﻿Yol. 
  53.] 
  GEOLOGY 
  OP 
  PART 
  OF 
  THE 
  NUBIAN 
  DESERT. 
  

  

  363 
  

  

  places, 
  is 
  abundant, 
  the 
  water 
  of 
  the 
  Nubian 
  Desert 
  is 
  directly 
  

   dependent 
  on 
  the 
  rainfall. 
  This 
  is 
  irregular 
  and 
  may 
  not 
  occur 
  for 
  

   several 
  years, 
  during 
  which 
  time 
  the 
  wells 
  become 
  very 
  low 
  and 
  their 
  

   water 
  strongly 
  saline. 
  1 
  The 
  sources 
  of 
  supply 
  in 
  this 
  Nubian 
  Desert 
  

   are 
  two 
  : 
  — 
  firstly, 
  from 
  wells 
  (Bir), 
  and 
  secondly 
  from 
  reservoirs 
  

   (Makhzan). 
  

  

  The 
  first 
  of 
  these 
  are 
  sunk 
  to 
  a 
  depth 
  of 
  from 
  8 
  to 
  20 
  feet 
  in 
  the 
  

   detritus 
  of 
  the 
  valleys, 
  and 
  are 
  fed 
  by 
  the 
  water 
  which 
  percolates 
  

   into 
  them 
  from 
  the 
  neighbouring 
  rocks, 
  being 
  full 
  after 
  heavy 
  rain 
  

   and 
  diminishing 
  as 
  the 
  drought 
  continues. 
  As 
  a 
  consequence, 
  nearly 
  

   all 
  these 
  wells 
  contain 
  a 
  considerable 
  amount 
  of 
  mineral 
  matter 
  

   in 
  solution, 
  and 
  the 
  waters 
  of 
  some 
  are 
  almost 
  undrinkable. 
  

  

  Rounded 
  boulders 
  in 
  pot-hole, 
  reservoir 
  of 
  Wadi 
  On 
  

   (Jebel 
  Raft). 
  

  

  Risha 
  

  

  [Height 
  represented 
  in 
  figure 
  = 
  about 
  20 
  feet.] 
  

  

  The 
  second 
  source 
  of 
  supply, 
  the 
  rainwater-reservoirs, 
  are 
  deep 
  

   holes 
  in 
  the 
  gullies 
  and 
  ravines 
  which 
  intersect 
  the 
  crystalline 
  hills 
  ; 
  

   these 
  are 
  filled 
  by 
  rainstorms, 
  and 
  as 
  they 
  are 
  partly 
  screened 
  from 
  

   the 
  sun 
  by 
  the 
  steep 
  side 
  of 
  the 
  ravine 
  they 
  often 
  retain 
  the 
  water 
  

   for 
  nearly 
  a 
  year. 
  Some 
  of 
  these 
  in 
  Jebel 
  Raft 
  are 
  of 
  considerable 
  

   size, 
  that 
  in 
  the 
  Wadi 
  Om 
  Risha 
  being 
  33 
  x 
  44 
  x 
  20 
  feet 
  in 
  cubical 
  

   capacity. 
  The 
  origin 
  of 
  these 
  deep 
  holes 
  worn 
  in 
  the 
  ravines 
  must 
  

   be 
  attributed 
  to 
  water-action, 
  and 
  in 
  the 
  reservoir 
  of 
  Medina 
  in 
  

   Jebel 
  Raft 
  lie 
  the 
  rounded 
  stones 
  which 
  have 
  assisted 
  in 
  forming 
  this 
  

   pot-hole. 
  (See 
  the 
  accompanying 
  figure.) 
  It 
  is 
  hardly 
  conceivable 
  

   that 
  this 
  has 
  been 
  effected 
  by 
  the 
  rare, 
  though 
  often 
  violent, 
  rain- 
  

   storms 
  of 
  the 
  present 
  time, 
  and 
  it 
  must, 
  I 
  think, 
  be 
  attributed 
  to 
  

   an 
  earlier 
  period 
  when 
  the 
  rainfall 
  was 
  heavier 
  and 
  carved 
  out 
  the 
  

   ravines 
  and 
  valleys 
  which 
  we 
  see 
  at 
  this 
  day. 
  Similar 
  rainwater- 
  

   reservoirs 
  occur 
  in 
  the 
  hills 
  on 
  the 
  western 
  coast 
  of 
  the 
  Red 
  Sea. 
  

  

  1 
  Within 
  the 
  last 
  few 
  years 
  the 
  only 
  rains 
  capable 
  of 
  filling 
  all 
  the 
  wells 
  in 
  

   the 
  neighbourhood 
  of 
  Murrat 
  fell 
  in 
  November 
  1891 
  and 
  August 
  1896. 
  In 
  

   this 
  long 
  interval 
  only 
  a 
  few 
  showers 
  fell. 
  

  

  