here 
  and 
  there. 
  

   The 
  rainwater- 
  reservoirs 
  of 
  Wadi 
  Mogharin 
  occur 
  in 
  this 
  rock. 
  

  

  It 
  was 
  not 
  seen 
  elsewhere, 
  so 
  that 
  its 
  true 
  position 
  could 
  not 
  be 
  

   determined, 
  but 
  it 
  appeared 
  to 
  be 
  older 
  than 
  the 
  Nubian 
  Sandstone 
  

   which 
  was 
  seen 
  lying 
  close 
  to 
  it 
  (if 
  not 
  overlying 
  it). 
  The 
  latter, 
  

   however, 
  has 
  a 
  basal 
  bed 
  of 
  quartz-conglomerate 
  with 
  pebbles 
  of 
  

   no 
  great 
  size 
  and 
  very 
  different 
  in 
  appearance 
  from 
  this 
  coarse 
  

   conglomerate. 
  

  

  (4) 
  The 
  Nubian 
  Sandstone. 
  

  

  The 
  Nubian 
  Sandstone 
  is 
  laid 
  down 
  on 
  the 
  flanks 
  of 
  the 
  crystalline 
  

   hills, 
  with 
  little 
  or 
  no 
  dip. 
  It 
  shows 
  very 
  slight 
  variation 
  in 
  

   composition, 
  being 
  a 
  quartzose 
  sandstone 
  of 
  different 
  degrees 
  of 
  

   fineness, 
  passing 
  from 
  a 
  pure 
  white 
  colour 
  through 
  all 
  the 
  shades 
  

   of 
  yellow, 
  brown, 
  and 
  red, 
  from 
  staining 
  by 
  oxides 
  of 
  iron 
  and 
  

   manganese. 
  Black 
  nodular 
  concretions 
  of 
  sand 
  cemented 
  by 
  these 
  

   minerals 
  are 
  very 
  common, 
  and, 
  in 
  consequence 
  of 
  their 
  power 
  of 
  

   resistance 
  to 
  weathering, 
  lie 
  everywhere 
  on 
  the 
  surface 
  of 
  the 
  desert. 
  

   Sometimes 
  a 
  more 
  argillaceous 
  bed 
  is 
  seen, 
  but 
  it 
  has 
  no 
  great 
  

   extent, 
  and 
  no 
  beds 
  could 
  be 
  found 
  sufficiently 
  constant 
  to 
  be 
  of 
  

   any 
  stratigraphical 
  value. 
  No 
  rock 
  was 
  anywhere 
  seen 
  intrusive 
  in 
  

   the 
  sandstone. 
  

  

  The 
  main 
  obstacle 
  in 
  traversing 
  the 
  Nubian 
  Desert 
  is 
  the 
  want 
  of 
  

   water 
  and 
  the 
  uncertainty 
  of 
  finding 
  a 
  sufficient 
  supply 
  at 
  any 
  of 
  the 
  

   wells. 
  Unlike 
  the 
  western 
  desert, 
  where 
  the 
  water 
  is 
  derived 
  from 
  

   $eep-seated 
  sources, 
  and 
  the 
  supply, 
  though 
  only 
  available 
  at 
  a 
  few 
  

  

  