﻿8 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [Nov. 
  4, 
  

  

  cayed 
  twigs 
  of 
  the 
  tamarisk, 
  which 
  grows 
  abundantly 
  in 
  the 
  district. 
  

   These 
  sand-hills 
  have 
  a 
  rounded 
  appearance, 
  and 
  vary 
  as 
  to 
  size, 
  many 
  

   being 
  fully 
  90 
  feet 
  in 
  height 
  : 
  on 
  their 
  sides 
  and 
  tops 
  are 
  tamarisk- 
  

   bushes, 
  which, 
  being 
  constantly 
  enveloped 
  in 
  the 
  sand-drift, 
  get 
  

   packed 
  so 
  closely 
  that 
  a 
  perpendicular 
  section 
  of 
  the 
  mound 
  has 
  the 
  

   exact 
  appearance 
  of 
  a 
  hay-rick, 
  the 
  small 
  twigs 
  being 
  deposited 
  any- 
  

   how, 
  whilst 
  fresh 
  bushes 
  are 
  sprouting 
  up 
  on 
  the 
  surface. 
  To 
  all 
  

   appearance, 
  these 
  accumulations 
  are 
  very 
  old. 
  Again, 
  along 
  the 
  

   edge 
  of 
  the 
  desert, 
  and 
  in 
  the 
  arable 
  tracts, 
  the 
  acacia 
  and 
  other 
  

   bushy 
  trees 
  and 
  shrubs 
  form 
  obstacles 
  to 
  the 
  sand, 
  which 
  is 
  seen 
  

   extending 
  in 
  long 
  tails 
  and 
  wreaths 
  behind 
  them. 
  It 
  takes 
  much 
  

   labour 
  to 
  prevent 
  the 
  encroachment 
  of 
  the 
  Nubian 
  desert, 
  and 
  now 
  

   that 
  the 
  population 
  has 
  been 
  decreasing 
  of 
  late 
  years 
  by 
  the 
  emi- 
  

   gration 
  of 
  the 
  able-bodied 
  men 
  to 
  Lower 
  Egypt, 
  the 
  advance 
  of 
  the 
  

   sand 
  on 
  the 
  cultivated 
  districts 
  is 
  becoming 
  more 
  apparent 
  ; 
  for 
  in- 
  

   stance, 
  north 
  of 
  Ibrim, 
  65 
  miles 
  north 
  of 
  Wadi 
  Halfeh, 
  I 
  noticed 
  

   that 
  the 
  desert 
  has 
  covered 
  a 
  great 
  alluvial 
  plain, 
  which 
  had 
  formerly 
  

   been 
  under 
  cultivation, 
  and 
  is 
  approaching 
  the 
  river, 
  so 
  that 
  the 
  

   trunks 
  of 
  the 
  palm-trees 
  are 
  completely 
  surrounded 
  with 
  sand 
  for 
  

   upwards 
  of 
  15 
  feet 
  from 
  their 
  roots. 
  

  

  2. 
  Alluvial 
  Heaps 
  of 
  lateral 
  Water-courses. 
  — 
  Although 
  rain 
  seldom 
  

   falls 
  in 
  Nubia, 
  yet, 
  when 
  such 
  is 
  the 
  case, 
  the 
  fall 
  is 
  remarkable 
  

   for 
  its 
  violence, 
  as 
  testified 
  by 
  the 
  water-courses 
  and 
  their 
  heaps 
  of 
  

   alluvium, 
  &c. 
  At 
  the 
  embouchures 
  of 
  these 
  torrents 
  high 
  banks 
  of 
  soil 
  

   are 
  to 
  be 
  seen, 
  doubtless 
  the 
  washings 
  from 
  the 
  ancient 
  Nile-deposits 
  

   on 
  the 
  higher 
  grounds, 
  where 
  the 
  organic 
  remains, 
  to 
  be 
  noticed 
  pre- 
  

   sently, 
  are 
  found. 
  A 
  breccia 
  of 
  great 
  hardness, 
  and 
  formed 
  of 
  rounded 
  

   as 
  well 
  as 
  angular 
  fragments, 
  may 
  be 
  often 
  traced 
  at 
  intervals 
  along 
  the 
  

   bottom 
  and 
  sides 
  of 
  these 
  water- 
  courses, 
  and, 
  from 
  its 
  always 
  under- 
  

   lying 
  the 
  alluvial 
  deposits, 
  it 
  would 
  appear 
  to 
  be 
  more 
  ancient. 
  I 
  have 
  

   no 
  doubt 
  that 
  it 
  has 
  also 
  been 
  formed 
  by 
  these 
  torrents, 
  which, 
  how- 
  

   ever, 
  are 
  often 
  local, 
  and 
  only 
  flow 
  occasionally 
  and 
  during 
  thunder- 
  

   storms. 
  The 
  alluvial 
  heaps 
  have 
  been 
  mistaken 
  for 
  ancient 
  deposits 
  of 
  

   the 
  Nile 
  ; 
  it 
  is 
  therefore 
  important 
  to 
  distinguish 
  them 
  from 
  what 
  

   are, 
  without 
  question, 
  the 
  result 
  of 
  the 
  river's 
  currents. 
  There 
  is 
  a 
  

   peculiarity 
  of 
  these 
  torrent-made 
  heaps 
  which 
  I 
  found 
  to 
  be 
  regular 
  ; 
  

   they 
  are 
  situated 
  at 
  the 
  entrances, 
  and 
  have 
  been 
  deposited 
  on 
  one 
  

   side, 
  as 
  if 
  caused 
  by 
  back-water, 
  — 
  a 
  result 
  of 
  the 
  current 
  receiving 
  

   a 
  check, 
  and 
  being 
  dammed 
  back 
  by 
  the 
  cultivated 
  land, 
  which 
  runs 
  

   across 
  the 
  mouths 
  of 
  the 
  ravines, 
  and 
  is 
  often 
  very 
  much 
  higher 
  than 
  

   the 
  bed 
  of 
  the 
  torrent. 
  On 
  the 
  surface, 
  and 
  throughout 
  the 
  alluvial 
  

   banks, 
  is 
  observed 
  a 
  hard, 
  concretionary, 
  tufaceous, 
  and 
  nodular 
  

   substance, 
  which 
  effervesces 
  readily 
  with 
  acid, 
  and 
  has 
  all 
  the 
  ap- 
  

   pearance 
  of 
  containing 
  a 
  large 
  amount 
  of 
  carbonate 
  of 
  lime 
  ; 
  it 
  is 
  

   usually 
  met 
  with 
  in 
  small 
  fragments, 
  which 
  are 
  strewed 
  over 
  the 
  sur- 
  

   face, 
  or 
  it 
  runs 
  in 
  veins 
  throughout 
  the 
  bed 
  ; 
  sometimes 
  it 
  is 
  found 
  

   in 
  masses 
  upwards 
  of 
  a 
  foot 
  in 
  thickness, 
  or 
  forming 
  sticks* 
  or 
  

   small 
  tubes 
  run 
  together 
  t 
  in 
  irregular- 
  shaped 
  masses. 
  It 
  is 
  plen- 
  

   tiful 
  all 
  over 
  the 
  alluvial 
  banks 
  of 
  Nubia 
  and 
  northwards, 
  wherever 
  

   * 
  See 
  Specimens 
  Nos. 
  3 
  and 
  4. 
  f 
  See 
  Specimen 
  No. 
  5. 
  

  

  